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

  • call ExcelAsyncUtil.Initialize() in your AutoOpen().
  • 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")](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");
            refB1.SetValue("Done!");
        }));
}

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 Excel-DNA, the one that is actually registered is in exceldna.cpp.)

A new NativeWindow on Excel’s main event loop traps WM_SYNCMACRO and WM_TIMER 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.)