未处理System.InvalidOperationException

Question

 double value = 0.0;
            inPut_4 inPut4 = new inPut_4();
            inPut4.Open("COM3");
            value = (double)inPut4.getInPut4_Temp();
            txtCurrent.Text = value.ToString("f");
            inPut4.Close();
            inPut4 = null;
            value = double.Parse(txtCurrent.Text);
            double temp = (value - 4) / 16 * 70 - 10;
            txtTemp.Text = temp.ToString("f1") + "℃";