private delegate void d_SetText(string str);

private void SetText(string str)
{
    this.textBox1.Text += str;
}

private void serialPort1_DataReceived(object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
{
    int len = serialPort1.BytesToRead;
    byte[] buf = new byte[len];
    serialPort1.Read(buf, 0, buf.Length);
    this.Invoke(new d_SetText(SetText), new object[] { System.Text.Encoding.Default.GetString(buf) });
}

private void button1_Click(object sender, EventArgs e)
{
    serialPort1.Write("这里写串口命令");  //注意有时候需要加Char(13)或者Char(10)，具体看通讯协议
}

------解决方案--------------------

请参照一下吧，下载实例看看：http://www.shilidata.com/search.aspx?type=&searchid=11055


关于C#中关于串口的监听  
http://www.shilidata.com/showtopic-280.aspx


串口通讯System.IO.Ports.Serialport  
http://www.shilidata.com/showtopic-182.aspx