Method overloading

很多编程语言都支持默认/可选参数. 这意味着你可以设定一个或多个可选的参数, 并给它们一个默认的值. 这在给已存在的代码增加功能时非常有用.

For instance, you may wish to add functionality to an existing function, which requires one or more parameters to be added. By doing so, you would break existing code calling this function, since they would now not be passing the required amount of parameters. To work around this, you could define the newly added parameters as optional, and give them a default value that corresponds to how the code would work before adding the parameters.

Default parameters were introduced in C# version 4.0, but up until that, C# coders have been using a different technique, which basically does the same, called method overloading. It allows the programmer to define several methods with the same name, as long as they take a different set of parameters. When you use the classes of the .NET framework, you will soon realize that method overloading is used all over the place. A good example of this, is the Substring() method of the String class. It has an extra overload, like this:

string Substring (int startIndex)
string Substring (int startIndex, int length)

You can call it with either one or two parameters. If you only call it with one parameter, the length parameter is assumed to be the rest of the string, saving us time whenever we simply want to get the last part of a string.

So, by defining several versions of the same function, how do we avoid having the same code several places? It's actually quite simple: We let the simple versions of the method make the complex version of it do all the work. Consider the following example:

class SillyMath
{
    public static int Plus(int number1, int number2)
    {
        return Plus(number1, number2, 0);
    }

    public static int Plus(int number1, int number2, int number3)
    {
        return number1 + number2 + number3;
    }
}

We define a Plus method, in two different versions. The first one takes two paramaters, for adding two numbers, while the second version takes three numbers. The actual work is done in the version that takes three numbers - if we only wish to add two, we call the three parameter version, and simply use 0 as the third paramater, acting as a default value. I know, I know, it's a silly example, as indicated by the name of the class, but it should give you an idea about how it all works.

Now, whenever you feel like doing advanced math by adding a total of four numbers (just kidding here), it's very simple to add a new overload:

class SillyMath
{
    public static int Plus(int number1, int number2)
    {
        return Plus(number1, number2, 0);
    }

    public static int Plus(int number1, int number2, int number3)
    {
        return Plus(number1, number2, number3, 0);
    }

    public static int Plus(int number1, int number2, int number3, int number4)
    {
        return number1 + number2 + number3 + number4;
    }
}

The cool thing about this, is that all your existing calls to the Plus method will continue working, as if nothing had been changed. The more you use C#, the more you will learn to appreciate method overloading.