C# developer interview questions and answers

Software development hiring managers and potential interviewees will find these open-ended C# proficiency interview questions and answers useful.

Good help is hard to find. There is an art to finding a developer who fits well in your organization in terms of personality and work ethic; fortunately, it's more straightforward to determine their technical expertise.

I worked at a couple of places where development managers loved drilling job candidates on syntax while having them write code, and it was clearly a stressful experience for the interviewee. I do not like asking specific syntax questions during interviews since most developers do not know language syntax or class names off the top of their heads.

In terms of the basic interview, I prefer to ask open-ended questions where the candidate can explain concepts and how they would attack problems. Some organizations like to give candidates tests or have them eyeball code snippets to spot problems, and I think those are good evaluation tools.

This is the first installment in our series of programming language-specific development interview questions and answers. Here is a list of questions (and the answers to those questions) that will help you get a feel for a candidate's proficiency with C#. You can ask follow-up questions based on their replies.

Note: This content is also available as a downloadable PDF.

What are namespaces, and how they are used?

Namespaces are used to organize classes within the .NET Framework. They dictate the logical structure of the code. They are analogous to Java packages, with the key difference being Java packages define the physical layout of source files (directory structure) while .NET namespaces do not. However, many developers follow this approach and organize their C# source files in directories that correlate with namespaces. The .NET Framework has namespaces defined for its many classes, such as System.Xml—these are utilized via the using statement. Namespaces are assigned to classes via the namespace keyword.

What is a constructor?

A constructor is a class member executed when an instance of the class is created. The constructor has the same name as the class, and it can be overloaded via different signatures. Constructors are used for initialization chores.

What is the GAC, and where is it located?

The GAC is the Global Assembly Cache. Shared assemblies reside in the GAC; this allows applications to share assemblies instead of having the assembly distributed with each application. Versioning allows multiple assembly versions to exist in the GAC—applications can specify version numbers in the config file. The gacutil command line tool is used to manage the GAC.

Why are strings in C# immutable?

Immutable means string values cannot be changed once they have been created. Any modification to a string value results in a completely new string instance, thus an inefficient use of memory and extraneous garbage collection. The mutable System.Text.StringBuilder class should be used when string values will change.

What is DLL Hell, and how does .NET solve it?

DLL Hell describes the difficulty in managing DLLs on a system; this includes multiple copies of a DLL, different versions, and so forth. When a DLL (or assembly) is loaded in .NET, it is loaded by name, version, and certificate. The assembly contains all of this information via its metadata. The GAC provides the solution, as you can have multiple versions of a DLL side-by-side.

How are methods overloaded?

Methods are overloaded via different signatures (number of parameters and types). Thus, you can overload a method by having different data types, different number of parameters, or a different order of parameters.

How do you prevent a class from being inherited?

The sealed keyword prohibits a class from being inherited.

What is the execution entry point for a C# console application?

The Main method.

How do you initiate a string without escaping each backslash?

You put an @ sign in front of the double-quoted string.

String ex = @"This has a carriage return\r\n"

What is the difference between a struct and a class?

Structs cannot be inherited. Structs are passed by value and not by reference. Structs are stored on the stack not the heap. The result is better performance with Structs.

What is a singleton?

A singleton is a design pattern used when only one instance of an object is created and shared; that is, it only allows one instance of itself to be created. Any attempt to create another instance simply returns a reference to the first one. Singleton classes are created by defining all class constructors as private. In addition, a private static member is created as the same type of the class, along with a public static member that returns an instance of the class. Here is a basic example:

public class SingletonExample {
 private static SingletonExample _Instance;
 private SingletonExample () { }
 public static SingletonExample GetInstance() {
  if (_Instance == null)  {
    _Instance = new SingletonExample ();
   return _Instance;

What is boxing?

Boxing is the process of explicitly converting a value type into a corresponding reference type. Basically, this involves creating a new object on the heap and placing the value there. Reversing the process is just as easy with unboxing, which converts the value in an object reference on the heap into a corresponding value type on the stack. The unboxing process begins by verifying that the recipient value type is equivalent to the boxed type. If the operation is permitted, the value is copied to the stack.

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Tony Patton has worn many hats over his 15+ years in the IT industry while witnessing many technologies come and go. He currently focuses on .NET and Web Development while trying to grasp the many facets of supporting such technologies in a productio...

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