C# 组合键 判断，文本框不接受纯回车enter

//TextBox 必须是多行文本框
private void Txt_Billcode_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.Control && e.KeyCode == Keys.Enter) //当按下Ctrl和Enter 输入回车
            {
                e.SuppressKeyPress = false;
                e.Handled = false;
                return;
            }
            else if (e.KeyCode == Keys.Enter)//单纯按下Enter不在文本框中输入回车
            {
                e.SuppressKeyPress = true;
                e.Handled = true;
            }

}        

当输入内容时 并弹框提示时， 上面的代码会失效，

if (e.Control && e.KeyCode == Keys.Enter)
            {
                Txt_Billcode.AcceptsReturn = true;
                e.SuppressKeyPress = false;
                e.Handled = false;
                return;
            }
            else if (e.KeyCode == Keys.Enter)
            {
                Txt_Billcode.AcceptsReturn = false;
                Txt_Billcode.Enabled = false;
                string strBillcode = Txt_Billcode.Text;
                if (true)
                {

                    MessageBox.Show(this, "提示成功");
                }
                else
                {
                    MessageBox.Show(this, "提示失败！");
                }
                e.SuppressKeyPress = true;
                e.Handled = true;
                Txt_Billcode.Enabled = true;
                Txt_Billcode.Text = strBillcode;

            }