用 底层键盘钩子 钩按键消息 写在 了 Textbox 中 但是每次按一个键 都把该键值重复2次 大印出来


键盘处理函数是这样的

 private int KeyboardHookProc(int nCode, Int32 wParam, IntPtr lParam)
        {
            KeyMSG keymsg = (KeyMSG)Marshal.PtrToStructure(lParam, typeof(KeyMSG));
            switch ((Keys)keymsg.vkCode)
            {
                case Keys.A: textBox1.Text += "A"; break;
                case Keys.B: textBox1.Text += "B"; break;
                case Keys.C: textBox1.Text += "C"; break;
                case Keys.D: textBox1.Text += "D"; break;
                case Keys.E: textBox1.Text += "E"; break;
                case Keys.F: textBox1.Text += "F"; break;
                case Keys.G: textBox1.Text += "G"; break;
                case Keys.H: textBox1.Text += "H"; break;
                case Keys.I: textBox1.Text += "I"; break;
                case Keys.J: textBox1.Text += "J"; break;
                case Keys.K: textBox1.Text += "K"; break;

                case Keys.L: textBox1.Text += "L"; break;
                case Keys.M: textBox1.Text += "M"; break;
                case Keys.N: textBox1.Text += "N"; break;
                case Keys.O: textBox1.Text += "O"; break;
                default: textBox1.Text += "---OVER"; break;
            }
            return 0;
        }


请问是什么原因

在线等答案!!!!