System::String str = this->textBox1->Text;
int matches = 0;
for (int i = 0; i < str.Length; i++) {
    if (System::Char::IsDigit(str, i)) ++matches;
}
if (matches != str.Length) {
    // error
} else {
    // ok
}

System::String str = this->textBox1->Text;
int matches = 0;
for (int i = 0; i < str.Length; i++) {
    if (System::Char::IsDigit(str.Chars[i])) ++matches;
}
if (matches != str.Length) {
    // error
} else {
    // ok
}