FIX: Nesting OLE Automation Calls Causes GP Fault

The information in this article applies to:

- Standard and Professional Editions of Microsoft Visual Basic for

  Windows, version 3.0

SYMPTOMS

Nesting multiple OLE Automation property accesses and/or method invocations within the same procedure may lead to temporary memory loss or a general protection (GP) fault.

CAUSE

When Visual Basic makes the cross process calls to perform the property access or method invocation it does not release the temporary space allocated until the procedure completes execution.

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This problem has been corrected in Visual Basic version 4.0.

MORE INFORMATION

Multiple IDispatch Calls Use Up Memory

In order to access a property or execute a method on a OLE Automation object, automation controllers (such as Visual Basic) make IDispatch calls to the server application. Each time a IDispatch call is made, Visual Basic creates a temporary variable referencing the object. Visual Basic version 3.0 does not release the memory for these temporary variables until the procedure in which the property access or method invocation resides completes execution.

Therefore, if you have multiple property accesses or method invocations within the same procedure, the amount of free memory steadily decreases until the procedure completes execution. Nesting multiple OLE Automation statements that require IDispatch calls within the same procedure can produce a GP fault in module VBOA300.DLL at 0001:0D03.

It could be useful to estimate the number of IDispatch calls Visual Basic makes. The number of IDispatch calls Visual Basic executes for an individual statement is directly related to the number of properties or methods that are combined by using the dot notation to perform the statement. Thus a good rule of thumb is to count the number of "dots" in the statement. For example, the following statement makes two IDispatch calls:

   oXLSheet.Range("A1").Value = "Price"

The following statement makes five IDispatch calls - three explicit "dots" and two implicit calls to the Item method:

   oXLApp.Workbooks(1).Sheets(1).Range("A1").Value = "Price"

Steps to Reproduce Problem

1. Start Microsoft Excel. Then Start a new project in Visual Basic. Form1 is created by default.

2. Add a command button (Command1) to Form1.

3. Add the following code to the Command1_Click event procedure:

      Sub Command1_Click ()
         Dim ExcelSheet As Object
         Dim StartCell As Object
         Dim EndCell As Object
         Dim I as Integer
         Set ExcelSheet = CreateObject("excel.sheet.5")
         ' The following For Next loop makes 4000 IDispatch calls:
         For i = 1 to 1000
            Set StartCell=ExcelSheet.Cells(i,1)
            Set EndCell=ExcelSheet.Cells(i,5)
            ExcelSheet.Range(StartCell,EndCell).FormulaArray = "=3"
         Next
         ExcelSheet.Application.Quit
         Set ExcelSheet = Nothing
      End Sub
   
   

4. Press the F5 key to run program. Then click the command button.