Existing ToArray declarion in Enumerable

Enumerable.cs

public static TSource[] ToArray<TSource>(this IEnumerable<TSource> source)
{
    if (source != null)
    {
        Buffer<TSource> buffer = new Buffer<TSource>(source);
        return buffer.ToArray();
    }
    else
    {
        throw Error.ArgumentNull("source");
    }
}