Difference between IEnumerable Count() and Length

Question

What are the key differences between IEnumerable Count() and Length?

Answer 1

Little addition to Jon Skeet's comment.

Here is the source code of the Count() extension method:

.NET 3:

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

.NET 4:

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