delegate in c# .what its purpose?

Actually delegates are istantiated, see, for instance: "Delegates Tutorial at MSDN"[^].

A delegate is not an instance - it is a variable holding a reference to a method.
As a result, you cannot instantiate it by using the new operator: that would imply that you are creating a new method, which is plainly silly!

If you think of a delegate as a "Pointer to a method" then you are there. It is a way to access a method later without needing to know it's name at that point.

public string GetScreenName() { return "Screen"; }
public string GetPrinterName() { return "Printer"; }
public delegate string GetNameMethod();
public GetNameMethod GetName;
...
            if (isUsingScreen)
                {
                GetName = GetScreenName;
                }
            else
                {
                GetName = GetPrinterName;
                }
            Console.WriteLine(GetName());

Delegates is a type that reference Method. When it assigned by a method it behaves exactly same as that method. It can be used as parameters and can be returned a value. So it has same what are the methods have.

For more Read this article,

http://www.pathumf.blogspot.com/2011/11/delegates-in-c.html

have a look at:
What are the advantages of delegates in C#.NET?[^]