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用 RichTextBox 做聊天内容显示的哥们，估计都遇到过这个问题。
使用SendMessage API向RichTextBox控件发送消息是最简单的一种解决方法。

using System.Runtime.InteropServices;

[DllImport("User32.dll",EntryPoint="SendMessage")]
private static extern int SendMessage(
IntPtr hWnd, // handle to destination window
uint Msg, // message
uint wParam, // first message parameter
uint lParam // second message parameter
);

private const int WM_VSCROLL = 0x0115;
private const int SB_BOTTOM = 7;

private void btnSend_Click(object sender, System.EventArgs e)
{
this.richTextBox1.AppendText("1");
SendMessage(this.richTextBox1.Handle, WM_VSCROLL, SB_BOTTOM, 0);
}