What is simpliest way to get Line number from char position in String?

What is simpliest way to get Line number from char position in String in C#? (or get Position of line (first char in line) ) Is there any built-in function ? If there are no such function is it good solution to write extension like :

public static class StringExt {
    public static int LineFromPos(this String S, int Pos) { 
        int Res = 1;
        for (int i = 0; i <= Pos - 1; i++)
            if (S[i] == '\n') Res++;
        return Res;                
    }

    public static int PosFromLine(this String S, int Pos) { .... }

}

?

Edited: Added method PosFromLine