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How to debug data binding problems on Windows Phone

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[[Category:Windows Phone]][[Category:Silverlight]][[Category:Debugging]][[Category:Files/Data]]
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{{Abstract|This article explains how to debug data binding issues by inspecting the elements before they are due to be displayed by the control.}}
 
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|signing= <!-- Signing requirements - empty or one of: Self-Signed, DevCert, Manufacturer -->
 
|capabilities= <!-- Capabilities required by the article/code example (e.g. Location, NetworkServices. -->
 
|capabilities= <!-- Capabilities required by the article/code example (e.g. Location, NetworkServices. -->
|keywords= IValueConverter
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|keywords= IValueConverter, data binding, xaml
 
|language= <!-- Language category code for non-English topics - e.g. Lang-Chinese -->
 
|language= <!-- Language category code for non-English topics - e.g. Lang-Chinese -->
 
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Revision as of 16:53, 18 October 2012

This article explains how to debug data binding issues by inspecting the elements before they are due to be displayed by the control.

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Article Metadata
Compatibility
Platform(s): Windows Phone 7.5
Windows Phone 8
Windows Phone 7.5
Article
Keywords: IValueConverter, data binding, xaml
Created: influencer (12 Oct 2012)
Last edited: influencer (18 Oct 2012)

Contents

Introduction

Data binding allows you to link a data source defined in code to a UI description in XAML (target). While defining a binding is quite straightforward, if something goes wrong it is useful to be able to tell whether the problem is in your source data or in the binding definition.

The easiest way to do this is to create a custom value converter (IValueConverter), which will be called when the data from the binding source arrives - before it gets delivered to the control. By setting a breakpoint in the converter you can confirm that the source is sending the expected data and contains the correct properties.

Tip.pngTip: If the converter doesn't get called then this is a sign that the binding has failed earlier than expected, perhaps in the definition of the source property or an incorrect DataContext. If a wrong property name in the binding declaration is given a message in the debug output window of VS appears.

Worked example

First, consider the case where you have a ListBox definition in XAML as shown below:

<ListBox x:Name="MainListBox" Margin="0,0,-12,0" ItemsSource="{Binding Items}" SelectionChanged="MainListBox_SelectionChanged">
<ListBox.ItemTemplate>
<DataTemplate>
<StackPanel Margin="0,0,0,17" Width="432" Height="78">
<TextBlock Text="{Binding LineOne}" TextWrapping="Wrap" Style="{StaticResource PhoneTextExtraLargeStyle}"/>
<TextBlock Text="{Binding LineTwo}" TextWrapping="Wrap" Margin="12,-6,12,0" Style="{StaticResource PhoneTextSubtleStyle}"/>
</StackPanel>
</DataTemplate>
</ListBox.ItemTemplate>
</ListBox>

To implement the converter you first have to derive a class from IValueConverter and implement it's methods:

public class DatabindingDebugConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, string language)
{
return value;
}
 
public object ConvertBack(object value, Type targetType, object parameter, string language)
{
return value;
}
}

Then you have to declare the converter as a static resource in your page's XAML:

<phone:PhoneApplicationPage.Resources>
<local:DataBindingDebugConverter x:Key="DatabindingDebugConverter"/>
</phone:PhoneApplicationPage.Resources>

You also have to change the XAML data binding statement to call the converter for a particular binding:

<TextBlock Text="{Binding LineOne, Converter={StaticResource DataBindingDebugConverter}}" 
TextWrapping="Wrap" Style="{StaticResource PhoneTextExtraLargeStyle}"/>

Note that this assumes you have declared the local namespace in the page tag:

xmlns:local="clr-namespace:EncryptionSample"

Next you set a breakpoint in the Convert() method and voilà, you can see the values that arrive from binding, if any.

Sample code

The sample code for the How to encrypt your application data in Windows Phone article uses this technique.

Version Hint

Windows Phone: [[Category:Windows Phone]]
[[Category:Windows Phone 7.5]]
[[Category:Windows Phone 8]]

Nokia Asha: [[Category:Nokia Asha]]
[[Category:Nokia Asha Platform 1.0]]

Series 40: [[Category:Series 40]]
[[Category:Series 40 1st Edition]] [[Category:Series 40 2nd Edition]]
[[Category:Series 40 3rd Edition (initial release)]] [[Category:Series 40 3rd Edition FP1]] [[Category:Series 40 3rd Edition FP2]]
[[Category:Series 40 5th Edition (initial release)]] [[Category:Series 40 5th Edition FP1]]
[[Category:Series 40 6th Edition (initial release)]] [[Category:Series 40 6th Edition FP1]] [[Category:Series 40 Developer Platform 1.0]] [[Category:Series 40 Developer Platform 1.1]] [[Category:Series 40 Developer Platform 2.0]]

Symbian: [[Category:Symbian]]
[[Category:S60 1st Edition]] [[Category:S60 2nd Edition (initial release)]] [[Category:S60 2nd Edition FP1]] [[Category:S60 2nd Edition FP2]] [[Category:S60 2nd Edition FP3]]
[[Category:S60 3rd Edition (initial release)]] [[Category:S60 3rd Edition FP1]] [[Category:S60 3rd Edition FP2]]
[[Category:S60 5th Edition]]
[[Category:Symbian^3]] [[Category:Symbian Anna]] [[Category:Nokia Belle]]

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