Force a COM server to remain open

Question

I have a COM automation server hosted by a VB6 exe.

The COM API is used from Excel VBA:

Dim o as MyCOMAPI.MyCOMType
Set o = new MyCOMAPI.MyCOMType
o.DoSomething

When I create objects in VBA the exe is started along with COM automation and VBA can use the API.

But the exe is closed quickly and "randomly" by Excel, I guess when Excel decides it doesn't need the COM API anymore.

This behaviour is causing random errors.

The simple solution is to start the exe before running the VBA code ; in this case all is working fine as the exe won't stop running until closed by the user.

Have you some information/documentation about the way Excel VBA manages calls to hosted APIs?

Is there a way to avoid this behaviour and have the exe kept open until the VBA code decides to stop it?

Answer 1

This would be the default behavior for a COM automation server when the last object is dereferenced, meaning that the variable that points to the server is set to nothing.

Now, if your code looks something like this today:

Sub MyFunction()
...
Dim o as MyCOMAPI.MyCOMType
Set o = new MyCOMAPI.MyCOMType
o.DoSomething
End Sub

Then, the server life is connected to the life of the o variable. That variable gets set to nothing when the function is finished, and then the server will be shut down (unless there are other variables keeping it alive).

To make sure that your COM server is kept alive for a longer time, simply define the variable as a Public variable as in the sample below.

This sample will start and show Excel and keep it open until the ShutdownExcel function is called.

Public o As Excel.Application

Sub MakeSureExcelIsRunning()
    If o Is Nothing Then
        Set o = New Excel.Application
        o.Visible = True
    End If
End Sub

Sub ShutdownExcel()
    Set o = Nothing
End Sub