Question

In the image below, why is FindAncestor needed and why is AncestorType not UserControl?

If the UserControl was inside a StackPanel, would AncestorType be StackPanel?

In other words, what does the parent control have to do with binding to the viewmodel.Message? Why is all the other xaml needed in the Binding?

Answer 1

It wouldn’t need to specify AncestorType=UserControl, since that’s the default context. If you omit the “RelativeSource” parameter, it will bind to the local DataContext.

In this case, the UserControl and the Window it is contained in must have different DataContexts. Since a UserControl generally doesn’t know the name of its parent at design time, you can at the least usually assume it’s only going to be in a single Window and create a binding to the Window’s DataContext using RelativeSource as shown.

This can be common because in an application, the Window might have a DataContext for information about the application as a whole, but the current UserControl might be focused on a particular piece of data (such as an Employee). In this case all the default bindings point to the Employee, but the UserControl might want to bind to something in the parent DataContext too.