DOCUMENT:Q180714 05-NOV-2000 [vbwin] TITLE :HOWTO: Pass a Picture Object Remotely Using COM PRODUCT :Microsoft Visual Basic for Windows PROD/VER::5.0,6.0 OPER/SYS: KEYWORDS:kbcode kbVBp500 kbVBp600 kbGrpDSVBDB ====================================================================== ------------------------------------------------------------------------------- The information in this article applies to: - Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0 ------------------------------------------------------------------------------- SUMMARY ======= This article discusses how to use the Component Object Model (COM) to pass objects remotely. This process is commonly referred to as cross-process object marshaling. The article also examines the difference between passing objects by reference (ByRef) and by value (ByVal), and demonstrates a custom marshaling approach to remotely pass a Picture object from one machine to another (ByVal) using COM. MORE INFORMATION ================ You can pass objects to a remote machine using COM, but you must pass the objects ByRef. (Note: An ActiveX Data Objects (ADO) recordset is the current exception to this rule because a custom proxy was built for this object type.) However, passing an object by reference requires network traffic for each subsequent method call because the object resides on the remote machine. You can work around this limitation using the Variant data type. The following sample passes a Picture object to a remote machine using a COM callback and the Variant data type. Step-by-Step Procedure ---------------------- The following sections contain the sample code for the server and client projects. The Server Project: 1. Start a new ActiveX EXE project. Class1 is added by default. 2. From Project menu, click Project Properties, and change the Project Name to "PicServer" (without the quotation marks). 3. In the Name property of the Class1 module, change the name to clsPassPicture. 4. Paste the following code into clsPassPicture's code section: Option Explicit Private gbconnected As Boolean Private objClient As Object ' Receive the callback reference from the client. Public Function AddObjectReference(Caller As Object) As Boolean On Error GoTo AddObjectReferenceError ' Store the remote client reference for later use. Set objClient = Caller AddObjectReference = True gbconnected = True ' Global connected flag. Exit Function AddObjectReferenceError: #If DEBUG_ON Then MsgBox Error$, vbOKOnly + vbExclamation, _ "AddObjectReference - Error " & Err.Number #End If AddObjectReference = False Exit Function End Function ' Receive the drop request from the client and destroy the ' reference. Public Function DropObjectReference(Caller As Object) As Boolean On Error GoTo DropObjReferenceError If objClient Is Caller Then Set objClient = Nothing gbconnected = False DropObjectReference = True Else ' Debug.Print "Caller not the same as ObjRef. Unable to ' quit." DropObjectReference = False End If Exit Function DropObjReferenceError: #If DEBUG_ON Then MsgBox Error$, vbOKOnly + vbExclamation, _ "DropObjectReference - Error" & Str$(Err) #End If DropObjectReference = False Exit Function End Function ' Public function that receives the pass picture request from ' the client. Public Function ServerIcon() As Boolean Dim i As Integer Dim MyFile As String Dim tb() As Byte On Error GoTo IconError If gbconnected Then ' Test for existing client reference. MyFile = "C:\TestPass.bmp" Load MyForm SavePicture MyForm.Image1.Picture, MyFile ' Save to temp ' file. Open MyFile For Binary As #1 ' Read into byte array. i = 0 i = LOF(1) ' Get the file size. ReDim tb(i + 1) As Byte ' Resize the array. i = 0 Do Until EOF(1) Get #1, , tb(i) ' Read data into array. i = i + 1 Loop Close #1 Kill MyFile ' Clean up the temp file. Unload MyForm 'Unload the Form objClient.ReceivePic tb ' Pass the array to the client ' using a reference. ServerIcon = True End If Exit Function IconError: #If DEBUG_ON Then MsgBox Error$, vbOKOnly + vbExclamation, _ "Get Server Icon - Error" & Str$(Err) #End If ServerIcon = False End Function 5. From Project menu, click Add Form to add a new form, and change the name and caption of the Form to "MyForm" (without the quotation marks). 6. Add an Image control to MyForm, Image1 is added by default. Set the Picture property to a .bmp file on your computer. 7. Compile the Project into an executable file. The Client Project: 1. Start a new ActiveX EXE project. Class1 is added by default. 2. From Project menu, click Project Properties, and change the Project Name to "PicClient" (without the quotation marks). 3. From the Project menu, click References, and then click PicServer. 4. In the Name property of the Class1 module, change the name to "clsReceivePicture" (without the quotation marks). 5. Add the following code to clsReceivePicture's code section: Option Explicit Public Sub ReceivePic(PassedPic As Variant) Dim i As Integer, j As Integer Dim MyFile As String Dim tb() As Byte ' This public sub routine shows how to pass an object using an ' icon. On Error GoTo BadObj i = UBound(PassedPic) ' Repack the variant into the byte array for binary write. ReDim tb(i) As Byte For j = LBound(PassedPic) To UBound(PassedPic) tb(j) = PassedPic(j) Next j ' Write to a local file. MyFile = "C:\TestPass.bmp" Open MyFile For Binary As #1 For j = LBound(tb) To UBound(tb) Put #1, , tb(j) Next j Close #1 ' Recreate the object from a file. Set frmDCOMDemoClient.picPassIcon.Picture = LoadPicture(MyFile) Kill MyFile Exit Sub BadObj: MsgBox Err.Number & ": " & Err.Description & _ " - " & "Object Pass Failed..." End Sub 6. From Project menu, click Add Form to add a new form, and change the name and caption of the Form to "frmDCOMDemoClient" (without the quotation marks). 7. Add a Command button to frmDCOMDemoClient. Change its name property to "cmdGetObject" (without the quotation marks), and change its Caption property to "Get Server Icon" (without the quotation marks). 8. Add a PictureBox control to frmDCOMDemoClient, and change its Name property to "picPassIcon" (without the quotation marks). 9. Paste the following code into frmDCOMDemoClient's code section: Option Explicit Public mbIsConnected As Boolean Public objMyClassInstance As Object Private oServer As PicServer.clsPassPicture Private Sub cmdGetObject_Click() ' This subroutine calls the server to initiate the icon pass. Dim rf As Boolean ' Connect to your server, selected in the Project References ' dialog box. Set oServer = New PicServer.clsPassPicture ServerConnect 'Set up the callback. If mbIsConnected Then rf = oServer.ServerIcon 'Call remote server method. If rf Then MsgBox "Icon Passed Successfully..." Else MsgBox "Icon Pass Failed..." End If ServerDisconnect ' Clean up after the callback. Else MsgBox "Callback Connection Failed" End If End Sub Public Sub ServerConnect() Dim bsuccess As Integer Dim bRetVal As Boolean Dim iCounter As Integer ' Used by the OLE Collision Handler. Dim nCurErrorCount As Integer Const MAX_ERROR_COUNT = 10 On Error GoTo cmdButtonError Screen.MousePointer = vbHourglass Set objMyClassInstance = New clsReceivePicture If oServer.AddObjectReference(objMyClassInstance) Then mbIsConnected = True Else mbIsConnected = False End If Screen.MousePointer = vbDefault Exit Sub cmdButtonError: ' When using asynchronous callbacks between two objects, you need ' this error checking code to deal with the possibility of a ' collision. This collision can occur when a client and server ' attempt to call each other at the same time. This error handler ' forces the client to wait for a random period of time and retry the ' failed operation. During this time, the server should complete its ' call to the client, allowing the client to succeed when it retries ' the call to the server. You should also include the same error ' handling code in the server object. If Erl = 100 And Err = &H80010001 Then If nCurErrorCount >= MAX_ERROR_COUNT Then MsgBox "Unable to obtain server reference. Retry later.", _ vbExclamation, "Remote Server Connect Error" Resume EndOfError Else For iCounter = 1 To 2000 * Rnd() DoEvents Next iCounter Resume End If End If Screen.MousePointer = vbDefault MsgBox Error$, vbCritical, "cmdButton Error" EndOfError: End Sub Public Sub ServerDisconnect() Dim bsuccess As Integer Dim bRetVal As Boolean Dim iCounter As Integer ' Used by the OLE Collision Handler. Dim nCurErrorCount As Integer Const MAX_ERROR_COUNT = 10 On Error GoTo cmdButtonError Screen.MousePointer = vbHourglass If oServer.DropObjectReference(objMyClassInstance) Then mbIsConnected = False 'Dereference the local object. Set objMyClassInstance = Nothing Set oServer = Nothing MsgBox "Disconnect Successful..." Else MsgBox "Disconnect Failed..." End If Screen.MousePointer = vbDefault Exit Sub cmdButtonError: If Erl = 100 And Err = &H80010001 Then If nCurErrorCount >= MAX_ERROR_COUNT Then MsgBox "Unable to release server reference. Retry later.", _ vbExclamation, "Remote Server Disconnect Error" Resume EndOfError Else For iCounter = 1 To 2000 * Rnd() DoEvents Next iCounter Resume End If End If Screen.MousePointer = vbDefault MsgBox Error$, vbCritical, "cmdButton Error" EndOfError: End Sub 10. From the Project menu, click Add Module. Module1 is added by default. 11. Paste the following code into Module1's code section: Option Explicit Sub Main() Load frmDCOMDemoClient frmDCOMDemoClient.Show End Sub 12. From the Project menu, click PicClient Properties. On the General tab, change Startup Object to Sub Main. On the Component tab, change Start Mode to Standalone. 13. Compile the Project into an executable. 14. Run PicClient.exe. Click Get Server icon. You should see the icon appear in the PictureBox. 15. In order to distribute the server project to another machine, please refer to the following articles in the Microsoft Knowledge Base: Q266717 HOWTO: Create a DCOM Client/Server Application by Using VB Q161837 HOWTO: Create a DCOM Client/Server Application in VB5 REFERENCES ========== For additional information, click the article numbers below to view the articles in the Microsoft Knowledge Base: Q183607 HOWTO: Configure DCOM for Visual Basic 5.0 Using DCOMCNFG.EXE Q266717 HOWTO: Create a DCOM Client/Server Application by Using Visual Basic Q267836 HOWTO: Create a DCOM Client/Server with Events by Using Visual Basic Q268550 HOWTO: Use Dcomcnfg for a Visual Basic DCOM Client/Server Application Q269330 HOWTO: Troubleshoot DCOM for Visual Basic Client/Server Applications Additional query words: callback marshaling ====================================================================== Keywords : kbcode kbVBp500 kbVBp600 kbGrpDSVBDB Technology : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVB500 kbVB600 Version : :5.0,6.0 Issue type : kbhowto ============================================================================= THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. 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