Parameter array

Parameter arrays allow to provide variable number of parameters to supply to a method. It starts with params keyword and must be last parameter. An example might be Console.WriteLine method

public class Console
{
    public static void WriteLine(String format, params Object[] arg) { ... }
}

In the method, the behavior is exactly same as with any other array. However it allows two different calling styles

Console.WriteLine("Format: {0}, {1}", x, y);
object[] args = new object[] { x, y };
Console.WriteLine("Format: {0}, {1}", args);

If you look into Reference Source for Console class it shows number of methods for common use-cases. This is because the params array gets created every time the method is called and can be inefficient in some tight loops or other heavy duty scenarios.