.net – ImageSourceConverter error for Source=null

exception handlingimageivalueconverternetwpf

I'm binding the Source property of an Image to a string. This string may be null in which case I just don't want to display an Image. However, I'm getting the following in my Debug output:

System.Windows.Data Error: 23 : Cannot
convert '<null>' from type '<null>' to
type
'System.Windows.Media.ImageSource' for
'en-AU' culture with default
conversions; consider using Converter
property of Binding.
NotSupportedException:'System.NotSupportedException:
ImageSourceConverter cannot convert
from (null). at
System.ComponentModel.TypeConverter.GetConvertFromException(Object
value) at
System.Windows.Media.ImageSourceConverter.ConvertFrom(ITypeDescriptorContext
context, CultureInfo culture, Object
value) at
MS.Internal.Data.DefaultValueConverter.ConvertHelper(Object
o, Type destinationType,
DependencyObject targetElement,
CultureInfo culture, Boolean
isForward)'

I'd prefer if this wasn't displayed as it's just noise – is there any way to suppress it?

Best Answer

@AresAvatar is right in suggesting you use a ValueConverter, but that implementation does not help the situation. This does:

public class NullImageConverter :IValueConverter
{
    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value == null)
            return DependencyProperty.UnsetValue;
        return value;
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        // According to https://msdn.microsoft.com/en-us/library/system.windows.data.ivalueconverter.convertback(v=vs.110).aspx#Anchor_1
        // (kudos Scott Chamberlain), if you do not support a conversion 
        // back you should return a Binding.DoNothing or a 
        // DependencyProperty.UnsetValue
        return Binding.DoNothing;
        // Original code:
        // throw new NotImplementedException();
    }
}

Returning DependencyProperty.UnsetValue also addresses the performance issues from throwing (and ignoring) all those exceptions. Returning a new BitmapSource(uri) would also get rid of the exceptions, but there is still a performance hit (and it isn't necessary).

Of course, you'll also need the plumbing:

In resources:

<local:NullImageConverter x:Key="nullImageConverter"/>

Your image:

<Image Source="{Binding Path=ImagePath, Converter={StaticResource nullImageConverter}}"/>
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