I have an old dll written in vc6 and have used it in vb6. Everything
works fine.
The vb6 declaration just and client code just like below:
Public Const DEF_D_OPERATER = 1    

Public Type CRNO
   No1 As String * 3  
   No2 As String * 5  
End Type

Public Type DLG_IOSTR
   keiban As CRNO          
   f10ban As CRNO          
   OpeID As String * 31    
   OpeName As String * 15  
   lpStr1 As Long          
   lpStr2 As Long          
   flag As Long            
End Type

Public Declare Function dialog Lib "ktccomm" Alias "KTCDlgBox" (ByVal
hwnd As Long, ByVal kind As Long, dlg As DLG_IOSTR) As Long

   dlg.lpStr1 = 0
   dlg.lpStr2 = 0
   ret = dialog(graphic_main.hwnd, DEF_D_OPERATER, dlg)
   If ret = 1 Then
       MsgBox dlg.OpeName
   End If

now, I need to use this function in an .net application. But when i
run the .net program, a nullreference exception will be throwed.
My .net source code just like this:
Module OperatorDlg
   Public Const DEF_D_OPERATER As Integer = 1

   <StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Ansi)>
Public Structure CRNO
       <MarshalAs(UnmanagedType.ByValArray, SizeConst:=3)> _
       Public No1() As Byte  
       <MarshalAs(UnmanagedType.ByValArray, SizeConst:=5)> _
       Public No2() As Byte
       Public Sub CRNO()
           No1 = New Byte(3) {}
           No2 = New Byte(5) {}
       End Sub
   End Structure

   <StructLayout(LayoutKind.Sequential, CharSet:=CharSet.Ansi)>
Public Structure DLG_IOSTR
       Public keiban As CRNO          
       Public f10ban As CRNO          
       <MarshalAs(UnmanagedType.ByValArray, SizeConst:=15)> _
       Public OpeID() As Byte    
       <MarshalAs(UnmanagedType.ByValArray, SizeConst:=31)> _
       Public OpeName() As Byte  
       Public lpStr1 As String    
       Public lpStr2 As String    
       Public flag As Long        
       Public Sub dlg_iostr()
           OpeID = New Byte(15) {}
           OpeName = New Byte(31) {}
       End Sub
   End Structure

   Declare Function OpInput Lib "ktccomm.dll" Alias "KTCDlgBox"
(ByVal hwnd As Long, ByVal kind As Long,
<MarshalAs(UnmanagedType.Struct)> ByRef dlg As DLG_IOSTR) As Long

   Dim dlgs As DLG_IOSTR
   dlgs.lpStr1 = "1"
   dlgs.lpStr2 = "2"
   dlgs.keiban.No1 = New Byte(2) {}
   dlgs.keiban.No2 = New Byte(4) {}
   dlgs.f10ban.No1 = New Byte(2) {}
   dlgs.f10ban.No2 = New Byte(4) {}
   dlgs.OpeID = New Byte(14) {}
   dlgs.OpeName = New Byte(30) {}
   Dim ret As Long = OpInput(0, DEF_D_OPERATER, dlgs) // This is the
statement that throws an exception
   If ret = 1 Then
          MessageBox.Show(dlgs.OpeName.ToString)
   End If
Can anybody tell me what is the problem? Thanks a lot for your reply.