InvalidOperationException: dispatcher processing has been suspended, but messages are still being processed

Question

We are having multiple issues with this exception but I cannot find technical documentation about the real cause of the issue, all the possible sources for this error and what should we avoid to avoid having the exception.

I have read the following:

The dispatcher processing is suspended to avoid reentrancy problems when updating the visual tree.

But I'm unsure about what means 'updating the visual tree' and what causes a message to be sent to the Dispatcher and reproduces the issue.

The following sample code reproduces the issue:

XAML

<Window x:Class="SuspendedPOF.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" 
        Height="350" 
        Width="525">
    <StackPanel>
        <Button Content="1" x:Name="Button1"  IsVisibleChanged="Button1_OnIsVisibleChanged" />
    </StackPanel>
</Window>

C# Code

using System.Windows;

namespace SuspendedPOF
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private void Button1_OnIsVisibleChanged(object sender, DependencyPropertyChangedEventArgs e)
        {
            MessageBox.Show("Hello");
        }
    }
}

The MessageBox call is not the only one that fires this exception, things like Focus sometimes are problematic as well.

Any help would be great.

Answer 1

As far as I know there is no way to know when you are handling an event that can modify the visual state of the control nor there is a way knowing if your actions will modify the visual state of the UI so it is just a matter of putting BeginInvoke() calls whenever you find a crash ... :S