This post is also available in french here. 

After reading this post from Eric Lippert, and reminding me that in the samples for this post, I'm using IEnumerable<T>.Count() where I'm actually not using the return value, therefore I'm enumerating through the whole collection for almost nothing.

I could have used IEnumerable<T>.Any(), which after looking at the IL code, just starts the enumeration, and stops just after one element if there is one and returns true, or if there isn't, returns false.

This is more effective, even without PLinq :)