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") + "℃";