x:Bind image with null string

问题:

In XAML I have the following line:

<Image x:Name="MainImage" 
       Source="{x:Bind ViewModel.MainPic,Mode=OneWay,TargetNullValue={x:Null}}"
       Stretch="UniformToFill"/>

In ViewModel:

public string MainPic
{
    get
    {
        if (Data == null)
            return default(string);
        else
            return Data.Photos.ElementAtOrDefault(0).url;
    }
}

App compiles fine but during execution (since Data is populated after few seconds), the app crashes with the following exception:

System.ArgumentException: The parameter is incorrect.

Debugger breaks at:

            private void Update_ViewModel_MainPic(global::System.String obj, int phase)
            {
                if((phase & ((1 << 0) | NOT_PHASED | DATA_CHANGED)) != 0)
                {
 /*HERE>>*/          XamlBindingSetters.Set_Windows_UI_Xaml_Controls_Image_Source(this.obj23, (global::Windows.UI.Xaml.Media.ImageSource) global::Windows.UI.Xaml.Markup.XamlBindingHelper.ConvertValue(typeof(global::Windows.UI.Xaml.Media.ImageSource), obj), null);
                }
            }

Apparently, this occurs since MainPic is returning null.

Now, this code works fine in WP8.1. I have tried returning uri which results in Compile time error. I believe only string can be binded to image source in Win 10 (?) I just want a blank white area until data is populated hence I don't wish to give a local image source as fallback. Can someone help me port this for Win 10?

---

UPDATE:

Thanks to the users who answered, following conclusion is drawn (for UWP):

• If you're binding image source to a string, it cannot be null or empty "". A singe character "x" or a space " " would work.
• If you bind to a BitmapImage, returning null works.
• You can use any of the methods mentioned by @Justin-xl . For me, changing all vm's to stop returning null was hard. So adding a simple convertor to xaml also does the trick.

Here's the converter code:

public object Convert(object value, Type targetType, object parameter, string language)
{
    if (string.IsNullOrEmpty(value as string))
    {
        return null;
    }
    else return new BitmapImage(new Uri(value as string, UriKind.Absolute));
}

回答1:

If you use x:Bind, the Source of the Image needs to bind to a property of the exact same type ImageSource (e.g. BitmapImage) instead of string, otherwise it will throw a compile-time error, which is exactly a compile-time binding is supposed to do. The old binding allows strings 'cause it uses Reflection to resolve the type for you during run-time.

Turns out my explicit type theory was wrong (thanks to @igrali for pointing it out). The Source does take a string as long it's is not null or ''. So it leaves us two options to fix this.

Option 1

Keep your uri as a string, but do a check in your vm, once it's null or '', return some dummy text (even returning an a letter x would work!).

Option 2

Change the uri from a string to a BitmapImage. Then you can use TargetNullValue and FallbackValue to handle nulls and invalid bindings.

... FallbackValue='http://Assets/SplashScreen.png' TargetNullValue='http://Assets/SplashScreen.png'}"