Question

I have the following WPF code:

    public Window1()
    {
        InitializeComponent();
        FlowDocument fd = new FlowDocument();
        fd.LoadFromWordML("../../testdoc.docx");

FlowDocument (in System.Windows.Documents namespace) does not have a LoadFromWordML method. Rather this method is defined locally as follows:

    public static void LoadFromWordML(this FlowDocument doc, string path)

Yet as you can see, LoadFromWordML is being called as if it is a method of FlowDocument class, and in fact the ‘this’ argument is being suppressed.

What C# language facility allows this? (And a comment against language design: this makes it very difficult to read legacy code.)

Answer 1

This is called extension methods

Extension methods enable you to “add” methods to existing types without creating a new derived type, recompiling, or otherwise modifying the original type. Extension methods are a special kind of static method, but they are called as if they were instance methods

I have encountered these problems using them

1. Hunting documentation to figure out where the hell they are defined.
2. They cannot be defined to be virtual so it can lead to problems. In one of the projects I worked on we had Arc deriving from Circle. Somebody decided that Transform method should be an extension method. So most of the time it worked. Transforming a Circle yielded a transformed Circle and same with Arc. But it lead to subtle errors when you transformed an Arc when it was being used as an Circle reference.