C# .NET interview questions

Good for preparation and general self-testing, but too specific for the actual job interview. This was sent in by a job applicant getting ready to step into the .NET field in India.

  1. Are private class-level variables inherited? - Yes, but they are not accessible, so looking at it you can honestly say that they are not inherited. But they are.
  2. Why does DllImport not work for me? - All methods marked with the DllImport attribute must be marked as public static extern.
  3. Why does my Windows application pop up a console window every time I run it? - Make sure that the target type set in the project properties setting is set to Windows Application, and not Console Application. If you’re using the command line, compile with /target:winexe, not /target:exe.
  4. 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)
  5. Why do I get a syntax error when trying to declare a variable called checked? - The word checked is a keyword in C#.
  6. Why do I get a security exception when I try to run my C# app? - Some security exceptions are thrown if you are working on a network share. There are some parts of the frameworks that will not run if being run off a share (roaming profile, mapped drives, etc.). To see if this is what’s happening, just move the executable over to your local drive and see if it runs without the exceptions. One of the common exceptions thrown under these conditions is System.Security.SecurityException. To get around this, you can change your security policy for the intranet zone, code group 1.2, (the zone that running off shared folders falls into) by using the caspol.exe tool.
  7. Why do I get a CS5001: does not have an entry point defined error when compiling? - The most common problem is that you used a lowercase ‘m’ when defining the Main method. The correct way to implement the entry point is as follows: class test { static void Main(string[] args) {} }
  8. What optimizations does the C# compiler perform when you use the /optimize+ compiler option? - The following is a response from a developer on the C# compiler team: We get rid of unused locals (i.e., locals that are never read, even if assigned). We get rid of unreachable code. We get rid of try-catch with an empty try. We get rid of try-finally with an empty try. We get rid of try-finally with an empty finally. We optimize branches over branches: gotoif A, lab1 goto lab2: lab1: turns into: gotoif !A, lab2 lab1: We optimize branches to ret, branches to next instruction, and branches to branches.
  9. What is the syntax for calling an overloaded constructor within a constructor (this() and constructorname() does not compile)? - The syntax for calling another constructor is as follows: class B { B(int i) { } } class C : B { C() : base(5) // call base constructor B(5) { } C(int i) : this() // call C() { } public static void Main() {} }
  10. What is the equivalent to regsvr32 and regsvr32 /u a file in .NET development? - Try using RegAsm.exe. Search MSDN on Assembly Registration Tool.
  11. 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.
  12. My switch statement works differently than in C++! 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 as continuation of case 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;
    		}
    	}
    }
    
  13. Is there regular expression (regex) support available to C# developers? - Yes. The .NET class libraries provide support for regular expressions. Look at the System.Text.RegularExpressions namespace.
  14. Is there any sample C# code for simple threading? - Yes:
    using System;
    using System.Threading;
    class ThreadTest
    {
    	public void runme()
    	{
    		Console.WriteLine("Runme Called");
    	}
    	public static void Main(String[] args)
    	{
    		ThreadTest b = new ThreadTest();
    		Thread t = new Thread(new ThreadStart(b.runme));
    		t.Start();
    	}
    }
  15. Is there an equivalent of exit() for quitting a C# .NET application? - Yes, you can use System.Environment.Exit(int exitCode) to exit the application or Application.Exit() if it’s a Windows Forms app.
  16. Is there a way to force garbage collection? - Yes. Set all references to null and then call System.GC.Collect(). If you need to have some objects destructed, and System.GC.Collect() doesn’t seem to be doing it for you, you can force finalizers to be run by setting all the references to the object to null and then calling System.GC.RunFinalizers().
  17. Is there a way of specifying which block or loop to break out of when working with nested loops? - The easiest way is to use goto:
    using System;
    class BreakExample
    {
    	public static void Main(String[] args) {
    		for(int i=0; i<3; i++)
    		{
    			Console.WriteLine("Pass {0}: ", i);
    			for( int j=0 ; j<100 ; j++ )
    			{
    				if ( j == 10)
    					goto done;
    				Console.WriteLine("{0} ", j);
    			}
    			Console.WriteLine("This will not print");
    		}
    		done:
    			Console.WriteLine("Loops complete.");
    	}
    }
  18. Is it possible to restrict the scope of a field/method of a class to the classes in the same namespace? - There is no way to restrict to a namespace. Namespaces are never units of protection. But if you’re using assemblies, you can use the ‘internal’ access modifier to restrict access to only within the assembly.
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14 Comments on C# .NET interview questions

  1. Dale Gammage
    Posted 4/4/2005 at 1:33 pm | Permalink

    12. is wrong. C# does support fall through. The following code is valid:

    switch (x)
    {
    case 0: // do something
    case 1: // do something as continuation of case 0
    default: // do something in common with
    //0, 1 and everything else
    {
    break;
    }
    }

    What C# does not allow is falling out of one code block into another like this:

    // will not compile because each code block needs
    // a break statement.
    switch (x)
    {
    case 0:
    {
    Console.WriteLine(”0″);
    }

    case 1:
    {
    Console.WriteLine(”0″);
    }
    }

  2. Posted 4/12/2005 at 12:42 am | Permalink

    i have more and tipical questions of C# and ado.net

  3. shabweb
    Posted 7/27/2005 at 11:44 am | Permalink

    switch (x)
    {
    case 0: // do something
    case 1: // do something as continuation of case 0
    default: // do something in common with
    //0, 1 and everything else
    {
    break;
    }
    }

    I tried running the code..gives me error “Control cannot fall through”.

  4. Nishi
    Posted 10/26/2005 at 12:26 pm | Permalink

    C# does not support explicit fall through for the Case ….
    you can use goto:Case/goto:default to make a fall through
    ex
    swich(x)

    case 1:
    //do spmething ;
    break;
    case 2: //do somethin
    goto case1;

    case 3 : //do spmething ;
    break;
    case default : //do spmething ;
    break;

  5. John
    Posted 1/10/2006 at 9:22 am | Permalink

    Fall through is achieved like this. Do not put any code except in the case 2 to achieve the fall through.
    switch(i)
    {
    case 0:
    case 1:
    case 2:
    //Some code here
    break;

    case 5:
    //some code here
    break;
    default:
    }

  6. Tara
    Posted 10/1/2006 at 2:03 am | Permalink

    what is defferent Between This Instruction (Thread.sleep(0)) and this:
    While (Thread.IsAlive())
    {
    Application.DoEvents();
    }

  7. vijaya kumar.p
    Posted 8/30/2007 at 8:57 am | Permalink

    ##### TAAVAT TECHNOLGIES BANGLORE #######
    public enum enumtypes
    {
    bol=1,
    template=2,
    draft=3
    }
    string s= enumtypes.bol.Tostring();
    consle.Writeline(s);

    Ans::
    bol

  8. shrikant
    Posted 11/14/2007 at 1:15 am | Permalink

    My switch statement works differently than in C++! 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 as continuation of case 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;
    }
    }
    }
    Is there regular expression (regex) support available to C# developers? - Yes. The .NET class libraries provide support for regular expressions. Look at the System.Text.RegularExpressions namespace.
    Is there any sample C# code for simple threading? - Yes:
    using System;
    using System.Threading;
    class ThreadTest
    {
    public void runme()
    {
    Console.WriteLine(”Runme Called”);
    }
    public static void Main(String[] args)
    {
    ThreadTest b = new ThreadTest();
    Thread t = new Thread(new ThreadStart(b.runme));
    t.Start();
    }
    }Is there an equivalent of exit() for quitting a C# .NET application? - Yes, you can use System.Environment.Exit(int exitCode) to exit the application or Application.Exit() if it’s a Windows Forms app.
    Is there a way to force garbage collection? - Yes. Set all references to null and then call System.GC.Collect(). If you need to have some objects destructed, and System.GC.Collect() doesn’t seem to be doing it for you, you can force finalizers to be run by setting all the references to the object to null and then calling System.GC.RunFinalizers().
    Is there a way of specifying which block or loop to break out of when working with nested loops? - The easiest way is to use goto:
    using System;
    class BreakExample
    {
    public static void Main(String[] args) {
    for(int i=0; i

  9. Posted 12/2/2007 at 2:50 am | Permalink

    Dale Gammage [comment #1] is correct.

    The following snippet is not legal C# [i.e., case 0 cannot fall through to default's break]:

    switch (x)
    {
    case 0: // do something
    Console.WriteLine(x);
    case 1: // do something as continuation of case 0
    default: // do something in common with
    //0, 1 and everything else
    break;
    }

    See Gunnerson’s “what’s with those break statements?”: http://blogs.msdn.com/ericgu/archive/2004/11/12/256777.aspx

  10. Posted 12/2/2007 at 3:04 am | Permalink

    #16’s answer may be misleading. You cannot force garbage collection; you can only [strongly] suggest that it runs.

    #17: Control flow transfer should be limited to break and continue as per common design guidelines. Goto works but should be discouraged for object oriented and code maintenance reasons.

  11. David Banks
    Posted 1/14/2008 at 10:57 am | Permalink

    Switch doesn’t support fallthrough

    Yes, it does. You fall through as so:

    switch (x)
    {
    case 1:
    case 2:
    //blah, blah, blah
    break;
    default:
    }

  12. Ranjith Reddy B
    Posted 1/17/2008 at 4:16 am | Permalink

    Input string was not in correct formate
    Hi help me……..
    VendorDetailsEL1.Mobile = (Convert.Toint16(txtmobileno.Text));
    in VendorDetailsEL1

    public int Mobile
    {
    get { return _Mobile; }
    set { _Mobile = value; }
    }

    hear error is :Input string was not in a correct format

  13. kavithareddy
    Posted 8/16/2008 at 12:45 am | Permalink

    hi
    convert.tostring(a)
    a.tostring

    what is the diffarence

  14. Ashok Kumar
    Posted 11/12/2008 at 3:13 am | Permalink

    The basic difference between them is “Convert” function handles NULLS while “i.ToString()” does not it will throw a NULL reference exception error. So as good coding practice using “convert” is always safe.

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