Don't run Command CanExecute on incorrect inherited binding context type (#572)

* Allow Command CanExecute to recover when run on inherited bindingcontext

* Make exception handler more generic

* Checking types in Command delegates to avoid exception in the first place

* Adding type chekc to other Command constructor

* Use nameof for ArgumentNullExceptions

* Add unit tests for null parameters, handle value types and Nullable<T>

2 files changed, 110 insertions, 0 deletions

diff --git a/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Bugzilla47971.cs b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Bugzilla47971.cs
new file mode 100644
index 00000000..351028d4
--- /dev/null
+++ b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Bugzilla47971.cs
@@ -0,0 +1,109 @@
+using System;
+using System.Collections.Generic;
+using System.Collections.ObjectModel;
+using System.ComponentModel;
+using System.Runtime.CompilerServices;
+using Xamarin.Forms.CustomAttributes;
+using Xamarin.Forms.Internals;
+
+namespace Xamarin.Forms.Controls.Issues
+{
+	[Preserve(AllMembers = true)]
+	[Issue(IssueTracker.Bugzilla, 47971, "UWP doesn't display list items when binding a CommandParameter to BindingContext in a DataTemplate and including a CanExecute method", PlatformAffected.WinRT)]
+	public class Bugzilla47971 : TestContentPage
+	{
+		protected override void Init()
+		{
+			var viewModel = new _47971ViewModel();
+
+			var lv = new ListView { BindingContext = viewModel };
+
+			lv.SetBinding(ListView.ItemsSourceProperty, new Binding("Models"));
+			lv.SetBinding(ListView.SelectedItemProperty, new Binding("SelectedModel"));
+
+			lv.ItemTemplate = new DataTemplate(() =>
+			{
+				var tc = new TextCell();
+
+				tc.SetBinding(TextCell.TextProperty, new Binding("Name"));
+				tc.SetBinding(TextCell.CommandParameterProperty, new Binding("."));
+				tc.SetBinding(TextCell.CommandProperty, new Binding("BindingContext.ModelSelectedCommand", source: lv));
+
+				return tc;
+			});
+
+			var layout = new StackLayout { Spacing = 10 };
+			var instructions = new Label {Text = "The ListView below should display three items (Item1, Item2, and Item3). If it does not, this test has failed." };
+
+			layout.Children.Add(instructions);
+			layout.Children.Add(lv);
+
+			Content = layout;
+		}
+
+		[Preserve(AllMembers = true)]
+		internal class _47971ViewModel : INotifyPropertyChanged
+		{
+			_47971ItemModel _selectedModel;
+			Command<_47971ItemModel> _modelSelectedCommand;
+			ObservableCollection<_47971ItemModel> _models;
+
+			public ObservableCollection<_47971ItemModel> Models
+			{
+				get { return _models; }
+				set
+				{
+					_models = value;
+					OnPropertyChanged();
+				}
+			}
+
+			public _47971ItemModel SelectedModel
+			{
+				get { return _selectedModel; }
+				set
+				{
+					_selectedModel = value;
+					OnPropertyChanged();
+				}
+			}
+
+			public Command<_47971ItemModel> ModelSelectedCommand => _modelSelectedCommand ??
+				(_modelSelectedCommand = new Command<_47971ItemModel>(ModelSelectedCommandExecute, CanExecute));
+
+			bool CanExecute(_47971ItemModel itemModel)
+			{
+				return true;
+			}
+
+			void ModelSelectedCommandExecute(_47971ItemModel model)
+			{
+				System.Diagnostics.Debug.WriteLine(model.Name);
+			}
+
+			public _47971ViewModel()
+			{
+				_models = new ObservableCollection<_47971ItemModel>(
+					new List<_47971ItemModel>()
+					{
+						new _47971ItemModel() { Name = "Item 1"},
+						new _47971ItemModel() { Name = "Item 2"},
+						new _47971ItemModel() { Name = "Item 3"}
+					});
+			}
+
+			public event PropertyChangedEventHandler PropertyChanged;
+
+			protected virtual void OnPropertyChanged([CallerMemberName] string propertyName = null)
+			{
+				PropertyChanged?.Invoke(this, new PropertyChangedEventArgs(propertyName));
+			}
+		}
+
+		[Preserve(AllMembers = true)]
+		internal class _47971ItemModel
+		{
+			public string Name { get; set; }
+		}
+	}
+}
\ No newline at end of file
diff --git a/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems
index 3b728f4d..08c26ba1 100644
--- a/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems
+++ b/Xamarin.Forms.Controls.Issues/Xamarin.Forms.Controls.Issues.Shared/Xamarin.Forms.Controls.Issues.Shared.projitems
@@ -139,6 +139,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla46494.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla44476.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla46630.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Bugzilla47971.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)CarouselAsync.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla34561.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Bugzilla34727.cs" />