LINQCount.cs

public static int Count<TSource>(this IEnumerable<TSource> source) {
   if (source == null) {
      throw Error.ArgumentNull("source");
   }
   var collectionT = source as ICollection<TSource>;
   if (collectionT != null) {
      return collectionT.Count;
   }
   var collection = source as ICollection;
   if (collection != null) {
      return collection.Count;
   }
   int num = 0;
   using (IEnumerator<TSource> enumerator = source.GetEnumerator()) {
      while (enumerator.MoveNext()) {
         num++;
      }
   }
   return num;
}