Form 上有三个文本框textBox1、textBox2 和textBox3,分别表示加数、被加数和和,要求边输入被加数边求和,同时在被加数文本框只能输入0到9的数字,请完成下面的程序 private void textBox2_ (1) (object sender, EventArgs e) { string str = textBox1.Text; string str1 = textBox2.Text; if ( str != "" && str1 != "" ) { int sum = int. (2) (str) + int. (2) (str1); textBox3.Text = sum. (3) ; } } private void textBox2_ (4) (object sender, KeyPressEventArgs e) { char ch = (5) ; if (ch >= '0' && ch <= '9') { } else { (6) ; } }