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February 16, 2012

Interview Questions for C# Developers

Useful for preparation, but too specific to be used in the interview.
  1. Is it possible to inline assembly or IL in C# code? - No.
  2. Is it possible to have a static indexer in C#? - No. Static indexers are not allowed in C#.
  3. If I return out of a try/finally in C#, does the code in the finally-clause run? - Yes. The code in the finally always runs. If you return out of the try block, or even if you do a “goto” out of the try, the finally block always runs:
    using System; 
    class main
     public static void Main()
       Console.WriteLine(\"In Try block\");
       Console.WriteLine(\"In Finally block\");
    Both “In Try block” and “In Finally block” will be displayed. Whether the return is in the try block or after the try-finally block, performance is not affected either way. The compiler treats it as if the return were outside the try block anyway. If it’s a return without an expression (as it is above), the IL emitted is identical whether the return is inside or outside of the try. If the return has an expression, there’s an extra store/load of the value of the expression (since it has to be computed within the try block).
  4. I was trying to use an “out int” parameter in one of my functions. How should I declare the variable that I am passing to it? - You should declare the variable as an int, but when you pass it in you must specify it as ‘out’, like the following: int i; foo(out i); where foo is declared as follows: [return-type] foo(out int o) { }
  5. 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 );
    Null Object is []
    Real Object is [StringTest]
    i is [10]
    foo == bar ? False
    bar == bar ? True
  6. 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.
  7. How do you mark a method obsolete? -
    [Obsolete] public int Foo() {...}
    [Obsolete(\"This is a message describing why this method is obsolete\")]
     public int Foo() {...}
    Note: The O in Obsolete is always capitalized.
  8. How do you implement thread synchronization (Object.Wait, Notify,and CriticalSection) in C#? - You want the lock statement, which is the same as Monitor Enter/Exit:
    lock(obj) { // code }
    translates to
    try {
     // code
  9. 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
     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.
  10. How do I simulate optional parameters to COM calls? - You must use the Missing class and pass Missing.Value (in System.Reflection) for any values that have optional parameters.

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