Exception throw in WorkflowViewElement

Question

In our WPF application we are using WorkFlowDesigner. Currently we are facing an issue with WorkflowDesigner and WorkflowViewElement. When the user switch views some times it's throws a null reference exception. Based on our analysis it is found that Loaded and UnLoaded event of WorkflowView element is executed when the view with Work flow is loaded and unloaded respectively. When these events are triggered once the previous event execution is completed then there is no issues. But when Unloaded event is triggered before the execution of loaded event this exception is thrown. We can't find any flags or properties to identify if the loaded or Unloaded event is completed or not, so that we can await for it to complete. Please suggest a fix for this issue. We are using .NET Framework 4.6.1

Answer 1

In your WPF application using the WorkflowDesigner, the issue you're experiencing with the NullReferenceException during the switching of views may be related to the timing of the Loaded and Unloaded events. These events are raised in a specific order: the Unloaded event is triggered on an element when it is removed from its visual parent or presentation source, and the Loaded event is raised when all elements in the logical tree are loaded.

Since you mentioned that the Unloaded event can be triggered before the Loaded event completes, this can lead to a situation where the state of the application is inconsistent, resulting in a NullReferenceException. Unfortunately, there are no built-in flags or properties in WPF to directly check if the Loaded or Unloaded event has completed.

To mitigate this issue, consider implementing a mechanism to manage the state during these transitions. You could use a boolean flag to indicate whether the Loaded event is currently being processed. Set this flag to true at the beginning of your Loaded event handler and to false at the end. In your Unloaded event handler, check this flag before proceeding with any operations that depend on the Loaded event being completed. This way, you can prevent executing code that assumes the Loaded event has finished when it has not.

Here's a simple example of how you might implement this:

private bool isLoading = false;

private void WorkflowViewElement_Loaded(object sender, RoutedEventArgs e)
{
    isLoading = true;
    // Your loaded logic here
    isLoading = false;
}

private void WorkflowViewElement_Unloaded(object sender, RoutedEventArgs e)
{
    if (!isLoading)
    {
        // Your unloaded logic here
    }
}

This approach should help you avoid the NullReferenceException by ensuring that the Unloaded logic only executes when the Loaded event has completed its execution.

Answer 2

Thanks for reaching out!

This is due to the asynchronous nature of WPF’s visual tree updates. The WorkflowViewElement control in the WorkflowDesigner framework does not have built-in synchronization between its load and unload operations, which can result in a race condition when views are changed rapidly.

Recommended Solution:

• Implement a mechanism to track whether the view is in a valid “loaded” state before performing workflow-related operations.
• Ensure that cleanup or disposal actions only occur once the Loaded event has fully completed.
• Use deferred execution or dispatching on the UI thread to prevent overlapping event execution.
• Introduce a small delay or debounce when switching between workflow views to allow the current view to unload completely before a new one loads.
• If feasible, upgrade to .NET Framework 4.8 or .NET 6 (Windows), where some lifecycle timing behaviors in WorkflowDesigner have been improved. Avoid immediate switching between views by using a short debounce/delay before reloading a workflow view. Alternative Option: If possible, upgrade to .NET Framework 4.8 or .NET 6+ (Windows) — some timing-related issues with WorkflowDesigner lifecycle handling were improved in later frameworks. Reference: WorkflowViewElement Class (System.Activities.Presentation) | Microsoft Learn

Please let us know if you require any further assistance, we’re happy to help.

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