c#中未将对象引用设置到对象的实例。改了好久改不来 求大神指导

private void butkeyverify_Click(object sender, EventArgs e)
{

        try
        {
            byte a, b;
            string c = null;

                //生成密钥类型
                {
                    if (cmbkeytype.SelectedValue.ToString() == "密钥A")
                    {
                        a = 0x60;
                    }
                    else
                    {
                        a = 0x61;
                    }
                }
                //生成扇区号
                {
                    if (cmbcardsection.SelectedValue.ToString() == "扇区0")
                    {
                        b = 0x00;
                    }
                    else if (cmbcardsection.SelectedValue.ToString() == "扇区1")
                    {
                        b = 0x01;
                    }
                    else if (cmbcardsection.SelectedValue.ToString() == "扇区2")
                    {
                        b = 0x02;
                    }
                    else if (cmbcardsection.SelectedValue.ToString() == "扇区3")
                    {
                        b = 0x03;
                    }
                    else if (cmbcardsection.SelectedValue.ToString() == "扇区4")
                    {
                        b = 0x04;
                    }
                    else if (cmbcardsection.SelectedValue.ToString() == "扇区5")
                    {
                        b = 0x05;
                    }
                    else if (cmbcardsection.SelectedValue.ToString() == "扇区6")
                    {
                        b = 0x06;
                    }
                    else if (cmbcardsection.SelectedValue.ToString() == "扇区7")
                    {
                        b = 0x07;
                    }
                    else if (cmbcardsection.SelectedValue.ToString() == "扇区8")
                    {
                        b = 0x08;
                    }
                    else if (cmbcardsection.SelectedValue.ToString() == "扇区9")
                    {
                        b = 0x09;
                    }
                    else if (cmbcardsection.SelectedValue.ToString() == "扇区10")
                    {
                        b = 0x10;
                    }
                    else if (cmbcardsection.SelectedValue.ToString() == "扇区11")
                    {
                        b = 0x11;
                    }
                    else if (cmbcardsection.SelectedValue.ToString() == "扇区12")
                    {
                        b = 0x12;
                    }
                    else if (cmbcardsection.SelectedValue.ToString() == "扇区13")
                    {
                        b = 0x13;
                    }
                    else if (cmbcardsection.SelectedValue.ToString() == "扇区14")
                    {
                        b = 0x14;
                    }
                    else
                    {
                        b = 0x15;
                    }

                }
                //生成密钥
                c = txtkey.Text;

                     if (xf != null)
                {

                    ErrorCode code = ErrorCode.Success;
                    code = xf.KeyVerify(a, b, c);
                    if (code == ErrorCode.Success)
                    {
                        tbReadLog.Text += "密钥验证成功！\r\n";

                    }
                    else
                        tbReadLog.Text += "密钥验证失败！\r\n";

                }
            }


        catch (Exception err)
        {
            MessageBox.Show(err.Message);
        }
    }

f5调试方式运行，看出错停在哪一行。

另外
if (cmbcardsection.SelectedValue.ToString() == "扇区0")
{
b = 0x00;
}
else if (cmbcardsection.SelectedValue.ToString() == "扇区1")
{
b = 0x01;
}
。。。
写出这么啰嗦的代码难道没有想过改进下么？

else if (cmbcardsection.SelectedValue.ToString() == "扇区10")
{
b = 0x10;
}
这个你确认么？按照合乎逻辑的设计，应该是
else if (cmbcardsection.SelectedValue.ToString() == "扇区A")
{
b = 0xA;
}
或者
else if (cmbcardsection.SelectedValue.ToString() == "扇区10")
{
b = 10;
}
当然你的程序也不怎么合乎逻辑。

你贴出来的代码是完整的代码吗？
把string c = null;改成string c = string.Empty;试试