C# - Gross- Kleinbuchstaben in String schnell zählen

public void countUpperLower (string a, out int upper, out int lower) {
    lower = 0; upper = 0; 
    unsafe {
        fixed (char* cTmp = a) {
            char* c = cTmp; 
            char* end = c + a.Length; 
            while (c < end) {
                if (*c > 64 && *c < 91 )
                    upper++; 
                if (*c > 96 && *c < 123)
                    lower++; 
                c++; 
            }
        }
    }
}