vineri, 15 ianuarie 2010

C# Programming Interview Questions

General Questions

  1. Does C# support multiple-inheritance? No.
  2. Who is a protected class-level variable available to? It is available to any derived class.
  3. 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.
  4. Describe the accessibility modifier “protected internal”. It is available to classes that are within the same assembly or derived from the specified base class.
  5. What’s the top .NET class that everything is derived from? System.Object.
  6. 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.
  7. 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.
  8. 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.
  9. Can you store multiple data types in System.Array? No.
  10. What’s the difference between the System.Array.CopyTo() and System.Array.Clone()?
    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
    identacle object.
  11. How can you sort the elements of the array in descending order? By calling Sort() and then Reverse() methods.
  12. What’s the .NET collection class that allows an element to be accessed using a unique key? HashTable, Dictionary<,>.
  13. What class is underneath the SortedList class? A sorted HashTable. 
  14. Will the finally block get executed if an exception has not occurred? Yes.
  15. 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
  16. 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).
  17. 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).
  18. Why do I get a syntax error when trying to declare a variable called checked? - The word checked is a keyword in C#.
  19. 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
  20. Explain the "yield" keyword. What is the compiler doing internally?
  21. What is an intern pool ?

    Defination: A performance optimization based on using collections of pre-allocated resources such as objects or database connections
    the advent of the .NET platform writing code that pools objects and
    threads has become a simple task. By using the Threading and Collections namespaces you can create robust object pooling applications. This could also be done by implementing COM+ interop interfaces into your code
  22. How many generations of garbage collection are there? 3
  23. Why are catch all exception handlers bad? When/Why might you use them?
  24. How do you choose 1 entry point when C# project has more Main( ) method? CSC /main:classname filename.cs
  25. How do you refer parent classes in C#? Base
  26. What is difference between typeof() and .GetType()?
  27. What is an indexer in C#? 

Class Questions

  1. 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
  2. Can you prevent your class from being inherited by another class? Yes.  The keyword “sealed” will prevent the class from being inherited.
  3. 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.
  4. 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.
  5. 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.
  6. 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.
  7. Why can’t you specify the accessibility modifier for methods inside the interface? They all must be public, and are therefore public by default.
  8. Can you inherit multiple interfaces? Yes.  .NET does support multiple interfaces.
  9. What happens if you inherit multiple interfaces and they have conflicting method names?
  10. 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.
  11. What is the difference between a Struct and a Class? Structs
    are value-type variables and are thus saved on the stack, additional
    overhead but faster retrieval.  Another difference is that structs cannot inherit.

Method and Property Questions

  1. 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.
  2. What does the keyword “virtual” declare for a method or property?  The method or property can be overridden.
  3. 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.
  4. 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)
  5. What are the different ways a method can be overloaded? Different parameter data types, different number of parameters, different order of parameters.
  6. 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.

Events and Delegates

  1. What’s a delegate?  A delegate object encapsulates a reference to a method. A strongly typed function pointer.
  2. What’s a multicast delegate? A delegate that has multiple handlers assigned to it.  Each assigned handler (method) is called.

XML Documentation Questions

  1. Is XML case-sensitive? Yes.
  2. What’s the difference between // comments, /* */ comments and /// comments? Single-line comments, multi-line comments, and XML documentation comments.
  3. How do you generate documentation from the C# file commented properly with a command-line compiler?  Compile it with the /doc switch.

Debugging and Testing Questions

  1. What debugging tools come with the .NET SDK?
    1.   CorDBG – command-line debugger.  To use CorDbg, you must compile the original C# file using the /debug switch.
    2.   DbgCLR – graphic debugger.  Visual Studio .NET uses the DbgCLR.
  2. What does assert() method 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.
  3. What’s 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.
  4. 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.
  5. Where is the output of TextWriterTraceListener redirected? To the Console or a text file depending on the parameter passed to the constructor. 
  6. How do you debug an ASP.NET Web application? Attach the aspnet_wp.exe process to the DbgClr debugger.
  7. 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).
  8. 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.

ADO.NET and Database Questions

  1. 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
  2. What are advantages and disadvantages of Microsoft-provided data provider classes in ADO.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.
  3. 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%’.
  4. 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.
  5. What connections does Microsoft SQL Server support?
    Windows Authentication (via Active Directory) and SQL Server authentication (via Microsoft SQL Server username and password).
  6. Between Windows Authentication and SQL Server Authentication, which one is trusted and which one is untrusted?
    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.
  7. What does the Initial Catalog parameter define in the connection string? The database name to connect to.
  8. What does the Dispose method do with the connection object?
  9. 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.

Assembly Questions

  1. 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.
  2. What are the ways to deploy an assembly? An MSI installer, a CAB archive, and XCOPY command.
  3. 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.
  4. What namespaces are necessary to create a localized application? System.Globalization and System.Resources.
  5. What is the smallest unit of execution in .NET? an Assembly.
  6. 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.
  7. How do you convert a value-type to a reference-type? Use Boxing.
  8. 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.
  9. Why does DllImport not work for me? - All methods marked with the DllImport attribute must be marked as public static extern
  10. What’s a strong name? A strong name includes the name of the assembly, version number, culture identity, and a public key token.


  1. Write a simple Windows Forms MessageBox statement. System.Windows.Forms.MessageBox.Show("Hello, Windows Forms");
  2. How to find out how many calender days are there between two dates in
DateTime t1 = new DateTime(2008, 5, 13);
DateTime t2 = new DateTime(2004, 2, 22);

TimeSpan span = (t1 > t2 ? t1 - t2 : t2 - t1); Console.WriteLine(span.TotalDays);
  1. Which debugging window allows you to see the methods called in the order they were called? The Call Stack
  2. How you hide a form when Close button is pressed? Use in method registered to closing event of the form put the following code: public override void OnClosing(CancelEventArgs e) { e.Cancel = true; Hide(); }
  3. Can you write a class without specifying namespace? Which namespace does it belong to by default? Yes, you can, then the class belongs to global namespace which has no
    name. For commercial products, naturally, you wouldn’t want global
  4. How can you save the desired properties of Windows Forms application? .config files in .NET are supported through the API to allow storing
    and retrieving information. They are nothing more than simple XML
    files, sort of like what .ini files were before for Win32 apps.
  5. So how do you retrieve the customized properties of a .NET application
    from XML .config file?
    Initialize an instance of AppSettingsReader
    class. Call the GetValue method of AppSettingsReader class, passing in
    the name of the property and the type expected. Assign the result to
    the appropriate variable.
  6. My progress bar freezes up and dialog window shows blank, when an
    intensive background process takes over.
    Yes, you should’ve
    multi-threaded your GUI, with taskbar and main form being one thread,
    and the background process being the other.
  7. How can I run an EXE from within my application? 
    Use the Process class found in the System.Diagnostics namespace.


    Process m_process = new Process();
    m_process.StartInfo.FileName = @"Notepad.exe";
    m_process.StartInfo.Arguments = "";
  8. How can I find all programs with a GUI (not just arbitrary windows) that are running on my local machine? 
    You could use EnumWindows with p/Invoke, but using the static
    Process.GetProcesses() found in the System.Diagnostics namespace will
    avoid the interop overhead.
    foreach ( Process p in Process.GetProcesses(System.Environment.MachineNam e) )

       if( p.MainWindowHandle != IntPtr.Zero)
    //this is a GUI app 
      Console.WriteLine( p ); // string s = p.ToString();

  9. How can I make sure there is only one instance of my application running?

    Use Process class in System.Diagnostics to implement this
    functionality using code such as

    public static Process RunningInstance()

      Process current = Process.GetCurrentProcess();
      Process[] processes = Process.GetProcessesByName (current.ProcessName);
      //Loop through the running processes in with the same name

    foreach (Process process in processes)
         //Ignore the current process
         if (process.Id != current.Id)
             //Make sure that the process is running from the exe file.
             if (Assembly.GetExecutingAssembly().Location.Replace( "/", "\\") == current.MainModule.FileName)
                    //Return the other process instance.
                    return process;

    //No other instance was found, return null.

    return null;

  10. How do I get the path to my running EXE? Application.ExecutablePath
  11. How can I trigger a Paint event in System.Drawing? Use Invalidate property.


  1. How do you implement thread synchronization (Object.Wait, Notify,and CriticalSection) in C#? lock, Monitor.Enter() .Exit()


  1. What is the Lazy Design Pattern? The approach of the Lazy Design Pattern is not to create objects until
    a specific requirement matches, and when it matches, object
    creation is triggered.

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