Saturday, July 3, 2010

Debugging databinding

WPF and Silverlight are great thanks to the rich databinding support. Important to notice is that late bound data binding is used bindings in XAML files. This feature allows a DataContext to be set at run-time and the objects within that DataContext to resolve their property bindings then. This late binding enables cool features like DataTemplates, composable applications and run-time loading of loose XAML.

A side effect of late bound binding resolution that databinding will fail silently if a value can not be found.  So troubleshooting why a specific field is not loaded in a control can be painfull. Therefore some tips to get some useful debugging information.

View the Debug Output Window

If you have the DataContext set, any data bindings within that DataContext that can't be resolved will be listed in the Debug Output Window at run-time.

WPF Trace Settings, Data Binding

The default value of the WPF Trace Settings, Data Binding is "Error." To get more data binding information change the level to "Warning". If the value is set to "Error" you won't see any of the PresentationTraceSources.TraceLevel warning messages.

To change the WPF Trace Settings, Data Binding value to "Warning," navigate to Tools, Options, Debugging, Output Window, WPF Trace Settings, Data Binding and set the value to Warning.

PresentationTraceSources.TraceLevel

Introduced to WPF in .NET 3.5 SP1, the PresentationTraceSources.TraceLevel attached property allows developers to dial up the amount of information outputted in the Debugger Output Window for a binding.

The below XAML snippet shows how to apply the PresentationTraceSources.TraceLevel attached property to a binding. 

   1:  <Window 
   2:      xmlns:diag="clr-namespace:System.Diagnostics;assembly=WindowsBase"
   3:      xmlns:local="clr-namespace:DebugDataBindings"
   4:      x:Class="DebugDataBindings.MainWindow"
   5:      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
   6:      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
   7:      Title="MainWindow" Height="350" Width="525">
   8:      <Grid x:Name="layoutRoot">
   9:          <Grid.Resources>
  10:              <local:DebugConverter x:Key="debugConverter" /> 
  11:          </Grid.Resources>
  12:          <TextBox 
  13:              Text="{Binding Path=Customer.FirstName, diag:PresentationTraceSources.TraceLevel=High}"   
  14:              Height="23" HorizontalAlignment="Left" Margin="90,18,0,0" VerticalAlignment="Top" Width="120"/>
  15:      </Grid>
  16:  </Window>

Still not enough information? Have a look at the following blog posts:


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