解决方法@H_403_9@

使用RichTextBox v5. Visual Studio中的默认值为v4.它解决了这个问题.

public class RichText50W : RichTextBox
{
    [DllImport("kernel32.dll",CharSet = CharSet.Auto)]
    static extern IntPtr LoadLibrary(string lpFileName);
    protected override CreateParams CreateParams
    {
        get
        {
            CreateParams prams = base.CreateParams;
            if (LoadLibrary("msftedit.dll") != IntPtr.Zero)
            {
                prams.ClassName = "RICHEDIT50W";
            }
            return prams;
        }
    }
}