extension-methods

A "non-believer" of C# was asking me what the purpose to extension methods was. I explained that you could then add new methods to objects that were already defined, especially when you don't own/control the source to the original object. He brought up "Why not just add a method to your own class?" We've been going round and round (in a good way). My general response is that it is another tool in the toolbelt, and his response is it is a useless waste of a tool... but I thought I'd get a more "enlightened" answer. What are some scenarios that you've used extension methods that you couldn't

I have created an extension method for an ASP.NET MVC ViewPage, e.g: public static class ViewExtensions { public static string Method<T>(this ViewPage<T> page) where T : class { return "something"; } } When calling this method from a View (deriving from ViewPage ), I get the error " CS0103: The name 'Method' does not exist in the current context " unless I use the this keyword to call it: <%: Method() %> <!-- gives error CS0103 --> <%: this.Method() %> <!-- works --> Why is the this keyword required? Or does it work without it, but I'm missing something? (I think there must be a duplicate of

What would you consider to be the best exception type to throw when an extension method is called on a null instance (where the extension method does not allow it)? Since extension methods are nothing but static methods you could argue that it should be ArgumentNullException, but on the other hand they're used like instance methods so it might be more natural to use the NullReferenceException. Let's take the following example: public static string ToInvariantString(this IFormattable value, string format) { return value.ToString(format, CultureInfo.InvariantCulture); } This way a