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

Question

1. What is the use of default keyword in C#?
2. Is it introduced in C# 3.0 ?

Answer 1

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)
    }
}

Answer 2

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

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

Answer 3

The most common use is with generics; while it works for "regular" types (i.e. default(string) etc), this is quite uncommon in hand-written code.

I do, however, use this approach when doing code-generation, as it means I don't need to hard-code all the different defaults - I can just figure out the type and use default(TypeName) in the generated code.

In generics, the classic usage is the TryGetValue pattern:

public static bool TryGetValue(string key, out T value) {
    if(canFindIt) {
        value = ...;
        return true;
    }
    value = default(T);
    return false;
}

Here we have to assign a value to exit the method, but the caller shouldn't really care what it is. You can contrast this to the constructor constraint:

public static T CreateAndInit<T>() where T : ISomeInterface, new() {
    T t = new T();
    t.SomeMethodOnInterface();
    return t;
}