RelayCommand<T> throws NullReferenceException when T is a ValueType and CanExecute is used #3619
Description
Describe the bug
If you open a View (Page or Window) and you have DataBound the Command of a Button to a RelayCommand<T> where T is a ValueType and the command has a CanExecute action you get a NullReferenceException. Tested it in a UWP and in a WPF app. Both have this problem.
Without the CanExecute it works ok. If T is a reference type it also works ok.
Steps to Reproduce
Take this xaml
<Page
x:Class="App19.MainPage"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="using:App19"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
mc:Ignorable="d"
Background="{ThemeResource ApplicationPageBackgroundThemeBrush}">
<Grid RowSpacing="8" Padding="8">
<Grid.RowDefinitions>
<RowDefinition Height="Auto" />
<RowDefinition Height="1*" />
</Grid.RowDefinitions>
<Button Content="Test"
Command="{x:Bind TestCommand, Mode=OneTime}"
CommandParameter="{x:Bind listViewDemo.SelectedIndex, Mode=OneWay}"/>
<ListView x:Name="listViewDemo">
<ListViewItem Content="A" />
<ListViewItem Content="B" />
<ListViewItem Content="C" />
</ListView>
</Grid>
</Page>And this codebehind
using Microsoft.Toolkit.Mvvm.Input;
using Windows.UI.Popups;
using Windows.UI.Xaml.Controls;
// The Blank Page item template is documented at https://go.microsoft.com/fwlink/?LinkId=402352&clcid=0x409
namespace App19 {
/// <summary>
/// An empty page that can be used on its own or navigated to within a Frame.
/// </summary>
public sealed partial class MainPage : Page {
private RelayCommand<int> TestCommand { get; }
public MainPage() {
this.InitializeComponent();
TestCommand = new RelayCommand<int>(OnTest, index => index > 0);
}
private void OnTest(int index) {
_ = new MessageDialog($"Test: {index}").ShowAsync();
}
}
}Start the app using F5. You get the Exception. You don't get the Page/Window it happens from the constructor of the page/window.
This also happens in WPF apps, not only UWP.
Expected behavior
No Exception, just a working app.
Environment
NuGet Package(s):
Microsoft.Toolkit.Mvvm
Package Version(s):
7.0.0.0-preview4
Windows 10 Build Number:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [ ] October 2018 Update (17763)
- [ ] May 2019 Update (18362)
- [X] May 2020 Update (19041)
- [ ] Insider Build (build number: )
App min and target version:
- [ ] Fall Creators Update (16299)
- [ ] April 2018 Update (17134)
- [ ] October 2018 Update (17763)
- [ ] May 2019 Update (18362)
- [ ] May 2020 Update (19041)
- [ ] Insider Build (xxxxx)
Device form factor:
- [X] Desktop
- [ ] Xbox
- [ ] Surface Hub
- [ ] IoT
Visual Studio
- [ ] 2017 (version: )
- [X] 2019 (version: )
- [X] 2019 Preview (version: )
Additional context
Add any other context about the problem here.