Knowledge Base

How To Pass a Picture Object Remotely Using COM

Article ID: 180714

Article Last Modified on 7/2/2004

APPLIES TO

Microsoft Visual Basic 5.0 Enterprise Edition
Microsoft Visual Basic 6.0 Enterprise Edition

This article was previously published under Q180714

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:

Start a new ActiveX EXE project. Class1 is added by default.
From Project menu, click Project Properties, and change the Project Name to PicServer.
In the Name property of the Class1 module, change the name to clsPassPicture.

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

From Project menu, click Add Form to add a new form, and change the name and caption of the Form to MyForm.
Add an Image control to MyForm, Image1 is added by default. Set the Picture property to a .bmp file on your computer.
Compile the Project into an executable file.

The Client Project:

Start a new ActiveX EXE project. Class1 is added by default.
From Project menu, click Project Properties, and change the Project Name to PicClient.
From the Project menu, click References, and then click PicServer.
In the Name property of the Class1 module, change the name to clsReceivePicture.

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

From Project menu, click Add Form to add a new form, and change the name and caption of the Form to frmDCOMDemoClient.
Add a Command button to frmDCOMDemoClient. Change its name property to cmdGetObject, and change its Caption property to Get Server Icon.
Add a PictureBox control to frmDCOMDemoClient, and change its Name property to picPassIcon.

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

From the Project menu, click Add Module. Module1 is added by default.

Paste the following code into Module1's code section:

Option Explicit

Sub Main()
  Load frmDCOMDemoClient
  frmDCOMDemoClient.Show
End Sub

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.
Compile the Project into an executable.
Run PicClient.exe. Click Get Server icon. You should see the icon appear in the PictureBox.
In order to distribute the server project to another machine, please refer to the following articles in the Microsoft Knowledge Base:

266717 How To Create a DCOM Client/Server Application by Using VB

161837 How To 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:

183607 How To Configure DCOM for Visual Basic 5.0 Using DCOMCNFG.EXE

266717 How To Create a DCOM Client/Server Application by Using Visual Basic

267836 How To Create a DCOM Client/Server with Events by Using Visual Basic

268550 How To Use Dcomcnfg for a Visual Basic DCOM Client/Server Application

269330 How To Troubleshoot DCOM for Visual Basic Client/Server Applications

Additional query words: callback marshaling

Keywords: kbhowto kbcode KB180714

Microsoft KB Archive/180714

Contents