ADT: How to Direct DDE to a Specific Instance of a Server

• Microsoft Access versions 1.0, 1.1, 2.0
• Microsoft Access Distribution Kit version 1.1
• Microsoft Access Developer's Toolkit version 2.0

SUMMARY

Advanced: Requires expert coding, interoperability, and multiuser skills.

You can run multiple instances of the same application under Microsoft Windows. When you communicate with an application using DDE, it may be important to be able to identify which instance of an application should respond to your DDE commands.

You can initiate a DDE conversation with a specific instance of an application by appending the application's Task ID number to the application name argument in the DDE initiate command.

This article assumes that you are familiar with Access Basic and with creating Microsoft Access applications using the programming tools provided with Microsoft Access. For more information about Access Basic, please refer to the "Introduction to Programming" manual in Microsoft Access version 1.x, or the "Building Applications" manual, Chapter 3, "Introducing Access Basic" in version 2.0.

MORE INFORMATION

Each instance of each application started in Windows is assigned a unique number called a Task ID. This number can be used to distinguish among multiple instances of an application.

To initiate a DDE conversation using the System topic with an instance of Microsoft Excel that has a Task ID of 34567, you could use the following sample command:

   Chan = DDEInitiate("excel34567", "System")

How to Get the Task ID of an Application

You can get the Task ID of an application by doing one of the following:

• Use the Access Basic shell command to launch the application. The shell command will return the application's Task ID if the application is launched successfully.

  -or-

• Use an Access Basic function to find the Task ID. The following sample function can be used to find an application's Task ID:

Notes

• In the following sample code, an underscore (_) is used as a line- continuation character. Remove the underscore from the end of the line when re-creating this code in Access Basic.
• You may have some Microsoft Windows API functions defined in an existing Microsoft Access library; therefore, your declarations may be duplicates. If you receive a duplicate procedure name error message, remove or comment out the declarations statement in your code.

        Option Explicit
  

        Declare Function GetActiveWindow Lib "User" () As Integer
        Declare Function GetWindowWORD Lib "User" (ByVal hWnd As Integer, _
           ByVal nIndex As Integer) As Integer
        Const GWW_HINSTANCE = (-6)
  
        Function GetActiveTaskID ()
           GetActiveTaskID = GetWindowWORD(GetActiveWindow(), _
              GWW_HINSTANCE)
        End Function
  
  

   Function TestTaskID()
      Dim TaskID
      TaskID = Shell("excel", 1)
      MsgBox TaskID & " and " & GetActiveTaskID()
   End Function

How to Use an Application's Task ID in a DDE Conversation

The following example demonstrates how to use Task IDs to indicate which instance of an application should be used in a DDE conversation. This sample code will launch two instances of Microsoft Excel, start a DDE conversation with each instance, and poke data to Sheet1 of each instance:

   Function DDEWithTwoInstances ()
      Dim TaskID1, TaskID2
      Dim Chan1, Chan2

      ' Start 2 instances of Microsoft Excel and record Task ID numbers.
      TaskID1 = Shell("excel.exe", 1)
      TaskID2 = Shell("excel.exe", 1)

      ' Initiate DDE conversations with both instances using
      ' the Task ID numbers. Trap an error that may happen
      ' if we try to initiate a conversation and the copy of
      ' Microsoft Excel is still trying to start from the shell command.

      On Error GoTo ExcelNotYetReady
      Chan1 = DDEInitiate("excel" & TaskID1, "Sheet1")
      On Error GoTo 0

      On Error GoTo ExcelNotYetReady
      Chan2 = DDEInitiate("excel" & TaskID2, "Sheet1")
      On Error GoTo 0

      ' Poke information into each instance of Microsoft Excel.

      DDEPoke Chan1, "R1C1", "This is Task1!"
      DDEPoke Chan2, "R1C1", "This is Task2!"

      DDETerminateAll

      Exit Function

   ExcelNotYetReady:
      DoEvents
      Resume

   End Function

REFERENCES

Microsoft Access "Introduction to Programming," version 1.1, pages 139-147