Count() is an extension method for IEnumerable<T>. This means that Count() is a static method which takes an single argument of type IEnumerable<T> and ostensib

Count() is an extension method for IEnumerable<T>. This means that Count() is a static method which takes an single argument of type IEnumerable<T> and ostensibly only depends on members defined by IEnumerable<T> to work (but manojlds points out that this isn't really the case). C# provides syntactic sugar to make calling a static extension method look like calling an instance method. If you wanted to implement your own "Count" you would implement ICollection<T> which the IEnumerable<T> extension method would call into.