What is the use of `default` keyword in C#?

You can use default to obtain the default value of a Generic Type as well.

public T Foo<T>()
{
    .
    .
    .
    return default(T);
}

The default keyword is contextual since it has multiple usages. I am guessing that you are referring to its newer C# 2 meaning in which it returns a type's default value. For reference types this is null and for value types this a new instance all zero'd out.

Here are some examples to demonstrate what I mean:

using System;

class Example
{
    static void Main()
    {
        Console.WriteLine(default(Int32)); // Prints "0"
        Console.WriteLine(default(Boolean)); // Prints "False"
        Console.WriteLine(default(String)); // Prints nothing (because it is null)
    }
}