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Top C# interview questions

  1. Whatís the implicit name of the parameter that gets passed into the classí set method? Value, and its datatype depends on whatever variable weíre changing.
  2. How do you inherit from a class in C#? Place a colon and then the name of the base class. Notice that itís double colon in C++.
  3. Does C# support multiple inheritance? No, use interfaces instead.
  4. When you inherit a protected class-level variable, who is it available to? Classes in the same namespace.
  5. 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.
  6. Describe the accessibility modifier protected internal. Itís available to derived classes and classes within the same Assembly (and naturally from the base class itís declared in).
  7. C# provides a default constructor for me. I write a constructor that takes a string as a parameter, but want to keep the no parameter one. How many constructors should I write? Two. Once you write at least one constructor, C# cancels the freebie constructor, and now you have to write one yourself, even if thereís no implementation in it.
  8. Whatís the top .NET class that everything is derived from? System.Object.
  9. Howís method overriding different from overloading? When overriding, you change the method behavior for a derived class. Overloading simply involves having a method with the same name within the class.
  10. What does the keyword virtual mean in the method definition? The method can be over-ridden.
  11. Can you declare the override method static while the original method is non-static? No, you canít, the signature of the virtual method must remain the same, only the keyword virtual is changed to keyword override.
  12. Can you override private virtual methods? No, moreover, you cannot access private methods in inherited classes, have to be protected in the base class to allow any sort of access.
  13. Can you prevent your class from being inherited and becoming a base class for some other classes? Yes, thatís what keyword sealed in the class definition is for. The developer trying to derive from your class will get a message: cannot inherit from Sealed class WhateverBaseClassName. Itís the same concept as final class in Java.
  14. Can you allow class to be inherited, but prevent the method from being over-ridden? Yes, just leave the class public and make the method sealed.
  15. Whatís 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, itís a blueprint for a class without any implementation.
  16. When do you absolutely have to declare a class as abstract (as opposed to free-willed educated choice or decision based on UML diagram)? When at least one of the methods in the class is abstract. When the class itself is inherited from an abstract class, but not all base abstract methods have been over-ridden.
  17. Whatís an interface class? Itís an abstract class with public abstract methods all of which must be implemented in the inherited classes.
  18. Why canít you specify the accessibility modifier for methods inside the interface? They all must be public. Therefore, to prevent you from getting the false impression that you have any freedom of choice, you are not allowed to specify any accessibility, itís public by default.
  19. Can you inherit multiple interfaces? Yes, why not.
  20. And if they have conflicting method names? Itís 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 youíre okay.
  21. Whatís 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.
  22. How can you overload a method? Different parameter data types, different number of parameters, different order of parameters.
  23. If a base class has a bunch of overloaded constructors, and an inherited class has another bunch of overloaded constructors, can you enforce a call from an inherited constructor to an arbitrary 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.
  24. Whatís the difference between System.String and System.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.
  25. Whatís the advantage of using System.Text.StringBuilder over System.String? StringBuilder is more efficient in the cases, where a lot of manipulation is done to the text. Strings are immutable, so each time itís being operated on, a new instance is created.
  26. Can you store multiple data types in System.Array? No.
  27. Whatís 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.
  28. How can you sort the elements of the array in descending order? By calling Sort() and then Reverse() methods.
  29. Whatís the .NET datatype that allows the retrieval of data by a unique key? HashTable.
  30. Whatís class SortedList underneath? A sorted HashTable.
  31. Will finally block get executed if the exception had not occurred? Yes.
  32. Whatís the C# equivalent of C++ catch (Ö), which was a catch-all statement for 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 {}.
  33. Can multiple catch blocks be executed? No, once the proper catch code fires off, the control is transferred to the finally block (if there are any), and then whatever follows the finally block.
  34. Why is it a bad idea to throw your own exceptions? Well, if at that point you know that an error has occurred, then why not write the proper code to handle that error instead of passing a new Exception object to the catch block? Throwing your own exceptions signifies some design flaws in the project.
  35. Whatís a delegate? A delegate object encapsulates a reference to a method. In C++ they were referred to as function pointers.
  36. Whatís a multicast delegate? Itís a delegate that points to and eventually fires off several methods.
  37. Howís 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.
  38. What are the ways to deploy an assembly? An MSI installer, a CAB archive, and XCOPY command.
  39. Whatís 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.
  40. What namespaces are necessary to create a localized application? System.Globalization, System.Resources.
  41. Whatís the difference between // comments, /* */ comments and /// comments? Single-line, multi-line and XML documentation comments.
  42. How do you generate documentation from the C# file commented properly with a command-line compiler? Compile it with a /doc switch.
  43. Whatís the difference between <c> and <code> XML documentation tag? Single line code example and multiple-line code example.
  44. Is XML case-sensitive? Yes, so <Student> and <student> are different elements.
  45. What debugging tools come with the .NET SDK? CorDBG Ė command-line debugger, and DbgCLR Ė graphic debugger. Visual Studio .NET uses the DbgCLR. To use CorDbg, you must compile the original C# file using the /debug switch.
  46. What does the This window show in the debugger? It points to the object thatís pointed to by this reference. Objectís instance data is shown.
  47. 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.
  48. Whatís 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.
  49. Why are there five tracing levels in System.Diagnostics.TraceSwitcher? The tracing dumps can be quite verbose and for some applications that are constantly running you run the risk of overloading the machine and the hard drive there. Five levels range from None to Verbose, allowing to fine-tune the tracing activities.
  50. Where is the output of TextWriterTraceListener redirected? To the Console or a text file depending on the parameter passed to the constructor.
  51. How do you debug an ASP.NET Web application? Attach the aspnet_wp.exe process to the DbgClr debugger.
  52. What are three test cases you should go through in unit testing? Positive test cases (correct data, correct output), negative test cases (broken or missing data, proper handling), exception test cases (exceptions are thrown and caught properly).
  53. 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.
  54. Explain the three services model (three-tier application). Presentation (UI), business (logic and underlying code) and data (from storage or other sources).
  55. 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, but itís a .NET layer on top of OLE layer, so not the fastest thing in the world. ODBC.NET is a deprecated layer provided for backward compatibility to ODBC engines.
  56. Whatís 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.
  57. What is the wildcard character in SQL? Letís 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 ĎLa%í.
  58. Explain ACID rule of thumb for transactions. Transaction must be Atomic (it is one unit of work and does not dependent on previous and following transactions), Consistent (data is either committed or roll back, no ďin-betweenĒ case where something has been updated and something hasnít), Isolated (no transaction sees the intermediate results of the current transaction), Durable (the values persist if the data had been committed even if the system crashes right after).
  59. What connections does Microsoft SQL Server support? Windows Authentication (via Active Directory) and SQL Server authentication (via Microsoft SQL Server username and passwords).
  60. 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.
  61. Why would you use untrusted verificaion? Web Services might use it, as well as non-Windows applications.
  62. What does the parameter Initial Catalog define inside Connection String? The database name to connect to.
  63. Whatís the data provider name to connect to Access database? Microsoft.Access.
  64. What does Dispose method do with the connection object? Deletes it from the memory.
  65. 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.