[XF] EventToCommandBehavior.OnEventRaised NullReferenceException

[chowarth]

Description

Setting EventArgsParameterPath to a property that is declared in a derived class of the event arguments causes a NullReferenceException to be thrown.

e.g. WebView.Navigating event arguments WebNavigatingEventArgs derives from WebNavigationEventArgs. The Url property is declared within WebNavigatingEventArgs so when you set EventArgsParameterPath to Url it will cause a NullReferenceException to be thrown.

Steps to Reproduce

Best reproduced on iOS as the Navigating event doesn't fire on an initial page load for Android.

1. Create a new Prism Blank App
2. Replace MainPage with the following:

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
             xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
             xmlns:b="clr-namespace:Prism.Behaviors;assembly=Prism.Forms"
             x:Class="EventToCommandBehavior.Views.MainPage"
             Title="{Binding Title}">

    <WebView
        Source="{Binding Url}">
        <WebView.Behaviors>
            <b:EventToCommandBehavior
                EventName="Navigating"
                EventArgsParameterPath="Url"
                Command="{Binding NavigatingCommand}"/>
        </WebView.Behaviors>
    </WebView>

</ContentPage>

3. Replace MainPageViewModel with the following:

    public class MainPageViewModel : ViewModelBase
    {
        public DelegateCommand<string> NavigatingCommand { get; }

        public string Url
        {
            get => "https://prismlibrary.github.io";
        }

        public MainPageViewModel(INavigationService navigationService)
            : base(navigationService)
        {
            Title = "Main Page";

            NavigatingCommand = new DelegateCommand<string>(OnNavigating);
        }

        private void OnNavigating(string url)
        {
            Console.WriteLine($"[URL]:{url}");
        }
    }

Reproduction Link

EventToCommandBehavior.zip

Expected Behavior

Bound command is executed with the Url parameter

Actual Behavior

NullReferenceException is thrown

Basic Information

• Version with issue: Prism.Forms 7.0.0.396
• Last known good version: -
• Xamarin.Forms version: 2.5.0.122203
• IDE: Visual Studio 2017 (15.8.4)

Suggested Fix

Change:

Prism/Source/Xamarin/Prism.Forms/Behaviors/EventToCommandBehavior.cs

Line 186 in 9ee268e

var propInfo = propertyValue.GetType().GetTypeInfo().GetDeclaredProperty(propertyPathPart);

To:

var propInfo = propertyValue.GetType().GetProperty(propertyPathPart);

[brianlagunas]

Thank you for reporting this. unfortunately GetProperty is not available in NetStandard 1.0. We will need to determine another fix, or just recommend using a converter in NetStandard 1.0 and fix it for 2.0.

We could use GetRuntimeProperty, but that also gets non-public values, which may also cause issues and a possible perf hit (not tested just assumed).

[brianlagunas]

We went with the GetRuntimeProperty approach. Test it out in the latest CI build on MyGet and let us know if you are still having issues.

[chowarth]

Sounds good and I'll give it a whirl later today when I get chance. Didn't expect you to put a fix in so quickly haha.

While GetProperty is not available in NetStandard 1.0 Xamarin.Forms has its own internal extension method, see here and here, which is what I used along with the test below to confirm that everything worked OK.

[Fact]
public void Command_EventArgsParameterPath_BaseDeclaredProperty()
{
    var executedCommand = false;
    var behavior = new EventToCommandBehaviorMock
    {
        EventName = "Navigating",
        EventArgsParameterPath = "Url", // Url is declared in WebNavigationEventArgs, not WebNavigatingEventArgs
        Command = new DelegateCommand<string>(o =>
        {
            executedCommand = true;
            Assert.NotNull(o);
        })
    };
    var webView = new WebView();
    webView.Behaviors.Add(behavior);
    behavior.RaiseEvent(webView, new WebNavigatingEventArgs(new WebNavigationEvent(), null, "http://prismlibrary.github.io/"));
    Assert.True(executedCommand);
}

[lock]

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