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();
}
}
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();
}
}
}
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();
}
}
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();
}
}
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();
}
}
