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How to: Convert Bound Data

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This example shows how to apply conversion to data that is used in bindings.

To convert data during binding, you must create a class that implements the IValueConverter interface, which includes the Convert and ConvertBack methods.

Example

The following example shows the implementation of a date converter that converts the date value passed in so that it only shows the year, the month, and the day. When implementing the IValueConverter interface, it is a good practice to decorate the implementation with a ValueConversionAttribute attribute to indicate to development tools the data types involved in the conversion, as in the following example:

[ValueConversion(typeof(DateTime), typeof(String))]
public class DateConverter : IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        DateTime date = (DateTime)value;
        return date.ToShortDateString();
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        string strValue = value as string;
        DateTime resultDateTime;
        if (DateTime.TryParse(strValue, out resultDateTime))
        {
            return resultDateTime;
        }
        return DependencyProperty.UnsetValue;
    }
}

Public Class DateConverter
    Implements System.Windows.Data.IValueConverter

    Public Function Convert(ByVal value As Object,
                            ByVal targetType As System.Type,
                            ByVal parameter As Object,
                            ByVal culture As System.Globalization.CultureInfo) _
             As Object Implements System.Windows.Data.IValueConverter.Convert

        Dim DateValue As DateTime = CType(value, DateTime)
        Return DateValue.ToShortDateString

    End Function

    Public Function ConvertBack(ByVal value As Object,
                                ByVal targetType As System.Type,
                                ByVal parameter As Object,
                                ByVal culture As System.Globalization.CultureInfo) _
            As Object Implements System.Windows.Data.IValueConverter.ConvertBack

        Dim strValue As String = value
        Dim resultDateTime As DateTime
        If DateTime.TryParse(strValue, resultDateTime) Then
            Return resultDateTime
        End If
        Return DependencyProperty.UnsetValue

    End Function
End Class

Once you have created a converter, you can add it as a resource in your Extensible Application Markup Language (XAML) file. In the following example, src maps to the namespace in which DateConverter is defined.

<src:DateConverter x:Key="dateConverter"/>

Finally, you can use the converter in your binding using the following syntax. In the following example, the text content of the TextBlock is bound to StartDate, which is a property of an external data source.

<TextBlock Grid.Row="2" Grid.Column="0" Margin="0,0,8,0"
           Name="startDateTitle"
           Style="{StaticResource smallTitleStyle}">Start Date:</TextBlock>
<TextBlock Name="StartDateDTKey" Grid.Row="2" Grid.Column="1" 
    Text="{Binding Path=StartDate, Converter={StaticResource dateConverter}}" 
    Style="{StaticResource textStyleTextBlock}"/>

The style resources referenced in the above example are defined in a resource section not shown in this topic.

See also