Why this cause a out of range exception

Question

string str="my nam is milad";

/* why this line cuse a out of range error ---------------- */

string tmp=str.Substring(str.Length-4,str.Length-1);
Console.WriteLine(tmp);

Answer 1

you want to get a substring of length str.Length-=13 starting at position str.Length-4. but here the maximum remaining string has only length 4, so you try to read beyond the end of str.