I use Addin in VS2008, C#, and I need show messages (error messages and others).
I don\'t know the length of messages, and therefore I want use Scrollable MessageBox.
I was looking for a scrollable messagebox for WPF - and then I found MaterialMessageBox, which is an (in my opinion) nice looking alternative to FlexibleMessageBox for WPF. You can download it from GitHub here: https://github.com/denpalrius/Material-Message-Box or install it as a nuget package inside Visual Studio (https://www.nuget.org/packages/MaterialMessageBox/).
The only problem I found was that you can't use it from outside the main thread, so my solution to that was to invoke the main thread and show the message from there:
mainWindow.Dispatcher.Invoke(() => MaterialMessageBox.Show("message text"));