How To Pass String Data Between Applications Using SendMessage

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.

The next section shows you how to create a sample program that demonstrates passing string data from one application to another.

Steps to Create the Sample

To create this sample, you create two separate projects; a sending project and a target project.

Create the target application:

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.