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C# Interview Questions

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).

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.

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.

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");


•      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();
    }
}

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();
    }
}

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();
        }
    }
}

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();
    }
}

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();
    }
}

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();
    }
}


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