Class extension syntax for C#

string_extension.cs

namespace System
{
    public static class StringExtension
    {
        public static string TimesMe(this String str, int times)
        {
            StringBuilder sb = new StringBuilder();

            for (int i = 0; i < times; i++)
            {
                sb.Append(str);
            }
            
            return sb.ToString();
        }
    }
}