Friday, 8 June 2012

ExpressionTextBox in .NET 4.5

The ExpressionTextBox allows you to databind to an argument on your activity and has, up until 4.5, had an optional OwnerActivity attribute which seemed to be optional. Whilst all activities in the framework used it, some custom activities might have omitted it as it worked in 4.0 without needing to be explicitly defined.

Now with .NET 4.5 this attribute is no longer optional – check out what I get if I forget to add it (the 3rd textbox is the one with the missing attribute, and I’m in a C# project)…


Not only do you get the horrific “Enter a VB expression” text, which sends a chill down my spine each time I see it, you’ll also find that the textbox is read-only if you omit the OwnerActivity attribute. It’s easy to fix though, the following highlighted XAML is all you need to change…

Expression="{Binding ModelItem.Body, Converter={StaticResource conv}}"
OwnerActivity="{Binding ModelItem}" .../>

This change in behaviour might be a problem for anyone upgrading their solutions to 4.5, so hopefully someone will find this post useful.