如何将绑定中的FallbackValue设置为外部图像文件的路径？

Question

I'm trying to set FallbackValue in case when my converter cannot be call, but I'm not sure how to do that.

<Image Source="{Binding FallbackValue="Pictures/Unknown.png", Path=LatestPosition.DeviceFamily, Converter={x:Static conv:ConverterSet.DeviceTypeToImageSourceconverter}}" Name="image1" Stretch="Fill" Margin="5,8" Width="150" Height="150" Grid.RowSpan="4" />

Paths of external images in converter looks like that and when LatestPosition!=null the image is set in proper way.

private static readonly ImageSource Dev1 = new BitmapImage(new Uri("/Pictures/dev1.png", UriKind.Relative));
private static readonly ImageSource Dev2 = new BitmapImage(new Uri("/Pictures/dev2.png", UriKind.Relative));

Answer 1

For the Image control, when you Binding the Source property with a URI string, it will automatically convert the URI to a BitmapImage. But if you set the FallbackValue and TargetNullValue as URI string, it will not display.

You need to set it as BitmapImage:

<Window.Resources>
    <BitmapImage x:Key="DefaultImage" UriSource="/Resources;component/Images/Default.jpg" />
</Window.Resources>

<Image Width="128"
               Height="128"
               HorizontalAlignment="Left"
               VerticalAlignment="Top"
               Source="{Binding Photo,FallbackValue={StaticResource DefaultImage},
                                TargetNullValue={StaticResource DefaultImage}}" />

As we set the FallbackValue and the TargetNullValue as StaticResource of BitmapImage, It works.

Answer 2

For myself, I created the following example:

<!-- xmlns:sys="clr-namespace:System;assembly=mscorlib" -->

<Window.Resources>
    <!-- Test data -->
    <local:TestDataForImage x:Key="MyTestData" />

    <!-- Image for FallbackValue -->
    <sys:String x:Key="ErrorImage">pack://application:,,,/NotFound.png</sys:String>

    <!-- Image for NULL value -->
    <sys:String x:Key="NullImage">pack://application:,,,/NullImage.png</sys:String>
</Window.Resources>

<!-- Grid using DataContext -->
<Grid DataContext="{StaticResource MyTestData}">
    <Image Name="ImageNull" Width="100" Height="100" HorizontalAlignment="Left" VerticalAlignment="Bottom" Source="{Binding Path=NullString, TargetNullValue={StaticResource NullImage}}" />

    <Image Name="ImageNotFound" Width="100" Height="100" HorizontalAlignment="Left" VerticalAlignment="Top" Source="{Binding Path=NotFoundString, FallbackValue={StaticResource ErrorImage}}" />
</Grid>

The path to the images, I announced in resources. Images are stored in the root of the project. Listing of MyTestData :

public class TestDataForImage : DependencyObject
{
    public string NotFoundString
    {
        get
        {
            return (string)GetValue(NotFoundStringProperty);
        }

        set
        {
            SetValue(NotFoundStringProperty, value);
        }
    }

    public static readonly DependencyProperty NotFoundStringProperty = DependencyProperty.Register("NotFoundString", typeof(string), typeof(TestDataForImage), new PropertyMetadata(""));

    public string NullString
    {
        get
        {
            return (string)GetValue(NullStringProperty);
        }

        set
        {
            SetValue(NullStringProperty, value);
        }
    }

    public static readonly DependencyProperty NullStringProperty = DependencyProperty.Register("NullString", typeof(string), typeof(TestDataForImage), new PropertyMetadata(""));

    public TestDataForImage()
    {
        NotFoundString = "pack://application:,,,/NotExistingImage.png";
        NullString = null;
    }
}