[RESOLVED] VB6 and VB.NET Interprocess Comm.

Hello everyone,

I have two apps ,one in vb and another in vb.net .Now I'm trying to send messages from one another.I'm able to detect the receipt of messages both ways.I'm also able to send a structure of the form

VB Code:

Private Type MyStructure
  dwData As Long
  cbData As Long
  lpData As Long
End Type

from vb6 to vb.net but when i attempt to send a structure from vb.net to vb6,it is not able to convert the data back to its original structure.


The logic i used to convert is simple
On the sender side:

VB Code:

Private Type MyStructure
              dwData As Long
              cbData As Long
              lpData As Long
      End Type
 
      Private Const WM_COPYDATA = &H4A
 
  Public Function SendMessageToVBNET()
    Dim hwndTarget As Long
    Dim MessageId, i As Long
    Dim a As String
    Dim cds As MyStructure
    Dim ThWnd As Long
    Dim buf(1 To 255) As Byte
 
    ThWnd = FindWindow(vbNullString, "VB.NET Main Form")
    a$ = "I'm from VB6"
    Call CopyMemory(buf(1), ByVal a$, Len(a$))
    cds.dwData = 3
    cds.cbData = Len(a$) + 1
    cds.lpData = VarPtr(buf(1))   'Converting a string 'a' to long so we can send it with the message
 
    If WindowMessagingInitialised = False Then
      InitWindowMessaging                           'this starts the subclassing
    End If
 
    hwndTarget = VBNET_WindowHandle         'this gets the handle of target
 
    MessageId = VB6_TO_VBNET_MessageId    'this is an indicator for the target that message is from vb6
 
    If hwndTarget <> 0 Then
                i = SendMessage(hwndTarget, WM_COPYDATA, MainForm.hwnd, cds)
 
    End If
  End Function

On the receiver side:
Using Marshalling to convert pointer to structure

VB Code:

Protected Overrides Sub WndProc(ByRef m As System.Windows.Forms.Message)
        Dim Mypoint As MyStructure
 
 
        If m.Msg = WM_COPYDATA Then
            Mypoint = CType(Marshal.PtrToStructure(m.LParam, GetType(MyStructure)), MyStructure)
            MsgBox("Value of X> " & Mypoint.x & " <Value of Y> " & Mypoint.y & " <Value of XY> " & Mypoint.xy & " <-")
 
 
        End If
        MyBase.WndProc(m)
 
    End Sub

I have attached two zip files.
1.vb-vb.zip has two projects ,one sender and one receiver and they are working as expected.
2.vb-vbnet.zip has two projects, one in vb6 and another in vb.net using the same logic as above.

Do not send a pointer to your string across processes, this won't be marshalled by the OS.

Instead send a length and a byte array that contains each character of the string.

VB Code:

Private Type MyStructure
  dwData As Long
  cbData As Long
  lStrLen As Long
  bData[MAXLENGTH] as byte
End Type

The problem was indeed with the marshalling thing.This is how i solved it

VB Code:

public void SendMessageToVB6()
        {
            IntPtr hwndTarget;
            IntPtr MessageId;
            String message;
            hwndTarget = this.VB6_WindowHandle;
 
            message="Hi,Did You get my message";
            IntPtr ptr = Marshal.StringToHGlobalAnsi(message);
            COPYDATASTRUCT cds = new COPYDATASTRUCT();
            cds.dwData = IntPtr.Zero;
            cds.cbData = message.Length;
            cds.lpData = ptr;
            
            long result = SendMessage(hwndTarget,WM_COPYDATA,0,ref cds);
 
            Marshal.FreeHGlobal(ptr);
 
            
        }

Also i changed my sendmessage declaration to

VB Code:

public static extern long SendMessage(IntPtr hWnd, uint Msg,    uint wParam, ref COPYDATASTRUCT lParam  );

After a bit of googling ,i found out that COPYDATASTRUCT is a standard convention for sending messages across apps :

VB Code:

public struct COPYDATASTRUCT
        {
            public IntPtr dwData;
            public int cbData;
            public IntPtr lpData;
        }

Where cbdata holds the length and lpdata holds the string .