C#解决switch-case后面的代码遇到的问题：使用了未赋值的局部变量

void 计算()

{ 

            Calc clac;//声明一个类
            switch (cbChar.SelectedItem.ToString())
            {
                case "+":
                    clac = new CalcAddWay();
                    clac.Num1 = int.Parse(tbNum1.Text);
                    clac.Num2 = int.Parse(tbNum2.Text);
                    tbResult.Text = clac.CalcResult().ToString();
                    break;

                case "-":
                    clac = new CalcSubWay();
                    break;

                case "*":
                    clac = new CalcChengWay();
                    break;

                default:
                    clac = new CalcChuWay();
                    break;
            }
            clac.Num1 = int.Parse(tbNum1.Text);

           //如果不写默认的default方法，下面此处会报错:使用了未赋值的局部变量(也就是clac)
            clac.Num2 = int.Parse(tbNum2.Text);
            tbResult.Text = clac.CalcResult().ToString();

}