What is the wildcard character in SQL?
Let us say you want to query database with LIKE for all employees whose name starts with La. The wildcard character is %, the proper query with LIKE would involve La%.
What is the role of the DataReader class in ADO.NET connections?
It returns a read-only dataset from the data source when the command is executed.
What does the This window show in the debugger?
It points to the object that is pointed to by this reference. Object’s instance data is shown.
Describe the accessibility modifier protected internal?
It is available to derived classes and classes within the same Assembly (and naturally from the base class it is declared in).
What is an interface class?
It is an abstract class with public abstract methods all of which must be implemented in the inherited classes.
What is a multicast delegate?
It is a delegate that points to and eventually fires off several methods.
How does one compare strings in C#?
In the past, you had to call .ToString() on the strings when using the == or != operators to compare the strings' values. That will still work, but the C# compiler now automatically compares the values instead of the references when the == or != operators are used on string types. If you actually do want to compare references, it can be done as follows: if ((object) str1 == (object) str2) { ... } Here's an example showing how string compares work: using System;
public class StringTest
{
public static void Main(string[] args)
{
Object nullObj = null;
Object realObj = new StringTest();
int i = 10;
Console.WriteLine("Null Object is [" + nullObj + "]n" +
"Real Object is [" + realObj + "]n" +
"i is [" + i + "]n");
// Show string equality operators
string str1 = "foo";
string str2 = "bar";
string str3 = "bar";
Console.WriteLine("{0} == {1} ? {2}", str1, str2, str1 == str2 );
Console.WriteLine("{0} == {1} ? {2}", str2, str3, str2 == str3 );
}
}
Output: Null Object is []
Real Object is [StringTest]
i is [10]
foo == bar ? False
bar == bar ? True
What does assert() do?
In debug compilation, assert takes in a Boolean condition as a parameter, and shows the error dialog if the condition is false. The program proceeds without any interruption if the condition is true.
How do I get deterministic finalization in C#?
In a garbage collected environment, it's impossible to get true determinism. However, a design pattern that we recommend is implementing IDisposable on any class that contains a critical resource. Whenever this class is consumed, it may be placed in a using statement, as shown in the following example:
using(FileStream myFile = File.Open(@"c:temptest.txt",
FileMode.Open))
{
int fileOffset = 0;
while(fileOffset < myFile.Length)
{
Console.Write((char)myFile.ReadByte());
fileOffset++;
}
}
When myFile leaves the lexical scope of the using, its dispose method will be called.
How can I get around scope problems in a try/catch?
If you try to instantiate the class inside the try, it'll be out of scope when you try to access it from the catch block. A way to get around this is to do the following: Connection conn = null;
try
{
conn = new Connection();
conn.Open();
}
finally
{
if (conn != null) conn.Close();
}
By setting it to null before the try block, you avoid getting the CS0165 error (Use of possibly unassigned local variable 'conn').
Why do I get an error (CS1006) when trying to declare a method without specifying a return type?
If you leave off the return type on a method declaration, the compiler thinks you are trying to declare a constructor. So if you are trying to declare a method that returns nothing, use void. The following is an example: // This results in a CS1006 error public static staticMethod (mainStatic obj) // This will work as wanted public static void staticMethod (mainStatic obj)
How do I convert a string to an int in C#?
Here's an example: using System;
class StringToInt
{
public static void Main()
{
String s = "105";
int x = Convert.ToInt32(s);
Console.WriteLine(x);
}
}
How do you directly call a native function exported from a DLL?
Here's a quick example of the DllImport attribute in action: using System.Runtime.InteropServices;
class C
{
[DllImport("user32.dll")]
public static extern int MessageBoxA(int h, string m, string c, int type);
public static int Main()
{
return MessageBoxA(0, "Hello World!", "Caption", 0);
}
}
This example shows the minimum requirements for declaring a C# method that is implemented in a native DLL. The method C.MessageBoxA() is declared with the static and external modifiers, and has the DllImport attribute, which tells the compiler that the implementation comes from the user32.dll, using the default name of MessageBoxA. For more information, look at the Platform Invoke tutorial in the documentation.
What is the .NET datatype that allows the retrieval of data by a unique key?
HashTable.
How do you specify a custom attribute for the entire assembly (rather than for a class)?
Global attributes must appear after any top-level using clauses and before the first type or namespace declarations. An example of this is as follows:
using System;
[assembly : MyAttributeClass]
class X {}
Note that in an IDE-created project, by convention, these attributes are placed in
AssemblyInfo.cs.
What is the difference between a struct and a class in C#?
From language spec:
The list of similarities between classes and structs is as follows. Longstructs can implement interfaces and can have the same kinds of members as classes. Structs differ from classes in several important ways; however, structs are value types rather than reference types, and inheritance is not supported for structs. Struct values are stored on the stack or in-line. Careful programmers can sometimes enhance performance through judicious use of structs. For example, the use of a struct rather than a class for a Point can make a large difference in the number of memory allocations performed at runtime. The program below creates and initializes an array of 100 points. With Point implemented as a class, 101 separate objects are instantiated-one for the array and one each for the 100 elements.
What is the difference between the Debug class and Trace class?
Documentation looks the same. Use Debug class for debug builds, use Trace class for both debug and release builds.
How can you overload a method?
Different parameter data types, different number of parameters, different order of parameters.
What debugging tools come with the .NET SDK?
CorDBG - command-line debugger, and DbgCLR - graphic debugger. Visual Studio .NET uses the DbgCLR. To use CorDbg, you must compile the original C# file using the /debug switch.
What does Dispose method do with the connection object?
Deletes it from the memory.
How do you generate documentation from the C# file commented properly with a command-line compiler?
Compile it with a /doc switch.
When you inherit a protected class-level variable, who is it available to?
Classes in the same namespace.
How can I get the ASCII code for a character in C#?
Casting the char to an int will give you the ASCII value: char c = 'f'; System.Console.WriteLine((int)c); or for a character in a string: System.Console.WriteLine((int)s[3]); The base class libraries also offer ways to do this with the Convert class or Encoding classes if you need a particular encoding.
Is there an equivalent to the instanceof operator in Visual J++?
C# has the is operator:
expr is type
How do I create a Delegate/MulticastDelegate?
C# requires only a single parameter for delegates: the method address. Unlike other languages, where the programmer must specify an object reference and the method to invoke, C# can infer both pieces of information by just specifying the method's name. For example, let's use System.Threading.ThreadStart: Foo MyFoo = new Foo(); ThreadStart del = new ThreadStart(MyFoo.Baz); This means that delegates can invoke static class methods and instance methods with the exact same syntax!
How do destructors and garbage collection work in C#?
C# has finalizers (similar to destructors except that the runtime doesn't guarantee they'll be called), and they are specified as follows:
class C
{
~C()
{
// your code
}
public static void Main() {}
}
Currently, they override object.Finalize(), which is called during the GC process.
My switch statement works differently! Why?
C# does not support an explicit fall through for case blocks.
The following code is not legal and will not compile in C#: switch(x)
{
case 0:
// do something
case 1:
// do something in common with 0
default:
// do something in common with
//0, 1 and everything else
break;
}
To achieve the same effect in C#, the code must be modified
as shown below (notice how the control flows are explicit): class Test
{
public static void Main()
{
int x = 3;
switch(x)
{
case 0:
// do something
goto case 1;
case 1:
// do something in common with 0
goto default;
default:
// do something in common with 0, 1, and anything else
break;
}
}
}
How can I access the registry from C# code?
By using the Registry and RegistryKey classes in Microsoft.Win32, you can easily access the registry. The following is a sample that reads a key and displays its value:
using System;using Microsoft.Win32;
class regTest
{
public static void Main(String[] args)
{
RegistryKey regKey;
Object value;
regKey = Registry.LocalMachine;
regKey =
regKey.OpenSubKey("HARDWAREDESCRIPTIONSystemCentralProcessor ");
value = regKey.GetValue("VendorIdentifier");
Console.WriteLine("The central processor of this machine is: {0}.", value);
}
}
How can you sort the elements of the array in descending order?
By calling Sort() and then Reverse() methods.
How do you debug an ASP.NET Web application?
Attach the aspnet_wp.exe process to the DbgClr debugger.
How do you mark a method obsolete?
Assuming you've done a "using System;": [Obsolete]
public int Foo() {...}
or [Obsolete("This is a message describing why this method is obsolete")]
public int Foo() {...}
Note: The O in Obsolete is capitalized.
How is the DLL Hell problem solved in .NET?
Assembly versioning allows the application to specify not only the library it needs to run (which was available under Win32), but also the version of the assembly
What are the ways to deploy an assembly?
An MSI installer, a CAB archive, and XCOPY command.
Why does DllImport not work for me?
All methods marked with the DllImport attribute must be marked as public static extern.
What is a delegate?
A delegate object encapsulates a reference to a method. In C++ they were referred to as function pointers.
What is the difference between an interface and abstract class?
In the interface all methods must be abstract; in the abstract class some methods can be concrete. In the interface no accessibility modifiers are allowed, which is ok in abstract classes.
What is an abstract class?
A class that cannot be instantiated. A concept in C++ known as pure virtual method. A class that must be inherited and have the methods over-ridden. Essentially, it is a blueprint for a class without any implementation.
_break
Does C# support multiple-inheritance?
No.
Who is a protected class-level variable available to?
It is available to any sub-class (a class inheriting this class).
Can you store multiple data types in System.Array?
No.
What’s the top .NET class that everything is derived from?
System.Object.
What does the term immutable mean?
The data value may not be changed. Note: The variable value may be changed, but the original immutable data value was discarded and a new data value was created in memory.
What’s the difference between System.String and System.Text.StringBuilder classes?
System.String is immutable. System.StringBuilder was designed with the purpose of having a mutable string where a variety of operations can be performed.
What’s the advantage of using System.Text.StringBuilder over System.String?
StringBuilder is more efficient in cases where there is a large amount of string manipulation. Strings are immutable, so each time a string is changed, a new instance in memory is created.
What’s the difference between the System.Array.CopyTo() and System.Array.Clone()?
The first one performs a deep copy of the array, the second one is shallow. A shallow copy of an Array copies only the elements of the Array, whether they are reference types or value types, but it does not copy the objects that the references refer to. The references in the new Array point to the same objects that the references in the original Array point to. In contrast, a deep copy of an Array copies the elements and everything directly or indirectly referenced by the elements.
How can you sort the elements of the array in descending order?
By calling Sort() and then Reverse() methods.
What’s the .NET collection class that allows an element to be accessed using a unique key?
HashTable.
What class is underneath the SortedList class?
A sorted HashTable.
Will the finally block get executed if an exception has not occurred?
Yes.
What’s the C# syntax to catch any possible exception?
A catch block that catches the exception of type System.Exception. You can also omit the parameter data type in this case and just write catch {}.
Can multiple catch blocks be executed for a single try statement?
No. Once the proper catch block processed, control is transferred to the finally block (if there are any).
Explain the three services model commonly know as a three-tier application.
Presentation (UI), Business (logic and underlying code) and Data (from storage or other sources).
What is the syntax to inherit from a class in C#?
Place a colon and then the name of the base class.
Example: class MyNewClass : MyBaseClass
Can you prevent your class from being inherited by another class?
Yes. The keyword “sealed” will prevent the class from being inherited.
Can you allow a class to be inherited, but prevent the method from being over-ridden?
Yes. Just leave the class public and make the method sealed.
What’s an abstract class?
A class that cannot be instantiated. An abstract class is a class that must be inherited and have the methods overridden. An abstract class is essentially a blueprint for a class without any implementation.
When do you absolutely have to declare a class as abstract?
1. When the class itself is inherited from an abstract class, but not all base abstract methods have been overridden.
2. When at least one of the methods in the class is abstract.
What is an interface class?
Interfaces, like classes, define a set of properties, methods, and events. But unlike classes, interfaces do not provide implementation. They are implemented by classes, and defined as separate entities from classes.
Why can’t you specify the accessibility modifier for methods inside the interface?
They all must be public, and are therefore public by default.
What happens if you inherit multiple interfaces and they have conflicting method names?
It’s up to you to implement the method inside your own class, so implementation is left entirely up to you. This might cause a problem on a higher-level scale if similarly named methods from different interfaces expect different data, but as far as compiler cares you’re okay.
To Do: Investigate
What’s the difference between an interface and abstract class?
In an interface class, all methods are abstract - there is no implementation. In an abstract class some methods can be concrete. In an interface class, no accessibility modifiers are allowed. An abstract class may have accessibility modifiers.
What is the difference between a Struct and a Class?
Struts are value-type variables and are thus saved on the stack, additional overhead but faster retrieval. Another difference is that struts cannot inherit.
What’s the implicit name of the parameter that gets passed into the set method/property of a class?
Value. The data type of the value parameter is defined by whatever data type the property is declared as.
What does the keyword “virtual” declare for a method or property?
The method or property can be overridden.
How is method overriding different from method overloading?
When overriding a method, you change the behavior of the method for the derived class. Overloading a method simply involves having another method with the same name within the class.
Can you declare an override method to be static if the original method is not static?
No. The signature of the virtual method must remain the same. (Note: Only the keyword virtual is changed to keyword override)
What are the different ways a method can be overloaded?
Different parameter data types, different number of parameters, different order of parameters.
If a base class has a number of overloaded constructors, and an inheriting class has a number of overloaded constructors; can you enforce a call from an inherited constructor to a specific base constructor?
Yes, just place a colon, and then keyword base (parameter list to invoke the appropriate constructor) in the overloaded constructor definition inside the inherited class.
What’s a delegate?
A delegate object encapsulates a reference to a method.
What’s a multicast delegate?
A delegate that has multiple handlers assigned to it. Each assigned handler (method) is called.
Why are there five tracing levels in System.Diagnostics.TraceSwitcher?
The tracing dumps can be quite verbose. For applications that are constantly running you run the risk of overloading the machine and the hard drive. Five levels range from None to Verbose, allowing you to fine-tune the tracing activities.
Where is the output of TextWriterTraceListener redirected?
To the Console or a text file depending on the parameter passed to the constructor.
How do you debug an ASP.NET Web application?
Attach the aspnet_wp.exe process to the DbgClr debugger.
What are three test cases you should go through in unit testing?
1. Positive test cases (correct data, correct output).
2. Negative test cases (broken or missing data, proper handling).
3. Exception test cases (exceptions are thrown and caught properly).
Can you change the value of a variable while debugging a C# application?
Yes. If you are debugging via Visual Studio.NET, just go to Immediate window.
What is the role of the DataReader class in ADO.NET connections?
It returns a read-only, forward-only rowset from the data source. A DataReader provides fast access when a forward-only sequential read is needed.What are advantages and disadvantages of Microsoft-provided data provider classes in ADO.NET?
SQLServer.NET data provider is high-speed and robust, but requires SQL Server license purchased from Microsoft. OLE-DB.NET is universal for accessing other sources, like Oracle, DB2, Microsoft Access and Informix. OLE-DB.NET is a .NET layer on top of the OLE layer, so it’s not as fastest and efficient as SqlServer.NET.
What is the wildcard character in SQL?
Let’s say you want to query database with LIKE for all employees whose name starts with La. The wildcard character is %, the proper query with LIKE would involve ‘La%’.
Explain ACID rule of thumb for transactions.
A transaction must be:
1. Atomic - it is one unit of work and does not dependent on previous and following transactions.
2. Consistent - data is either committed or roll back, no “in-between” case where something has been updated and something hasn’t.
3. Isolated - no transaction sees the intermediate results of the current transaction).
4. Durable - the values persist if the data had been committed even if the system crashes right after.
What connections does Microsoft SQL Server support?
Windows Authentication (via Active Directory) and SQL Server authentication (via Microsoft SQL Server username and password).
Between Windows Authentication and SQL Server Authentication, which one is trusted and which one is untrusted?
Windows Authentication is trusted because the username and password are checked with the Active Directory, the SQL Server authentication is untrusted, since SQL Server is the only verifier participating in the transaction.
What does the Initial Catalog parameter define in the connection string?
The database name to connect to.
What does the Dispose method do with the connection object?
Deletes it from the memory.
What is a pre-requisite for connection pooling?
Multiple processes must agree that they will share the same connection, where every parameter is the same, including the security settings. The connection string must be identical.
How is the DLL Hell problem solved in .NET?
Assembly versioning allows the application to specify not only the library it needs to run (which was available under Win32), but also the version of the assembly.
What are the ways to deploy an assembly?
An MSI installer, a CAB archive, and XCOPY command.
What is a satellite assembly?
When you write a multilingual or multi-cultural application in .NET, and want to distribute the core application separately from the localized modules, the localized assemblies that modify the core application are called satellite assemblies.
What namespaces are necessary to create a localized application?
System.Globalization and System.Resources.
What is the smallest unit of execution in .NET?
an Assembly.
When should you call the garbage collector in .NET?
As a good rule, you should not call the garbage collector. However, you could call the garbage collector when you are done using a large object (or set of objects) to force the garbage collector to dispose of those very large objects from memory. However, this is usually not a good practice.
How do you convert a value-type to a reference-type?
Use Boxing.
What happens in memory when you Box and Unbox a value-type?
Boxing converts a value-type to a reference-type, thus storing the object on the heap. Unboxing converts a reference-type to a value-type, thus storing the value on the stack.Difference between directcast and ctype.
Answer1
DirectCast requires the run-time type of an object variable to bethe same as the specified type.The run-time performance ofDirectCast is better than that of CType, if the specified type and the run-time typeof the expression are the same. Ctype works fine if there is a valid conversion defined between the expression and the type.
Answer2
The difference between the two keywords is that CType succeeds as long as there is a valid conversion defined between the expression and the type, whereas DirectCast requires the run-time type of an object variable to be the same as the specified type. If the specified type and the run-time type of the expression are the same, however, the run-time performance of DirectCast is better than that of CType.
An example of a ctype and directcast.
In the preceding example, the run-time type of Q is Double. CType succeeds because Double can be converted to Integer, but DirectCast fails because the run-time type of Q is not already Integer
ctype(123.34,integer) - should it throw an error? Why or why not?
Answer1
It would work fine. As the runtime type of 123.34 would be double, and Double can be converted to Integer.
Answer2
the ctype(123.34,integer) will work fine no errors
directcast(123.34,integer) - should it throw an error? Why or why not?
It would throw an InvalidCast exception as the runtime type of 123.34 (double) doesnt match with Integer.
Difference between a sub and a function.
Answer1
A Sub does not return anything whereas a Function returns something.
Answer2
-A Sub Procedure is a method will not return a value
-A sub procedure will be defined with a “Sub” keyword
Sub ShowName(ByVal myName As String)
Console.WriteLine(”My name is: ” & myName)
End Sub
-A function is a method that will return value(s).
-A function will be defined with a “Function” keyword
Function FindSum(ByVal num1 As Integer, ByVal num2 As Integer) As Integer
Dim sum As Integer = num1 + num2
Return sum
End Function
Explain manifest & metadata.
Answer1
Manifest is metadata about assemblies. Metadata is machine-readable information about a resource, or “”data about data.” In .NET, metadata includes type definitions, version information, external assembly references, and other standardized information.
Answer2
Manifest: Manifest describes assembly itself. Assembly Name, version number, culture, strong name, list of all files, Type references, and referenced assemblies.
Metadata: Metadata describes contents in an assembly classes, interfaces, enums, structs, etc., and their containing namespaces, the name of each type, its visibility/scope, its base class, the nterfaces it implemented, its methods and their scope, and each method’s parameters, type’s properties, and so on.
Difference between imperative and interrogative code.
There are imperative and interrogative functions. Imperative functions are the one which return a value while the interrogative functions do not return a value.
Difference between value and reference type. what are value types and reference types?
Value type - bool, byte, chat, decimal, double, enum , float, int, long, sbyte, short, strut, uint, ulong, ushort
Value types are stored in the Stack
Reference type - class, delegate, interface, object, string
Reference types are stored in the Heap
What are the two kinds of properties.
Two types of properties in .Net: Get and Set
Explain constructor.
Constructor is a method in the class which has the same name as the class (in VB.Net its New()). It initializes the member attributes whenever an instance of the class is created.
Describe ways of cleaning up objects.
Answer1
There is a perfect tool provide by .net frameworks calls Garbage collector, where by mean of GC we can clean up the object and reclaim the memory. The namespace used is System.GC
Answer2
the run time will maintain a service called as garbage collector. This service will take care of deallocating memory corresponding to objects. it works as a thread with least priority. when application demands for memory the runtime will take care of setting the high priority for the garbage collector, so that it will be called for execution and memory will be released. the programmer can make a call to garbage collector by using GC class in system name space.
How can you clean up objects holding resources from within the code?
Call the dispose method from code for clean up of objects
Which controls do not have events?
Timer control.
What is the maximum size of the textbox?
65536.
Which property of the textbox cannot be changed at runtime?
Locked Property.
Which control cannot be placed in MDI?
The controls that do not have events.
What is the difference between proc. sent BY VAL and BY SUB?
BY VAL: changes will not be reflected back to the variable.
By REF: changes will be reflected back to that variable.( same as & symbol in c, c++)
General C# Interview Questions :
Does C# support multiple-inheritance?
No.
Who is a protected class-level variable available to?
It is available to any sub-class (a class inheriting this class).
Are private class-level variables inherited?
Yes, but they are not accessible. Although they are not visible or accessible via the class interface, they are inherited.
Describe the accessibility modifier “protected internal”.
It is available to classes that are within the same assembly and derived from the specified base class.
What’s the top .NET class that everything is derived from?
System.Object.
What does the term immutable mean?
The data value may not be changed. Note: The variable value may be changed, but the original immutable data value was discarded and a new data value was created in memory.
What’s the difference between System.String and System.Text.StringBuilder classes?
System.String is immutable. System.StringBuilder was designed with the purpose of having a mutable string where a variety of operations can be performed.
What’s the advantage of using System.Text.StringBuilder over System.String?
StringBuilder is more efficient in cases where there is a large amount of string manipulation. Strings are immutable, so each time a string is changed, a new instance in memory is created.
Can you store multiple data types in System.Array?
No.
What’s the difference between the System.Array.CopyTo() and System.Array.Clone()?
The Clone() method returns a new array (a shallow copy) object containing all the elements in the original array. The CopyTo() method copies the elements into another existing array. Both perform a shallow copy. A shallow copy means the contents (each array element) contains references to the same object as the elements in the original array. A deep copy (which neither of these methods performs) would create a new instance of each element's object, resulting in a different, yet identical object.
How can you sort the elements of the array in descending order?
By calling Sort() and then Reverse() methods.
Let us say you want to query database with LIKE for all employees whose name starts with La. The wildcard character is %, the proper query with LIKE would involve La%.
What is the role of the DataReader class in ADO.NET connections?
It returns a read-only dataset from the data source when the command is executed.
What does the This window show in the debugger?
It points to the object that is pointed to by this reference. Object’s instance data is shown.
Describe the accessibility modifier protected internal?
It is available to derived classes and classes within the same Assembly (and naturally from the base class it is declared in).
What is an interface class?
It is an abstract class with public abstract methods all of which must be implemented in the inherited classes.
What is a multicast delegate?
It is a delegate that points to and eventually fires off several methods.
How does one compare strings in C#?
In the past, you had to call .ToString() on the strings when using the == or != operators to compare the strings' values. That will still work, but the C# compiler now automatically compares the values instead of the references when the == or != operators are used on string types. If you actually do want to compare references, it can be done as follows: if ((object) str1 == (object) str2) { ... } Here's an example showing how string compares work: using System;
public class StringTest
{
public static void Main(string[] args)
{
Object nullObj = null;
Object realObj = new StringTest();
int i = 10;
Console.WriteLine("Null Object is [" + nullObj + "]n" +
"Real Object is [" + realObj + "]n" +
"i is [" + i + "]n");
// Show string equality operators
string str1 = "foo";
string str2 = "bar";
string str3 = "bar";
Console.WriteLine("{0} == {1} ? {2}", str1, str2, str1 == str2 );
Console.WriteLine("{0} == {1} ? {2}", str2, str3, str2 == str3 );
}
}
Output: Null Object is []
Real Object is [StringTest]
i is [10]
foo == bar ? False
bar == bar ? True
What does assert() do?
In debug compilation, assert takes in a Boolean condition as a parameter, and shows the error dialog if the condition is false. The program proceeds without any interruption if the condition is true.
How do I get deterministic finalization in C#?
In a garbage collected environment, it's impossible to get true determinism. However, a design pattern that we recommend is implementing IDisposable on any class that contains a critical resource. Whenever this class is consumed, it may be placed in a using statement, as shown in the following example:
using(FileStream myFile = File.Open(@"c:temptest.txt",
FileMode.Open))
{
int fileOffset = 0;
while(fileOffset < myFile.Length)
{
Console.Write((char)myFile.ReadByte());
fileOffset++;
}
}
When myFile leaves the lexical scope of the using, its dispose method will be called.
How can I get around scope problems in a try/catch?
If you try to instantiate the class inside the try, it'll be out of scope when you try to access it from the catch block. A way to get around this is to do the following: Connection conn = null;
try
{
conn = new Connection();
conn.Open();
}
finally
{
if (conn != null) conn.Close();
}
By setting it to null before the try block, you avoid getting the CS0165 error (Use of possibly unassigned local variable 'conn').
Why do I get an error (CS1006) when trying to declare a method without specifying a return type?
If you leave off the return type on a method declaration, the compiler thinks you are trying to declare a constructor. So if you are trying to declare a method that returns nothing, use void. The following is an example: // This results in a CS1006 error public static staticMethod (mainStatic obj) // This will work as wanted public static void staticMethod (mainStatic obj)
How do I convert a string to an int in C#?
Here's an example: using System;
class StringToInt
{
public static void Main()
{
String s = "105";
int x = Convert.ToInt32(s);
Console.WriteLine(x);
}
}
How do you directly call a native function exported from a DLL?
Here's a quick example of the DllImport attribute in action: using System.Runtime.InteropServices;
class C
{
[DllImport("user32.dll")]
public static extern int MessageBoxA(int h, string m, string c, int type);
public static int Main()
{
return MessageBoxA(0, "Hello World!", "Caption", 0);
}
}
This example shows the minimum requirements for declaring a C# method that is implemented in a native DLL. The method C.MessageBoxA() is declared with the static and external modifiers, and has the DllImport attribute, which tells the compiler that the implementation comes from the user32.dll, using the default name of MessageBoxA. For more information, look at the Platform Invoke tutorial in the documentation.
What is the .NET datatype that allows the retrieval of data by a unique key?
HashTable.
How do you specify a custom attribute for the entire assembly (rather than for a class)?
Global attributes must appear after any top-level using clauses and before the first type or namespace declarations. An example of this is as follows:
using System;
[assembly : MyAttributeClass]
class X {}
Note that in an IDE-created project, by convention, these attributes are placed in
AssemblyInfo.cs.
What is the difference between a struct and a class in C#?
From language spec:
The list of similarities between classes and structs is as follows. Longstructs can implement interfaces and can have the same kinds of members as classes. Structs differ from classes in several important ways; however, structs are value types rather than reference types, and inheritance is not supported for structs. Struct values are stored on the stack or in-line. Careful programmers can sometimes enhance performance through judicious use of structs. For example, the use of a struct rather than a class for a Point can make a large difference in the number of memory allocations performed at runtime. The program below creates and initializes an array of 100 points. With Point implemented as a class, 101 separate objects are instantiated-one for the array and one each for the 100 elements.
What is the difference between the Debug class and Trace class?
Documentation looks the same. Use Debug class for debug builds, use Trace class for both debug and release builds.
How can you overload a method?
Different parameter data types, different number of parameters, different order of parameters.
What debugging tools come with the .NET SDK?
CorDBG - command-line debugger, and DbgCLR - graphic debugger. Visual Studio .NET uses the DbgCLR. To use CorDbg, you must compile the original C# file using the /debug switch.
What does Dispose method do with the connection object?
Deletes it from the memory.
How do you generate documentation from the C# file commented properly with a command-line compiler?
Compile it with a /doc switch.
When you inherit a protected class-level variable, who is it available to?
Classes in the same namespace.
How can I get the ASCII code for a character in C#?
Casting the char to an int will give you the ASCII value: char c = 'f'; System.Console.WriteLine((int)c); or for a character in a string: System.Console.WriteLine((int)s[3]); The base class libraries also offer ways to do this with the Convert class or Encoding classes if you need a particular encoding.
Is there an equivalent to the instanceof operator in Visual J++?
C# has the is operator:
expr is type
How do I create a Delegate/MulticastDelegate?
C# requires only a single parameter for delegates: the method address. Unlike other languages, where the programmer must specify an object reference and the method to invoke, C# can infer both pieces of information by just specifying the method's name. For example, let's use System.Threading.ThreadStart: Foo MyFoo = new Foo(); ThreadStart del = new ThreadStart(MyFoo.Baz); This means that delegates can invoke static class methods and instance methods with the exact same syntax!
How do destructors and garbage collection work in C#?
C# has finalizers (similar to destructors except that the runtime doesn't guarantee they'll be called), and they are specified as follows:
class C
{
~C()
{
// your code
}
public static void Main() {}
}
Currently, they override object.Finalize(), which is called during the GC process.
My switch statement works differently! Why?
C# does not support an explicit fall through for case blocks.
The following code is not legal and will not compile in C#: switch(x)
{
case 0:
// do something
case 1:
// do something in common with 0
default:
// do something in common with
//0, 1 and everything else
break;
}
To achieve the same effect in C#, the code must be modified
as shown below (notice how the control flows are explicit): class Test
{
public static void Main()
{
int x = 3;
switch(x)
{
case 0:
// do something
goto case 1;
case 1:
// do something in common with 0
goto default;
default:
// do something in common with 0, 1, and anything else
break;
}
}
}
How can I access the registry from C# code?
By using the Registry and RegistryKey classes in Microsoft.Win32, you can easily access the registry. The following is a sample that reads a key and displays its value:
using System;using Microsoft.Win32;
class regTest
{
public static void Main(String[] args)
{
RegistryKey regKey;
Object value;
regKey = Registry.LocalMachine;
regKey =
regKey.OpenSubKey("HARDWAREDESCRIPTIONSystemCentralProcessor ");
value = regKey.GetValue("VendorIdentifier");
Console.WriteLine("The central processor of this machine is: {0}.", value);
}
}
How can you sort the elements of the array in descending order?
By calling Sort() and then Reverse() methods.
How do you debug an ASP.NET Web application?
Attach the aspnet_wp.exe process to the DbgClr debugger.
How do you mark a method obsolete?
Assuming you've done a "using System;": [Obsolete]
public int Foo() {...}
or [Obsolete("This is a message describing why this method is obsolete")]
public int Foo() {...}
Note: The O in Obsolete is capitalized.
How is the DLL Hell problem solved in .NET?
Assembly versioning allows the application to specify not only the library it needs to run (which was available under Win32), but also the version of the assembly
What are the ways to deploy an assembly?
An MSI installer, a CAB archive, and XCOPY command.
Why does DllImport not work for me?
All methods marked with the DllImport attribute must be marked as public static extern.
What is a delegate?
A delegate object encapsulates a reference to a method. In C++ they were referred to as function pointers.
What is the difference between an interface and abstract class?
In the interface all methods must be abstract; in the abstract class some methods can be concrete. In the interface no accessibility modifiers are allowed, which is ok in abstract classes.
What is an abstract class?
A class that cannot be instantiated. A concept in C++ known as pure virtual method. A class that must be inherited and have the methods over-ridden. Essentially, it is a blueprint for a class without any implementation.
_break
Does C# support multiple-inheritance?
No.
Who is a protected class-level variable available to?
It is available to any sub-class (a class inheriting this class).
Can you store multiple data types in System.Array?
No.
What’s the top .NET class that everything is derived from?
System.Object.
What does the term immutable mean?
The data value may not be changed. Note: The variable value may be changed, but the original immutable data value was discarded and a new data value was created in memory.
What’s the difference between System.String and System.Text.StringBuilder classes?
System.String is immutable. System.StringBuilder was designed with the purpose of having a mutable string where a variety of operations can be performed.
What’s the advantage of using System.Text.StringBuilder over System.String?
StringBuilder is more efficient in cases where there is a large amount of string manipulation. Strings are immutable, so each time a string is changed, a new instance in memory is created.
What’s the difference between the System.Array.CopyTo() and System.Array.Clone()?
The first one performs a deep copy of the array, the second one is shallow. A shallow copy of an Array copies only the elements of the Array, whether they are reference types or value types, but it does not copy the objects that the references refer to. The references in the new Array point to the same objects that the references in the original Array point to. In contrast, a deep copy of an Array copies the elements and everything directly or indirectly referenced by the elements.
How can you sort the elements of the array in descending order?
By calling Sort() and then Reverse() methods.
What’s the .NET collection class that allows an element to be accessed using a unique key?
HashTable.
What class is underneath the SortedList class?
A sorted HashTable.
Will the finally block get executed if an exception has not occurred?
Yes.
What’s the C# syntax to catch any possible exception?
A catch block that catches the exception of type System.Exception. You can also omit the parameter data type in this case and just write catch {}.
Can multiple catch blocks be executed for a single try statement?
No. Once the proper catch block processed, control is transferred to the finally block (if there are any).
Explain the three services model commonly know as a three-tier application.
Presentation (UI), Business (logic and underlying code) and Data (from storage or other sources).
What is the syntax to inherit from a class in C#?
Place a colon and then the name of the base class.
Example: class MyNewClass : MyBaseClass
Can you prevent your class from being inherited by another class?
Yes. The keyword “sealed” will prevent the class from being inherited.
Can you allow a class to be inherited, but prevent the method from being over-ridden?
Yes. Just leave the class public and make the method sealed.
What’s an abstract class?
A class that cannot be instantiated. An abstract class is a class that must be inherited and have the methods overridden. An abstract class is essentially a blueprint for a class without any implementation.
When do you absolutely have to declare a class as abstract?
1. When the class itself is inherited from an abstract class, but not all base abstract methods have been overridden.
2. When at least one of the methods in the class is abstract.
What is an interface class?
Interfaces, like classes, define a set of properties, methods, and events. But unlike classes, interfaces do not provide implementation. They are implemented by classes, and defined as separate entities from classes.
Why can’t you specify the accessibility modifier for methods inside the interface?
They all must be public, and are therefore public by default.
What happens if you inherit multiple interfaces and they have conflicting method names?
It’s up to you to implement the method inside your own class, so implementation is left entirely up to you. This might cause a problem on a higher-level scale if similarly named methods from different interfaces expect different data, but as far as compiler cares you’re okay.
To Do: Investigate
What’s the difference between an interface and abstract class?
In an interface class, all methods are abstract - there is no implementation. In an abstract class some methods can be concrete. In an interface class, no accessibility modifiers are allowed. An abstract class may have accessibility modifiers.
What is the difference between a Struct and a Class?
Struts are value-type variables and are thus saved on the stack, additional overhead but faster retrieval. Another difference is that struts cannot inherit.
What’s the implicit name of the parameter that gets passed into the set method/property of a class?
Value. The data type of the value parameter is defined by whatever data type the property is declared as.
What does the keyword “virtual” declare for a method or property?
The method or property can be overridden.
How is method overriding different from method overloading?
When overriding a method, you change the behavior of the method for the derived class. Overloading a method simply involves having another method with the same name within the class.
Can you declare an override method to be static if the original method is not static?
No. The signature of the virtual method must remain the same. (Note: Only the keyword virtual is changed to keyword override)
What are the different ways a method can be overloaded?
Different parameter data types, different number of parameters, different order of parameters.
If a base class has a number of overloaded constructors, and an inheriting class has a number of overloaded constructors; can you enforce a call from an inherited constructor to a specific base constructor?
Yes, just place a colon, and then keyword base (parameter list to invoke the appropriate constructor) in the overloaded constructor definition inside the inherited class.
What’s a delegate?
A delegate object encapsulates a reference to a method.
What’s a multicast delegate?
A delegate that has multiple handlers assigned to it. Each assigned handler (method) is called.
Why are there five tracing levels in System.Diagnostics.TraceSwitcher?
The tracing dumps can be quite verbose. For applications that are constantly running you run the risk of overloading the machine and the hard drive. Five levels range from None to Verbose, allowing you to fine-tune the tracing activities.
Where is the output of TextWriterTraceListener redirected?
To the Console or a text file depending on the parameter passed to the constructor.
How do you debug an ASP.NET Web application?
Attach the aspnet_wp.exe process to the DbgClr debugger.
What are three test cases you should go through in unit testing?
1. Positive test cases (correct data, correct output).
2. Negative test cases (broken or missing data, proper handling).
3. Exception test cases (exceptions are thrown and caught properly).
Can you change the value of a variable while debugging a C# application?
Yes. If you are debugging via Visual Studio.NET, just go to Immediate window.
What is the role of the DataReader class in ADO.NET connections?
It returns a read-only, forward-only rowset from the data source. A DataReader provides fast access when a forward-only sequential read is needed.What are advantages and disadvantages of Microsoft-provided data provider classes in ADO.NET?
SQLServer.NET data provider is high-speed and robust, but requires SQL Server license purchased from Microsoft. OLE-DB.NET is universal for accessing other sources, like Oracle, DB2, Microsoft Access and Informix. OLE-DB.NET is a .NET layer on top of the OLE layer, so it’s not as fastest and efficient as SqlServer.NET.
What is the wildcard character in SQL?
Let’s say you want to query database with LIKE for all employees whose name starts with La. The wildcard character is %, the proper query with LIKE would involve ‘La%’.
Explain ACID rule of thumb for transactions.
A transaction must be:
1. Atomic - it is one unit of work and does not dependent on previous and following transactions.
2. Consistent - data is either committed or roll back, no “in-between” case where something has been updated and something hasn’t.
3. Isolated - no transaction sees the intermediate results of the current transaction).
4. Durable - the values persist if the data had been committed even if the system crashes right after.
What connections does Microsoft SQL Server support?
Windows Authentication (via Active Directory) and SQL Server authentication (via Microsoft SQL Server username and password).
Between Windows Authentication and SQL Server Authentication, which one is trusted and which one is untrusted?
Windows Authentication is trusted because the username and password are checked with the Active Directory, the SQL Server authentication is untrusted, since SQL Server is the only verifier participating in the transaction.
What does the Initial Catalog parameter define in the connection string?
The database name to connect to.
What does the Dispose method do with the connection object?
Deletes it from the memory.
What is a pre-requisite for connection pooling?
Multiple processes must agree that they will share the same connection, where every parameter is the same, including the security settings. The connection string must be identical.
How is the DLL Hell problem solved in .NET?
Assembly versioning allows the application to specify not only the library it needs to run (which was available under Win32), but also the version of the assembly.
What are the ways to deploy an assembly?
An MSI installer, a CAB archive, and XCOPY command.
What is a satellite assembly?
When you write a multilingual or multi-cultural application in .NET, and want to distribute the core application separately from the localized modules, the localized assemblies that modify the core application are called satellite assemblies.
What namespaces are necessary to create a localized application?
System.Globalization and System.Resources.
What is the smallest unit of execution in .NET?
an Assembly.
When should you call the garbage collector in .NET?
As a good rule, you should not call the garbage collector. However, you could call the garbage collector when you are done using a large object (or set of objects) to force the garbage collector to dispose of those very large objects from memory. However, this is usually not a good practice.
How do you convert a value-type to a reference-type?
Use Boxing.
What happens in memory when you Box and Unbox a value-type?
Boxing converts a value-type to a reference-type, thus storing the object on the heap. Unboxing converts a reference-type to a value-type, thus storing the value on the stack.Difference between directcast and ctype.
Answer1
DirectCast requires the run-time type of an object variable to bethe same as the specified type.The run-time performance ofDirectCast is better than that of CType, if the specified type and the run-time typeof the expression are the same. Ctype works fine if there is a valid conversion defined between the expression and the type.
Answer2
The difference between the two keywords is that CType succeeds as long as there is a valid conversion defined between the expression and the type, whereas DirectCast requires the run-time type of an object variable to be the same as the specified type. If the specified type and the run-time type of the expression are the same, however, the run-time performance of DirectCast is better than that of CType.
An example of a ctype and directcast.
In the preceding example, the run-time type of Q is Double. CType succeeds because Double can be converted to Integer, but DirectCast fails because the run-time type of Q is not already Integer
ctype(123.34,integer) - should it throw an error? Why or why not?
Answer1
It would work fine. As the runtime type of 123.34 would be double, and Double can be converted to Integer.
Answer2
the ctype(123.34,integer) will work fine no errors
directcast(123.34,integer) - should it throw an error? Why or why not?
It would throw an InvalidCast exception as the runtime type of 123.34 (double) doesnt match with Integer.
Difference between a sub and a function.
Answer1
A Sub does not return anything whereas a Function returns something.
Answer2
-A Sub Procedure is a method will not return a value
-A sub procedure will be defined with a “Sub” keyword
Sub ShowName(ByVal myName As String)
Console.WriteLine(”My name is: ” & myName)
End Sub
-A function is a method that will return value(s).
-A function will be defined with a “Function” keyword
Function FindSum(ByVal num1 As Integer, ByVal num2 As Integer) As Integer
Dim sum As Integer = num1 + num2
Return sum
End Function
Explain manifest & metadata.
Answer1
Manifest is metadata about assemblies. Metadata is machine-readable information about a resource, or “”data about data.” In .NET, metadata includes type definitions, version information, external assembly references, and other standardized information.
Answer2
Manifest: Manifest describes assembly itself. Assembly Name, version number, culture, strong name, list of all files, Type references, and referenced assemblies.
Metadata: Metadata describes contents in an assembly classes, interfaces, enums, structs, etc., and their containing namespaces, the name of each type, its visibility/scope, its base class, the nterfaces it implemented, its methods and their scope, and each method’s parameters, type’s properties, and so on.
Difference between imperative and interrogative code.
There are imperative and interrogative functions. Imperative functions are the one which return a value while the interrogative functions do not return a value.
Difference between value and reference type. what are value types and reference types?
Value type - bool, byte, chat, decimal, double, enum , float, int, long, sbyte, short, strut, uint, ulong, ushort
Value types are stored in the Stack
Reference type - class, delegate, interface, object, string
Reference types are stored in the Heap
What are the two kinds of properties.
Two types of properties in .Net: Get and Set
Explain constructor.
Constructor is a method in the class which has the same name as the class (in VB.Net its New()). It initializes the member attributes whenever an instance of the class is created.
Describe ways of cleaning up objects.
Answer1
There is a perfect tool provide by .net frameworks calls Garbage collector, where by mean of GC we can clean up the object and reclaim the memory. The namespace used is System.GC
Answer2
the run time will maintain a service called as garbage collector. This service will take care of deallocating memory corresponding to objects. it works as a thread with least priority. when application demands for memory the runtime will take care of setting the high priority for the garbage collector, so that it will be called for execution and memory will be released. the programmer can make a call to garbage collector by using GC class in system name space.
How can you clean up objects holding resources from within the code?
Call the dispose method from code for clean up of objects
Which controls do not have events?
Timer control.
What is the maximum size of the textbox?
65536.
Which property of the textbox cannot be changed at runtime?
Locked Property.
Which control cannot be placed in MDI?
The controls that do not have events.
What is the difference between proc. sent BY VAL and BY SUB?
BY VAL: changes will not be reflected back to the variable.
By REF: changes will be reflected back to that variable.( same as & symbol in c, c++)
General C# Interview Questions :
Does C# support multiple-inheritance?
No.
Who is a protected class-level variable available to?
It is available to any sub-class (a class inheriting this class).
Are private class-level variables inherited?
Yes, but they are not accessible. Although they are not visible or accessible via the class interface, they are inherited.
Describe the accessibility modifier “protected internal”.
It is available to classes that are within the same assembly and derived from the specified base class.
What’s the top .NET class that everything is derived from?
System.Object.
What does the term immutable mean?
The data value may not be changed. Note: The variable value may be changed, but the original immutable data value was discarded and a new data value was created in memory.
What’s the difference between System.String and System.Text.StringBuilder classes?
System.String is immutable. System.StringBuilder was designed with the purpose of having a mutable string where a variety of operations can be performed.
What’s the advantage of using System.Text.StringBuilder over System.String?
StringBuilder is more efficient in cases where there is a large amount of string manipulation. Strings are immutable, so each time a string is changed, a new instance in memory is created.
Can you store multiple data types in System.Array?
No.
What’s the difference between the System.Array.CopyTo() and System.Array.Clone()?
The Clone() method returns a new array (a shallow copy) object containing all the elements in the original array. The CopyTo() method copies the elements into another existing array. Both perform a shallow copy. A shallow copy means the contents (each array element) contains references to the same object as the elements in the original array. A deep copy (which neither of these methods performs) would create a new instance of each element's object, resulting in a different, yet identical object.
How can you sort the elements of the array in descending order?
By calling Sort() and then Reverse() methods.
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