Q176058: HOWTO: Pass String Data Between Applications Using SendMessage
Article: Q176058
Product(s): Microsoft Visual Basic for Windows
Version(s): WINDOWS:5.0,6.0
Operating System(s):
Keyword(s): kbAPI kbSDKWin32 kbVBp500 kbVBp600
Last Modified: 11-JAN-2001
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The information in this article applies to:
- Microsoft Visual Basic Learning Edition for Windows, version 6.0
- Microsoft Visual Basic Professional Edition for Windows, version 6.0
- Microsoft Visual Basic Enterprise Edition for Windows, version 6.0
- Microsoft Visual Basic Control Creation Edition for Windows, version 5.0
- Microsoft Visual Basic Learning Edition for Windows, version 5.0
- Microsoft Visual Basic Professional Edition for Windows, version 5.0
- Microsoft Visual Basic Enterprise Edition for Windows, version 5.0
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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.
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.
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
Technology : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVBA500Search kbVBA500 kbVBA600 kbVB500 kbVB600 kbZNotKeyword3
Version : WINDOWS:5.0,6.0
Issue type : kbhowto
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