工程名为DDE，窗口名是FRM
创建一个PICTURE，名字是P，用来启动DDELINK
如果是多窗体，就在MDIFORM的LOAD中写，如果是单窗体，就在FORM的LOAD中写：
If App.PrevInstance = True Then
    Me.LinkMode = 0
    Me.LinkTopic = ""
    P.LinkMode = 0
    P.LinkTopic = "DDE|Frm"
    P.LinkMode = 2
    P.LinkExecute Command
    End
Else
    OpenFile(Command)'OpenFile是你的程序中用来打开文件的函数。
End If
然后，在LinkExecute中写：
OpenFile(CmdStr)
Cancel = False

这样就可以得到第二次打开的文件的名字啦。

做了一个完整的，不过不是多窗体的。
工程名：DDETRY，窗体名：FRMTRY，PICTURE名：P，RTFBOX名：TEXT1

Option Explicit

Private Sub Form_LinkExecute(CmdStr As String, Cancel As Integer)
Dim Tmp As String
If CmdStr <> "" Then
    Tmp = CmdStr
    If Left(Tmp, 1) = Chr(34) Then Tmp = Right(Tmp, Len(Tmp) - 1)
    If Right(Tmp, 1) = Chr(34) Then Tmp = Left(Tmp, Len(Tmp) - 1)
    Text1.Text = ""
    Text1.FileName = Tmp
    Me.Caption = Tmp
End If
Cancel = False
End Sub

Private Sub Form_Load()
Dim Tmp As String
If App.PrevInstance = True Then
    Me.LinkMode = 0
    Me.LinkTopic = ""
    P.LinkMode = 0
    P.LinkTopic = "DDETry|FrmTry"
    P.LinkMode = 2
    P.LinkExecute Command
    End
Else
    If Command <> "" Then
        Tmp = Command$
        If Left(Tmp, 1) = Chr(34) Then Tmp = Right(Tmp, Len(Tmp) - 1)
        If Right(Tmp, 1) = Chr(34) Then Tmp = Left(Tmp, Len(Tmp) - 1)
        Text1.FileName = Tmp
        Me.Caption = Tmp
    End If
End If
Text1.Top = 0
Text1.Left = 0
End Sub

Private Sub Form_Resize()
Text1.Height = Me.ScaleHeight
Text1.Width = Me.ScaleWidth
End Sub