c＃替换输入字符串中某个位置的char

Question

如果有一个电话号码输入，例如＆＃34; 555-GET-FOOD＆＃34;，我试图用相应的号码替换字母（即A，B和C = 1）。到目前为止，我有这个：

public enum LetterNumber
{
    A = 2, B = 2, C = 2, D = 3, E = 3, F = 3, G = 4, H = 4, I = 4,
    J = 5, K = 5, L = 5, M = 6, N = 6, O = 6, P = 7, Q = 7, R = 7,
    S = 7, T = 8, U = 8, V = 8, W = 9, Y = 9, Z = 9
}

private bool IsValidNumber(string str)
{
    const int VALID_LENGTH = 10;
    bool valid = true;

    if (str.Length == VALID_LENGTH)
    {
        foreach (char ch in str)
        {
            if (char.IsLower(ch))
            {
                valid = false;
            }
        }
    }
    else
    {
        valid = false;
    }

    return valid;
}

不幸的是，我不知道使用什么命令将数字替换给定位置的每个字符，或者如何使用该命令。任何帮助将不胜感激！