Is there a difference between Count() (linq extension) and List<T>.Count

Question

List<string> list = new List<string>() {"a", "b", "c"};
IEnumerable<string> enumerable = list;

int c1 = list.Count;
int c2 = list.Count();
int c3 = enumerable.Count();

Are there differences in performance and implementation between these last 3 statements? Will list.Count() perform worse or the same as list.Count, and does it matter if the reference is of type IEnumerable<string> ?

Answer 1

Let's look with Reflector:

public static int Count<TSource>(this IEnumerable<TSource> source)
{
    if (source == null)
    {
        throw Error.ArgumentNull("source");
    }
    ICollection<TSource> is2 = source as ICollection<TSource>;
    if (is2 != null)
    {
        return is2.Count;
    }
    ICollection is3 = source as ICollection;
    if (is3 != null)
    {
        return is3.Count;
    }
    int num = 0;
    using (IEnumerator<TSource> enumerator = source.GetEnumerator())
    {
        while (enumerator.MoveNext())
        {
            num++;
        }
    }
    return num;
}

So, if your IEnumerable<T> implements ICollection<T> or ICollection, it will return the Count property.

Answer 2

The Linq Count method is clever enough not to iterate over the underlying collection if it implements ICollection interface and therefore already has Count property.