In this article I will cover the list of best possible C# Interview Questions and Answers of the C# programming language and explain in as simple language as possible so it becomes understandable by every fellow programmer.
The list of C# Interview Questions and Answers is also available for download as PDF file.
C# is an enormous programming language with many concepts which is being constantly updated by Microsoft and hence the list of C# Interview Questions and Answers cannot be constant.
Thus this article and the downloadable PDF file will be periodically updated with the new questions that fit best into this list of C# Interview Questions and Answers.
C# Interview Questions and Answers
1. What is C#?
C# (pronounced as C-Sharp) is an Object Oriented programming language developed by Microsoft. This language is very similar to Java and was developed to bring together C++ features and the easy programming of Visual Basic (VB).
2. Difference between C++ and C#?
C#
• C# is a higher level programming language built over C++ programming language.
• It is similar to Java.
• It can be considered as a hybrid programming language consisting of features of C++ and ease of use of VB.Net.
C++
• C++ is a low level programming language built by adding object oriented concepts to the C programming language.
• It is NOT similar to Java.
• C++ itself is a language and does not inherit any features of other languages.
3. What is Struct and Class in C#?
Struct
• Value Type i.e. whenever a variable is created, it holds the value.
• Copying a Struct variable, copies the value.
• A new variable is created in Stack memory.
• Cannot be inherited.
• Cannot be of Abstract type.
• Cannot contain Private parameterless constructors.
• Doesn’t support Destructor.
Class
• Reference Type i.e. whenever a variable is created, it holds the reference (address) while the value is stored some location.
• Copying a Class variable, copies the reference while the value remains on same location.
• A new variable is created in Heap memory.
• Can be inherited.
• Can be of Abstract type.
• Can contain Private parameterless constructors.
• Supports Destructor.
4. Difference between String and StringBuilder in C#?
String
• Belongs to the System namespace.
• Immutable i.e. whenever you append a String object it creates a new object with new memory allocation.
• When changed, a new object is created and old object is destroyed.
• Avoid using inside loops, as if the loop executes N times. N memory allocation operations will be performed.
• Concat function is used for String concatenation.
Example:
string s = "Mudassar";
s = string.Concat(s, "Khan");
StringBuilder
• Belongs to the System.Text namespace.
• Mutable i.e. whenever you append a String object it does not creates a new object with new memory allocation.
• When changed, no new object is created or old object is destroyed.
• Recommended for loops, as only the size of object will change.
• Append function is used for String concatenation.
Example:
StringBuilder sb = new StringBuilder();
sb.Append("Mudassar");
sb.Append("Khan");
5. What is Static function and example of in-built Static function in .Net?
• Has the keyword static.
• A Static function is one that can be accessed without creating an object of the class.
• It can be accessed using the name of the class.
• It can belong to Static as well as Non-Static classes.
• One cannot use external Non-Static objects and functions inside a Static method.
• One can use Static function in Static as well as Non-Static classes.
• Keyword this is not supported.
• Example of one inbuilt Static function in C# is Convert.ToString().
Example:
public class A
{
public static void C()
{
//Invalid
this.D();
}
public void D()
{
//Valid
C();
}
}
6. What is Static class and its difference with Non-Static class?
A Static class is a class that cannot be instantiated i.e. its object cannot be created.
Static class
• Has the keyword static.
• Cannot be instantiated and no object can be created.
• Keyword new is not supported.
• Objects and Functions can be accessed by class name.
• Mandatory for functions and methods to be marked static i.e. can contain only Static members.
• Members cannot be accessed using this keyword.
• Only one instance can be created.
Example:
public static class B
{
public static object E;
public static void F()
{
}
public static void G()
{
//Valid
var q = E;
//Valid
F();
}
}
Non-Static class
• Does not have the keyword static.
• Can be instantiated and object can be created.
• Keyword new is supported.
• Objects and Functions cannot be accessed by class name unless marked static.
• Not mandatory for functions and methods to be marked static i.e. can contain Static as well as Non-Static members.
• Non-Static members can be accessed using this keyword. But Static members cannot be accessed using this keyword.
• Multiple instances can be created.
Example:
public class A
{
public object C;
public static object G;
public static void D()
{
//Invalid
var p = C;
//Valid
var q = G;
//Invalid
E();
}
public void E()
{
//Valid.
var p = this.C;
//Valid
var q = G;
//Valid
D();
}
}
7. What is Namespace in C#? Give example of in-built Namespace in .Net?
A Namespace is used to organize classes. Example of in-built .Net Namespace is the System Namespace.
• One can create his own Namespace and define the scope of the classes belonging to the Namespace.
• Keyword using is used in order to access a class belonging to a particular Namespace.
• A class belonging to a Namespace can be accessed only when the Namespace with the help of using directive.
• A namespace does not have any access modifiers i.e. namespace cannot be public, private, etc.
• A namespace can belong to multiple assemblies and also a single assembly can have multiple namespaces.
Example:
namespace MySpace
{
public class A
{
public static void Fun()
{
}
}
}
namespace YourSpace
{
//Accesing the Namespace
using MySpace;
public class B
{
public void Fun()
{
//Accessing the class of external namespace
A.Fun();
}
}
}
8. What is a Sealed class in C#?
A Sealed class is a class which cannot be inherited. The sealed keyword is used to prohibit inheritance of a particular class in C#.
A sealed class can be public as well as private.
Example:
public sealed class A
{
public void Fun()
{
}
}
//Compiler Error: 'B': cannot derive from sealed type 'A'
public class B : A
{
public static void Fun()
{
}
}
public class C
{
public static void Fun()
{
//Valid
A a = new A();
a.Fun();
}
}
9. What is an Internal class in C#?
An Internal class is a class which cannot be used outside its Assembly. The internal keyword is used to mark a particular class Internal i.e. it restrict its access outside the Assembly.
• An Assembly could be a Project, a DLL or an EXE.
• Inside the Assembly, the internal class is like public class.
• An internal class can be inherited within the Assembly.
Example:
internal class A
{
public void Fun()
{
}
}
//Valid
public class B : A
{
public static void Fun()
{
}
}
public class C
{
public static void Fun()
{
//Valid
A a = new A();
a.Fun();
}
}
10. What is an Abstract class in C#?
An Abstract class is a special class which is majorly used for inheritance and it cannot be instantiated.
• Cannot be instantiated i.e. object cannot be created using the new keyword.
• Can contain both Abstract and Non-Abstract members.
• Abstract members are simply declared and are defined in the classes deriving the Abstract class.
• Abstract members are defined by using the override keyword.
• Non-Abstract members are defined within the Abstract class.
• Non-Abstract members can be accessed within the derived classes only if marked public or protected.
• Private Non-Abstract members are not accessible outside the Abstract class.
• Abstract and Non-Abstract members can be accessed using the derived classes.
• Does not support Multiple Inheritance.
Example:
public abstract class A
{
public abstract void Fun1();
public void Fun4()
{
}
protected void Fun5()
{
}
private void Fun6()
{
}
}
public class B : A
{
public override void Fun1()
{
}
public void Fun2()
{
//Valid
Fun4();
Fun5();
//Compiler Error: Cannot create an instance of the abstract class or interface
Fun6();
}
}
public class C
{
public static void Fun3()
{
//Compiler Error: Cannot create an instance of the abstract class or interface 'A'
A a = new A();
//Valid
B b = new B();
b.Fun1();
b.Fun2();
b.Fun4();
}
}
11. What is an Interface in C#?
An Interface is similar to an Abstract class and it also cannot be instantiated.
• Cannot be instantiated i.e. object cannot be created using the new keyword.
• Can contain only declaration of Members and not definition.
• Members are simply declared and are defined in the classes deriving the Interface.
• Members are defined without using the override keyword.
• Cannot contain Access modifiers i.e. Members cannot be public, private, etc.
• The derived member can only be public.
• Does support Multiple Inheritance.
Example:
public interface A
{
void Fun1();
void Fun2();
}
public class B : A
{
public void Fun1()
{
}
//Error: 'B' does not implement interface member 'A.Fun2()'
//'B.Fun2()' cannot implement an interface member because it is not public
private void Fun2()
{
}
}
public class C
{
public static void Fun3()
{
//Compiler Error: Cannot create an instance of the abstract class or interface 'A'
A a = new A();
//Valid
B b = new B();
b.Fun1();
//Invalid
b.Fun2();
}
}
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