C# 4.0 : Optional Parameters

C# 4.0 has introduced a new feature of optional parameters, where we can provide a default value of a parameter in its declaration itself. Default parameters can be applied to methods, constructors, indexers or delegate.

Example1:

public void Hello(string strGreeting = "Hello!")

{

     Console.WriteLine(strGreeting);

}

Here pstrGreeting is an optional parameter and can be omitted during method call.

Hello();

Output:  Hello!

If we do not supply value of pstrGreeting, it will be default to “Hello!”

Example2:

void Hello (string pstrGreeting = “Hello!”, string strWin = “Winshuttle”)

{

                Console.WriteLine(pstrGreeting + “ “ + strWin);

}

Outputs for different versions are as follows:

Hello();                                  // Hello! Winshuttle

Hello("HELLO");                  // HELLO Winshuttle

Hello("HELLO", "INDIA");   // HELLO INDIA

Note:

• If we add a default parameter to a public method then any depended assembly will also required recompilation.
• Default value must be specified by a constant expression.
• Optional parameter can’ be ref or out.
• Mandatory parameters must come before optional except params (always come last).
• String.Empty cannot be used as default value as it is not a compile-time constant. It is a static property defined in the string class.