HOWTO: Pass String Data Between Applications Using SendMessage

• Microsoft Visual Basic Learning, Professional, and Enterprise Editions for Windows, version 6.0
• Microsoft Visual Basic Control Creation, Learning, Professional, and Enterprise Editions for Windows, version 5.0

SUMMARY

There are many ways to achieve inter-process communication using Visual Basic. Unless you establish an OLE Automation client server relationship, string data is difficult to handle cleanly. The main reason is that 32-bit applications run in a separate address space, so the address of a string in one application is not meaningful to another application in a different address space. Using the SendMessage() API function to pass a WM_COPYDATA message avoids this problem.

This article demonstrates how to pass string data from one application to another by using the SendMessage API function with the WM_COPYDATA message.

MORE INFORMATION

WARNING: One or more of the following functions are discussed in this article; VarPtr, VarPtrArray, VarPtrStringArray, StrPtr, ObjPtr. These functions are not supported by Microsoft Technical Support. They are not documented in the Visual Basic documentation and are provided in this Knowledge Base article "as is." Microsoft does not guarantee that they will be available in future releases of Visual Basic.

Visual Basic does not support pointers and castings in the manner of Visual C++. In order to pass string data from one Visual Basic application to another, the Unicode string must be converted to ASCII prior to passing it to the other application. The other application must then convert the ASCII string back to Unicode.

The following summarizes how to pass string data from one application to another.

Step-by-Step Example

1. Convert the string to a byte array using the CopyMemory() API.



2. Obtain the address of the byte array using the VarPtr() intrinsic function and copy the address and length of the byte array into a COPYDATASTRUCT structure.



3. Pass the COPYDATASTRUCT to another application using the WM_COPYDATA message, setting up the other application to receive the message.



4. Unpack the structure on the target system using CopyMemory(), and convert the byte array back to a string using the StrConv() intrinsic function.

Steps to Create the Sample

1. Start a new Standard EXE project in Visual Basic. Form1 is created by default. This project will be your target application.



2. Add a Label control to Form1.



3. Copy the following code to the Code window of Form1:




      Private Sub Form_Load()
          gHW = Me.hWnd
          Hook
          Me.Caption = "Target"
          Me.Show
          Label1.Caption = Hex$(gHW)
      End Sub

      Private Sub Form_Unload(Cancel As Integer)
          Unhook
      End Sub 

4. Add a module to the project and paste the following code in the Module1 code window:




     Type COPYDATASTRUCT
              dwData As Long
              cbData As Long
              lpData As Long
      End Type

      Public Const GWL_WNDPROC = (-4)
      Public Const WM_COPYDATA = &H4A
      Global lpPrevWndProc As Long
      Global gHW As Long

      'Copies a block of memory from one location to another.

      Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" _
         (hpvDest As Any, hpvSource As Any, ByVal cbCopy As Long)

      Declare Function CallWindowProc Lib "user32" Alias _
         "CallWindowProcA" (ByVal lpPrevWndFunc As Long, ByVal hwnd As _
         Long, ByVal Msg As Long, ByVal wParam As Long, ByVal lParam As _
         Long) As Long

      Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" _
         (ByVal hwnd As Long, ByVal nIndex As Long, ByVal dwNewLong As _
         Long) As Long

      Public Sub Hook()
          lpPrevWndProc = SetWindowLong(gHW, GWL_WNDPROC, _
          AddressOf WindowProc)
          Debug.Print lpPrevWndProc
      End Sub

      Public Sub Unhook()
          Dim temp As Long
          temp = SetWindowLong(gHW, GWL_WNDPROC, lpPrevWndProc)
      End Sub

      Function WindowProc(ByVal hw As Long, ByVal uMsg As Long, _
         ByVal wParam As Long, ByVal lParam As Long) As Long
          If uMsg = WM_COPYDATA Then
              Call mySub(lParam)
          End If
          WindowProc = CallWindowProc(lpPrevWndProc, hw, uMsg, wParam, _
             lParam)
      End Function

      Sub mySub(lParam As Long)
          Dim cds As COPYDATASTRUCT
          Dim buf(1 To 255) As Byte

          Call CopyMemory(cds, ByVal lParam, Len(cds))

          Select Case cds.dwData
           Case 1
              Debug.Print "got a 1"
           Case 2
              Debug.Print "got a 2"
           Case 3
              Call CopyMemory(buf(1), ByVal cds.lpData, cds.cbData)
              a$ = StrConv(buf, vbUnicode)
              a$ = Left$(a$, InStr(1, a$, Chr$(0)) - 1)
              Form1.Print a$
          End Select
      End Sub 

5. Save the project and minimize the Visual Basic IDE.

Create the Sending Application

1. Start a second instance of the Visual Basic IDE and create a new Standard EXE project in Visual Basic. Form1 is created by default.



2. Add a CommandButton to Form1.



3. Copy the following code to the Code window of Form1:




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

      Private Const WM_COPYDATA = &H4A

      Private Declare Function FindWindow Lib "user32" Alias _
         "FindWindowA" (ByVal lpClassName As String, ByVal lpWindowName _
         As String) As Long

      Private Declare Function SendMessage Lib "user32" Alias _
         "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal _
         wParam As Long, lParam As Any) As Long

      'Copies a block of memory from one location to another.
      Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" _
         (hpvDest As Any, hpvSource As Any, ByVal cbCopy As Long)

      Private Sub Command1_Click()
          Dim cds As COPYDATASTRUCT
          Dim ThWnd As Long
          Dim buf(1 To 255) As Byte

      ' Get the hWnd of the target application
          ThWnd = FindWindow(vbNullString, "Target")
          a$ = "It Works!"
      ' Copy the string into a byte array, converting it to ASCII
          Call CopyMemory(buf(1), ByVal a$, Len(a$))
          cds.dwData = 3
          cds.cbData = Len(a$) + 1
          cds.lpData = VarPtr(buf(1))
          i = SendMessage(ThWnd, WM_COPYDATA, Me.hwnd, cds)
      End Sub

      Private Sub Form_Load()
      ' This gives you visibility that the target app is running
      ' and you are pointing to the correct hWnd
          Me.Caption = Hex$(FindWindow(vbNullString, "Target"))
      End Sub 

4. Save the project.

Running the Sample

1. Restore the target application and press the F5 key to run the project. Note that the value of the hWnd displayed in the label.



2. Restore the sending application and press the F5 key to run the project. Verify that the hWnd in the form caption matches the hWnd in the label on the target application. Click the CommandButton and the text message should be displayed on the form of the target application.

REFERENCES

For more information, please see the following articles in the Microsoft Knowledge Base:

Q168795 HOWTO: Hook Into a Window's Messages Using AddressOf

Q129947 INFO: Win32 Replacement for the hmemcpy Function

Additional query words:

Keywords : kbAPI kbSDKWin32 kbVBp500 kbVBp600
Version : WINDOWS:5.0,6.0
Platform : WINDOWS
Issue type : kbhowto