近日研究文本控件和字符串，研究得头混脑胀，问题真是太多了。
　　这只是其中的一个问题，答案来自问专家网站和CSDN的TechnoFantasy(www.applevb.com) ，解决如下：

Public Const WM_USER = &H400
Public Const EM_SETTARGETDEVICE = (WM_USER + 72)

Public Declare Function GetDC Lib "user32" (ByVal hWnd As Long) As Long
Public Declare Function SendMessage Lib "user32" Alias "SendMessageA" _
  (ByVal hWnd As Long, _
   ByVal wMsg As Long, _
   ByVal wParam As Long, _
   lParam As Any) As Long


Public Sub AutoSwitchLine(ByRef RichText As Long, ByVal bSwitch As Boolean)
    If bSwitch Then
        '设置 RichTextBox 自动换行
        Call SendMessage(RichText.hWnd, EM_SETTARGETDEVICE, _
        GetDC(RichText.hWnd), RichText.Width / 15)
        If RichText.RightMargin = 0 Then
            RichText.RightMargin = 1
        Else
            RichText.RightMargin = 0
        End If
    Else
        '设置 RichTextBox 不自动换行
        Call SendMessage(RichText.hWnd, EM_SETTARGETDEVICE, 0, 1)
    End If
End Sub