WPF DataTemplate not throwing exception

Question

While designing a new WPF application I noticed exceptions not being thrown during data binding for controls with DataTemplates. To test I wrote the following simple user control with as little logic as possible. I'm using .NET 3.5 SP1, VS2008 SP1 running on WIN7.

When the DataContext is set, TestMethod is called and an exception is thrown in the codebehind but the application doesn't break.

Would someone mind explaining what is happening when the DataTemplate is being created? Specifically I'm interested in where the exception goes.

Below is the XAML code:

<UserControl x:Class="WPF2008.DataTemplateQuestion"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Height="300" Width="300">
    <UserControl.Resources>
        <DataTemplate x:Key="TESTKey">
            <Border BorderBrush="Black" BorderThickness="10">
                <Grid>
                    <Grid.RowDefinitions>
                        <RowDefinition></RowDefinition>
                        <RowDefinition></RowDefinition>
                        <RowDefinition></RowDefinition>
                        <RowDefinition></RowDefinition>
                    </Grid.RowDefinitions>
                    <TextBox Grid.Row="0" Text="{Binding Path=Txt}" />
                    <TextBox Grid.Row="1" Text="{Binding Path=Lbl}" />
                    <TextBox Grid.Row="2" Text="Am I disabled?" />
                    <TextBox Grid.Row="3" Text="I am disabled." IsEnabled="False" />
                </Grid>
            </Border>
        </DataTemplate>
    </UserControl.Resources>
    <Grid>
        <Label x:Name="lblTest" Content="{Binding Path=TestMethod}" ContentTemplate="{StaticResource TESTKey}" />
    </Grid>
</UserControl>

Below is the codebehind:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WPF2008
{
    public partial class DataTemplateQuestion : UserControl
    {
        public DataTemplateQuestion()
        {
            try
            {
                InitializeComponent();
                lblTest.DataContext = new TestClass();
            }
            catch (Exception e)
            {
                // ADDING A BREAKPOINT HERE DOESN'T CATCH THE EXCEPTION
                throw e; 
            }
        }
    }

    public class TestClass
    {
        public TestValue TestMethod
        {
            get
            {
                // BREAKPOINT WILL BE HIT WHEN DEBUGGING BUT THE EXCEPTION DISAPPEARS
                throw new Exception("WATCH ME DISAPPEAR");
                return new TestValue { Lbl = "Label", Txt = "Text" };
            }
        }

        public class TestValue
        {
            public string Lbl { get; set; }
            public string Txt { get; set; }
        }
    }
}

Answer 1

Exceptions get eaten by data bindings, and result in the data binding being disabled (I'm not 100% sure this is the case here, I'm too lazy to put it into VS).

Another possibility is that you may be hitting http://support.microsoft.com/kb/976038 if you are running a 64-bit OS.

Answer 2

Exceptions thrown by data bindings get translated into trace, which are delivered to the DataBindingSource TraceSource, which has a source name of "System.Windows.Data".

You can convert these into exceptions by creating a subclass of TraceListner that throws an exception on a trace, and add it to the Listeners collection of the TraceSourceSource. This can be done either in code or in your App.config file.

Here is how you would do it in code:

System.Diagnostics.PresentationTraceSources.DataBindingSource.Listeners.Add(
  new MyTraceListener());

See TraceSource and TraceListener documentation and samples for more details.