Why is an exception being raised when I mark a RoutedEventArgs event as Handled?

Question

I am trying to mark an event as handled but an exception gets thrown when I try doing so.

I have the following function that gets called on the LostFocus event of a TextBox:

private void TextBox_LostFocus(object sender, RoutedEventArgs e)
{
   // ...
   // do something
   // ...

   e.Handled = true;
}

However, the following exception gets raised as soon as e.Handled = true is executed:

An unhandled exception of type 'System.InvalidOperationException' occurred in PresentationCore.dll

Additional information: Every RoutedEventArgs must have a non-null RoutedEvent associated with it.

I have tried moving the e.Handled line before the other code in the function but that doesn't help.

Why is the exception being raised and what are some possible workarounds?

Answer 1

Figured it out. TextBox_LostFocus was being called from elsewhere in the application with the event parameter being null.

Hence, for some function calls to TextBox_LostFocus, e.Handled didn't work (since e was null) but worked in some other scenarios (when it was actually called by the framework).