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
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The information in this article applies to:
- Microsoft Visual Basic Enterprise Edition for Windows, versions 5.0, 6.0
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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
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Keywords : kbcode kbVBp500 kbVBp600 kbGrpDSVBDB
Technology : kbVBSearch kbAudDeveloper kbZNotKeyword6 kbZNotKeyword2 kbVB500Search kbVB600Search kbVB500 kbVB600
Version : :5.0,6.0
Issue type : kbhowto
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