public static string makeSid(string sid, int startIndex)
{
　string str = sid.Remove(0, startIndex);
　FSharpFunc<int, string> func = new rand@15();
　int length = str.Length;
　int num2 = 8;
　if (length >= num2)
　{
　　return (sid.Substring(0, startIndex) + str.Remove(7, 7).Insert(7, func.Invoke(7)));
　}
　return null;
}