Windows Workflow: “instance operation is not valid on workflow runtime thread” or “EventDeliveryFailedException: MessageQueueErrorCode QueueNotFound”

When you’ve created your ExternalDataExchange interface and class, remember, when you have a CallExternalMethod activity, to invoke the external method in a separate thread.

So if you have InvokeSendPreview that you want to invoke from a WF, then ensure that you encapsulate the call to the event inside a new thread, for instance, via the ThreadPool, as follows.

[Serializable]
public class CommunicationService : ICommunicationService
{
    #region ICommunicationService Members

    public event EventHandler<ExternalDataEventArgs> SendPreview;
    
    public void InvokeSendPreview()
    {
        Guid instanceId = WorkflowEnvironment.WorkflowInstanceId;
        ThreadPool.QueueUserWorkItem(delegate(object state)
        {

            SendPreview(null, new ExternalDataEventArgs(instanceId)); 
    
        });
    }

    #endregion
}

The error at the beginning of this post was caused by

public void InvokeSendPreview()
{
    Guid instanceId = WorkflowEnvironment.WorkflowInstanceId;
    SendPreview(null, new ExternalDataEventArgs(instanceId));  // ERROR HERE!   
}

This error is because the WF threads are not to be used to execute code outside WF.