ボックス化解除を使わない文字列型から数値型への変換

public static class StringExtensions
{
    public static int ToInt32(this string value)
    {
        int ret = 0;
 
        for(int n = 0; n < value.Length; n++)
        {
            ret = ret * 10 + (value[n] - '0');
        }
 
        return ret;
    }
}

public static class StringExtensions
{
    public static double ToDouble(this string value)
    {
        double ret = 0;
 
        double fraction = 1;
        bool isFraction = false;
 
        for (int n = 0; n  < value.Length; n++)
        {
            char target = value[n];
 
            if(value[n] == '.')
            {
                isFraction = true;
            }
            else
            {
                ret = ret * 10 + (value[n] - '0');
 
                if (isFraction)
                {
                    fraction = fraction * 10;
                }
            }
        }
 
        return ret / fraction;
    }
}