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It is important to only communicate with Excel when it is Ready. For example, if one displays a non modal dialog then the program may fail if it attempts to call a VBA routine in response to a button press while the user is also in the process of editing a cell. Intercepting Window Messages messages etc. can also produce errors even though they are on the main thread. And of course accessing Excel from a different thread is forbidden.

The best approach is to enqueue such work to execute on the main Excel thread when it is ready. Some support for initiating such cross-thread work is now (December 2012) implemented by Excel-DNA, and exposed as methods on the ExcelDna.Integration.ExcelAsyncUtil class.

To try it you need to
  1. call ExcelAsyncUtil.Initialize() in your AutoOpen().
  2. when you want Excel to do the work, call ExcelAsyncUtil.QueueAsMacro.

For example, this menu button starts a Task that takes a while, and
upon completion it updates cell B1 on Sheet1 using the C API (via an ExcelReference).

[ExcelCommand(MenuName="Async Test", MenuText="Run Later")] 
public static void RunLater() 
    Task.Factory.StartNew( () => Thread.Sleep(5000) ) 
    .ContinueWith(t => 
        ExcelAsyncUtil.QueueAsMacro(() => 
            var refB1 = new ExcelReference(0,0,1,1, "Sheet1"); 

Internally this is implemented by adding the QueueAsMacro delegate on a queue, and (normally) posting a WM_SYNCMACRO event. The delegate is then dequequed by a SyncMacro function that is run as an Excel xlfregister ed macro. (There are several functions called "SyncMacro" in the ExcelDNA, the one that is actually registered is in Exceldna.cpp.)

A new NativeWindow on Excel's main event loop traps WMSYNCMACRO and WMTIMER events and attempts to run SyncMacro on the main thread. If the attempt fails then the timer is reset for 250ms later. A test is performed to ensure SyncMacro is not run while the user is editing a formula. (The same NativeWindow is also used for RTD processing.)

Last edited Nov 25, 2013 at 5:25 AM by aberglas, version 9