Form 上有三个文本框 textBox1 , textBox2 , textBox3 ,分别表示加数,被加数和求和结果。要求边输入被加数边求和,且在被加数文本框只能输入 0 到 9 的数字,请补充完成下列程序: private void textBox2_ ( 1 ) ( object sender, EventArgs e) { string str1 = ( 2 ) ; string str2 = textBox2.Text; if (str1 != "" && str2 != "" ) { int sum = int . ( 3 ) (str1) + int . ( 3 ) (str2); textBox3.Text = sum. (4) (); } } private void textBox2_ (5) ( object sender, KeyPressEventArgs e) { char ch = (6) ; if ((ch >= '0' &&ch <= '9' )||ch==8) { } else { (7) ; } }