Pick a .NET class, any class. Take a class from the .NET Framework. Is there any method that you think is missing from that class?
Let’s say you picked the ArrayList. Too bad it does not have a Concat method, that will take the ToString() of every object in the ArrayList and concatenate it to one big string. The sample code below shows just that.
ArrayList list = new ArrayList();
list.Add(42);
list.Add(“The answer to everything”);
string all = list.Concat(‘;’);
Console.WriteLine(all);
Well, with extension methods you can create this method anyway. Remember, Concat is NOT a member of the original ArrayList implementation. You can extend the class with extra methods.
In order to get the Concat method into the ArrayList class (or any method into a certain class) we will need to create a class as a carrier for the extension methods. This class can have any name you like, but it must be declared static. We’ll call it ExtensionMethods. Every extension method is a regular static method, with one exception: it has a special first parameter. This first parameter is of the type which needs to be extended AND it has the this keyword defined before the type.
In our case we need a Concat method hooked up to the ArrayList. This bit of code will do that:
public namespace OurExtension
{
public static class ExtensionMethods
{
public static string Concat(this ArrayList list, char separator)
{
// Do the concatenation thingy
return concatenatedString();
}
}
}
Again, notice the three ingredients of the extension method: static class, static method, first parameter of method is of type to extended and has this before it.
Now we can use the ArrayList class and it will have a magical extra instance (!) method called Concat. There’s nothing much we need to do to make that work, except importing the namespace OurExtension.
using OurExtensions;
…
ArrayList list = new ArrayList();
string big = list.Concat(‘;’);
There are some rules that apply to these extension methods:
- Existing instance methods of the real class take precedence over the extension method.
This means that if the ArrayList class would have had a Concat method of the same signature itself, it would have used that instead of the extension method. - Inner namespace declarations take precedence over outer namespace declarations.
So, if you import a namespace that is below/inside your own, it will be used before any namespaces that are outside of your own namespace.
Lastly, you can also use a generic static method to make that extension method appear on a whole range of classes.
public
static T[] Slice(this T[] source, int index, int count)The declaration above will appear on all array types.
