28 September 2017

CreateObject caching (.NET example)

Thanks to a little experiment using .NET classes from GB, I discovered a bug in the caching of dispids for IDispatch objects created using the CreateObject() function. (An update of the OCX runtime is available.) For demonstration purposes I’ll show you code accessing .NET classes I used to test caching. Caching is only available for objects created by CreateObject. I discussed the peculiarities of CreateObject in an earlier blog and if you need to freshen your memory you might want to check it out.

CreateObject and caching
The CreateObject function not only differs from VB/VBScript in creating a COM class and connecting to the IID_IDISPATCH interface, but it also sets up a Hash table to store the dispatch identifiers of the properties and method names. Each(!) object created from CreateObject is encapsulated in another hidden IDispatch object. The Object returned by CreateObject is a pointer to this hidden GB-provided COM object. The hidden object implements all 6 IDispatch functions, but only provides custom functions for GetIDsOfNames and Release. The other four (QueryInterface, Add, GetTypeInfoCount, and GetTypeInfo) are simply routed to the automation object directly. The hidden object provides the caching.
Only objects created with CreateObject are affected, that is if the class is not a GFABASIC32 Ocx, StdFont, or StdPicture type. When these terms are met, the CreateObject function creates an empty hash table and stores it in the encapsulating IDispatch object. When a property or method is executed for the first time, the identifier of the name is looked up using IDispatch.GetIDsOfNames function and saved in the hash table for future use.

Why caching is useful
The compiler converts the execution of a  property or method into a call to a runtime-library function which performs the actual execution of the property/method. The property/method is directed to the IDispatch.Invoke function. However Invoke does not accept named properties/methods, but only numeric values identifying the property or method. This integer value is called dispid. (You can retrieve a dispid yourself using the _DispId() function.) Before Invoke is executed the dispid value associated with the property/method name must the obtained by calling IDispatch.GetIDsOfNames function. Calling GetIDsOfNames each time before calling Invoke, decreases performance. This is especially true for out-of-process servers; non-DLL servers, DLLs are in-process servers, loaded into the process memory. Communicating with an out-of-process server is time consuming. Even more when the communication requires two steps, obtaining the dispid and executing the function. Once a property or method is used and its disp-ID value is known, it is logical to store the ID for future use in an easy accessible Hash table.

The encapsulating object
All IDispatch properties and methods are executed by the same runtime-library function, which is unaware of cached IDs. The GetIDsOfNames function is executed always before calling Invoke. However, the hidden Object that encapsulates the automation object, supports a custom GetIDsOfNames and first searches the hash table for the name. If it finds the name it has the ID value available directly. If the hash element isn’t found, it calls GetIDsOfNames on the external COM object and then stores the ID in the hash. When the Object goes out of scope, the custom Release function erases the hash table and the encapsulating object is freed from memory.

Example using .NET
The following code creates a .NET classes that support an IDispatch interface. The classes used are located in the mscorlib.dll, which is loaded into the process-space of the GB-application. The performance gain measured is about 15%. It will be better for out-of-process servers, the example is simply as a test for caching.

' ArrayList coclass supports multiple dual intefaces:
'  IList, ICloneable, _Object, ICollection, IEnumerable
' Togeher their exposed properties/methods make-up
' the ArrayList's class interface.
Dim oArrList As Object, v As Variant, i As Int
Set oArrList = CreateObject("System.Collections.Arraylist")
' IList
oArrList.add 3                ' IList.Add
oArrList.add 4
oArrList.add 4
Trace oArrList(0)             ' IList.Item(index)
Trace oArrList.Contains(3)    ' ILIst.Contains
' ICloneable
v = oArrList.Clone            ' ICloneable.Clone
' v is copy of ArrayList:
Trace v
' _Object interface
Trace v.ToString              ' _Object.ToString
Trace v.GetHashCode           ' _Object.GetHashCode
Set v = Nothing
' ICollection
Trace oArrList.Count          ' ICollection.Count
Trace oArrList.SyncRoot       ' ICollection.SyncRoot
' IEnumerable
For Each v In oArrList
  Trace v

' Queue collection
Dim myQ As Object
Set myQ = CreateObject("System.Collections.Queue")
myQ.Enqueue "Hello"
myQ.Enqueue "World"
myQ.Enqueue "!"

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