Timing problem in WPF data binding in styles

[heckl]

Description

I wanted to have data validation at once in WPF. At the same time I wanted to detect LostFocus, this is the time I modify the DB. Moreover, I needed this in a DataGrid. I introduced a temp property. I use the PropertyChanged at the TempName so the validation happens at once and used the event handler at LostFocus. The event handler copied the TempName to the Name and triggered the DB change. This method worked.

I wanted to use Behavior instead of event handler.

FinalizeNameCommand copies the TempName to the Name. Visual Studio says there is a Binding problem at FinalizeNameCommand though the binding works.

The binding error says the DataContext is null, it should be the object behind the selected row, Person. FinalizeNameCommand is in that object and it is executed as expected. It seems the Binding is evaulated before the DataContext is set, that is why an error is shown. Later the DataContext is set and the binding is reevaulated (?), that is why FinalizeNameCommand works.

The problem is not only a wrong error message. This causes other problems in the code later.

Reproduction Steps

https://www.dropbox.com/scl/fi/pugbf1mxo72yjv64sqt8x/ImmediateValidationBindingTest.zip?rlkey=cflx26ayz6kusz3vino1mf90l&dl=0

Download the solution. Run the program See the binding error Modify a name and see that FinalizeNameCommand is executed

Expected behavior

There should be no binding error

Actual behavior

There is binding error

Regression?

No response

Known Workarounds

use events instead of behavior

Impact

No response

Configuration

.net8 windows x64

Other information

No response