WPF TextBlock binding deactivate & detach

Question

I have started seeing an unusual issue with some of the binding on my textblocks:

XAML

  <TextBlock Text="{Binding PartCount,FallbackValue=1, UpdateSourceTrigger=PropertyChanged, diag:PresentationTraceSources.TraceLevel=High}" Foreground="Black" Margin="-3"/>

I know for sure that my binding path resolution is correct because if I change this block to a label with content instead of text then it works just fine.

I have the diagnostic logger enabled for the binding and I can see it initialise correctly and set the initial value from its source:

DEBUG LOG

System.Windows.Data Warning: 67 : BindingExpression (hash=40999308): Resolving source (last chance) System.Windows.Data Warning: 70 : BindingExpression (hash=40999308): Found data context element: TextBlock (hash=5477191) (OK) System.Windows.Data Warning: 78 : BindingExpression (hash=40999308): Activate with root item LoadStack (hash=22395825) System.Windows.Data Warning: 107 : BindingExpression (hash=40999308): At level 0 using cached accessor for LoadStack.PartCount: RuntimePropertyInfo(PartCount) System.Windows.Data Warning: 104 : BindingExpression (hash=40999308): Replace item at level 0 with LoadStack (hash=22395825), using accessor RuntimePropertyInfo(PartCount) System.Windows.Data Warning: 101 : BindingExpression (hash=40999308): GetValue at level 0 from LoadStack (hash=22395825) using RuntimePropertyInfo(PartCount): 'Infinity' System.Windows.Data Warning: 80 : BindingExpression (hash=40999308): TransferValue - got raw value 'Infinity' System.Windows.Data Warning: 84 : BindingExpression (hash=40999308): TransferValue - implicit converter produced '∞' System.Windows.Data Warning: 89 : BindingExpression (hash=40999308): TransferValue - using final value '∞'

But then, when I navigate to the window that shows this text box I get this:

DEBUG LOG

System.Windows.Data Warning: 79 : BindingExpression (hash=40999308): Deactivate System.Windows.Data Warning: 103 : BindingExpression (hash=40999308): Replace item at level 0 with {NullDataItem} System.Windows.Data Warning: 63 : BindingExpression (hash=40999308): Detach

So after this point the visual element does not update, but I do still get information in the log when the source changes like so:

DEBUG LOG

System.Windows.Data Warning: 95 : BindingExpression (hash=53188122): Got PropertyChanged event from LoadStack (hash=22395825) System.Windows.Data Warning: 101 : BindingExpression (hash=53188122): GetValue at level 0 from LoadStack (hash=22395825) using RuntimePropertyInfo(PartCount): '50' System.Windows.Data Warning: 80 : BindingExpression (hash=53188122): TransferValue - got raw value '50' System.Windows.Data Warning: 84 : BindingExpression (hash=53188122): TransferValue - implicit converter produced '50' System.Windows.Data Warning: 89 : BindingExpression (hash=53188122): TransferValue - using final value '50'

So something is causing that "deactivate" & "detach" which means that it no longer updates the view. I have checked that the data context is consistent, which it is.

Is there anything that can cause the binding to respond in this way?

Remove this , UpdateSourceTrigger=PropertyChanged and never use it again until you understand what it does. — Andy
Show us your viewmodel. Does it implement inotifypropertychanged? What's your property look like. What exactly is it's initial value? What happens if you remove your fallbackvalue? — Andy

thorstenmz thorstenmz · Accepted Answer · 2021-03-18T19:44:20

Detachment behavior usually occurs when a value is set outside of the data binding.

The way to go is to check if you're setting TextBlock.Text somewhere else (e.g. in the code behind or in your XAML).

Another possibility is that your viewmodel instance gets replaced by another one.

WPF TextBlock binding deactivate & detach

1 Answers