Compiled Bindings: Compilation Error - Object reference not set to an instance of an object

[inforithmics]

Description

When using Compiled Bindings on a Type with a Generic Base Class and Binding on A Property with the Generic Type: A Compilation Error occurs with Object reference not set to an instance of an object.

Steps to Reproduce

1. Follwing Base Class

 public class TestViewModel : TestViewModelBase<string>
    {
        public TestViewModel()
        {
            this.Add("test");
            this.SelectedItem = this.First();
        }        
    }

    public class TestViewModelBase<T> : Collection<string>
    {
        public T SelectedItem { get; set; }
    }

Following xaml with Binding to the Generic Property SelectedItem

<?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:local="clr-namespace:App14"
             x:Class="App14.MainPage"
             x:DataType="local:TestViewModel">

    <StackLayout>
        <!-- Place new controls here -->        
        <Label Text="Number of Items:"/>
        <Label Text="{Binding SelectedItem}" />        
    </StackLayout>

</ContentPage>

Expected Behavior

The Binding Works and the Code compiles

Actual Behavior

The Code doesn't Compile, following error occurs:
Object reference not set to an instance of an object

Basic Information

• Version with issue: 3.4.0.1008975
• Last known good version: N/A
• IDE: Visual Studio 2017 15.9.1
• Platform Target Frameworks:
  • iOS: 12.1
  • Android: The version of the Android SDK you are compiling against, e.g. 9.0
  • UWP: 16299
• Android Support Library Version: v27.0.2.1
• Nuget Packages: N/A
• Affected Devices: N/A

Screenshots

Reproduction Link

App14.zip

Workaround

Override the Property in the ViewModel than it works.

    public class TestViewModel : TestViewModelBase<string>
    {
        public TestViewModel()
        {
            this.Add("test");
            this.SelectedItem = this.First();
        }

        public override string SelectedItem
        {
            get => base.SelectedItem;
            set => this.SelectedItem = value;
        }
    }

    public class TestViewModelBase<T> : Collection<string>
    {
        public virtual T SelectedItem { get; set; }
    }

[StephaneDelcroix]

I have to check, but I think this was resolved in #4352, and the fix will be released as part of the 3.4.0 release

[StephaneDelcroix]

y, I think that is. reopen if it's not fixed in 3.4.0

[inforithmics]

Can you reopen this issue because it is not a duplicate of #4348.

1. I used 3.4.0 Version as stated in Version with issue.
2. This binding works now (was bug in issue)
   <Label Text="{Binding Count}" />
   This binding does not work
   <Label Text="{Binding SelectedItem}" />
   throws Object reference not set to an instance of an object.

The Difference between the two are.

Count is a Property on a Generic Type.
SelectedItem is a Property on Generic Type with a Generic TypeParameter as Type.

So I guess Compiled Bindings tries to find out the Type of this Property and cannot because it is a TypeParameter Type.

Additional Info:

I tried it with the Nightly Nuget package 3.5.1006868 and the issue is still there

[StephaneDelcroix]

fixed in #4483

[inforithmics]

Thanks a lot for the fix.