IntelliSense is great for discovering what methods are available on an object. Frustration can arise when you wish to extend existing types that you do not control.

C# 3 solves this problem by allowing you to write methods that appear as if they are part of the type. In reality they are static methods declared on a static type with the special this keyword in front of the first parameter which indicates the type - or interface - they appear on.

Code

Now

Console.WriteLine(customer.GetSalutation());
// ...
static class CustomerHelper
{
    public static string GetSalutation(this Customer c)
    {
        return "Dear " + c.Title + " " + c.Surname;
    }
}

Before

Console.WriteLine(CustomerHelper.GetSalutation(customer));
// ...
static class CustomerHelper
{
    public static string GetSalutation(Customer c)
    {
        return "Dear " + c.Title + " " + c.Surname;
    }
}

Notes

• Extension methods can only access members that are accessible to the static class (public members, or internal ones within the same assembly) - never private or protected members
• Extension methods must be static methods of a non-nested, non-generic static class
• Extension methods can also be used to "add" methods to interfaces!
• Reserve extensions for common operations - do not pollute IntelliSense with every helper you can think of
• Extension members ^{C# 14.0} add extension properties and static extension members

The first parameter must be prefixed with this but it does not have to be a class or struct. You can also extend interface, enums, and generic types.

• enums with methods are very convenient and feel more like their rich Java cousins
• To extend an open generic type such as List<T> the method must itself be generic. Remember to place generic type constraints on the method to ensure it only appears for types it actually deals with.

More information

• C# Programming Guide