Look at this code which I surrounded by textchanged event of a text box:

        string testString = comboIPAddress.Text;
        string[] parts = testString.Split('.');
        int? ipclass = int.Parse(parts[0]);
        if (ipclass == null)
        {
            //do nothing
        }

        if (ipclass >= 1 && ipclass <= 126)
        {
            comboSubnet.Text = "255.0.0.0";
        }
        if (ipclass >= 192 && ipclass <= 223)
        {
            comboSubnet.Text = "255.255.255.0";
        }
        if (ipclass >= 128 && ipclass <= 191)
        {
            comboSubnet.Text = "255.255.0.0";
        }
        else
        {
            comboSubnet.Text = "";
        }

While executing the exe if I delete everything from IPAddress combo box, it is giving error(Input string was not in a correct format.). I don't know the other way to compare an int to null. Please help.