解决方法
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; } } }