How does Xaml create the string to BitmapImage value conversion when binding to Image.Source?

Question

I'm creating an Image.Source-String binding in code like:

var newBinding = new System.Windows.Data.Binding()
  {
    Path = new PropertyPath("MyImageUrl")
  };
BindingOperations.SetBinding(attachedObject, Image.SourceProperty, newBinding);

This approach works well for, for example, TextBlock.TextProperty-String bindings, but for the Image.Source-String I ideally would like the Binding to automatically insert a conversion for me - in the same way that the Xaml binding does when I use:

<Image Source="{Binding ImageUrl}" />

I realise I can add my own converter to mimic the Xaml binding behaviour, but I'd like to see if there's some way to do exactly what the Xaml does.

Is there some way to get the new Binding to automatically add it's own string->BitmapImage ValueConverter during the code-based bind evaluation?

Answer 1

System.Windows.Media.ImageSource has a TypeConverterAttribute

[TypeConverter(typeof(ImageSourceConverter))]

The binding will look for this and use the converter automatically.

If you look at the ImageSourceConverter you can see what types it can convert from:

if (sourceType == typeof(string) || 
    sourceType == typeof(Stream) || 
    sourceType == typeof(Uri) || 
    sourceType == typeof(byte[]))
{
    return true;
}

In order to mimic this process, you must add a TypeConverterAttribute on the Type of the property being bound to.

You can do this by 1. controlling the type, or 2. use the TypeDescriptor at runtime to add the attribute. There's a question about this here.