Writing Custom LINQ Extension Methods

Microsoft have done a great job with LINQ IMHO. However there are times when it might be handy to create your own LINQ extension methods. LINQ extension methods can be applied to any type provided that the source is of type IEnumerable<T>, so thats really the only requirement.

So how do we write an extension method. Well its quite simple. Here are 2 that I’ve written for a  IEnumerable<string> the first example takes a Predicate and the second one doesnt. This extension method will check whether a file name string is a valid image file, but you can do it for whatever your pupose is.


The only other thing to note is the cryptic “this” keyword used in the method signature. This means its being applied to the current IEnumerable<T> collection. Just always put it in your extension methods and you’ll be fine.

So how to use these extension methods, well providing your dealing with a  Enumerable<string> collection youll get the option coming up as an extension method.


And to actually use it you could use a standard LINQ query something like




  1. Yeah sure, but that would not have made a very good post on extension methods now would it 🙂


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