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>

1 files changed, 49 insertions, 13 deletions

diff --git a/Xamarin.Forms.Core/Command.cs b/Xamarin.Forms.Core/Command.cs
index 73ae1b08..d15574cf 100644
--- a/Xamarin.Forms.Core/Command.cs
+++ b/Xamarin.Forms.Core/Command.cs
@@ -1,22 +1,59 @@
 using System;
+using System.Reflection;
 using System.Windows.Input;
 
 namespace Xamarin.Forms
 {
-	public sealed class Command<T> : Command
+	public sealed class Command<T> : Command 
 	{
-		public Command(Action<T> execute) : base(o => execute((T)o))
+		public Command(Action<T> execute) 
+			: base(o =>
+			{
+				if (IsValidParameter(o))
+				{
+					execute((T)o);
+				}
+			})
 		{
 			if (execute == null)
-				throw new ArgumentNullException("execute");
+			{
+				throw new ArgumentNullException(nameof(execute));
+			}
 		}
 
-		public Command(Action<T> execute, Func<T, bool> canExecute) : base(o => execute((T)o), o => canExecute((T)o))
+		public Command(Action<T> execute, Func<T, bool> canExecute) 
+			: base(o =>
+			{
+				if (IsValidParameter(o))
+				{
+					execute((T)o);
+				}
+			}, o => IsValidParameter(o) && canExecute((T)o))
 		{
 			if (execute == null)
-				throw new ArgumentNullException("execute");
+				throw new ArgumentNullException(nameof(execute));
 			if (canExecute == null)
-				throw new ArgumentNullException("canExecute");
+				throw new ArgumentNullException(nameof(canExecute));
+		}
+
+		static bool IsValidParameter(object o)
+		{
+			if (o != null)
+			{
+				// The parameter isn't null, so we don't have to worry whether null is a valid option
+				return o is T;
+			}
+
+			var t = typeof(T);
+
+			// The parameter is null. Is T Nullable?
+			if (Nullable.GetUnderlyingType(t) != null)
+			{
+				return true;
+			}
+
+			// Not a Nullable, if it's a value type then null is not valid
+			return !t.GetTypeInfo().IsValueType;
 		}
 	}
 
@@ -28,7 +65,7 @@ namespace Xamarin.Forms
 		public Command(Action<object> execute)
 		{
 			if (execute == null)
-				throw new ArgumentNullException("execute");
+				throw new ArgumentNullException(nameof(execute));
 
 			_execute = execute;
 		}
@@ -36,13 +73,13 @@ namespace Xamarin.Forms
 		public Command(Action execute) : this(o => execute())
 		{
 			if (execute == null)
-				throw new ArgumentNullException("execute");
+				throw new ArgumentNullException(nameof(execute));
 		}
 
 		public Command(Action<object> execute, Func<object, bool> canExecute) : this(execute)
 		{
 			if (canExecute == null)
-				throw new ArgumentNullException("canExecute");
+				throw new ArgumentNullException(nameof(canExecute));
 
 			_canExecute = canExecute;
 		}
@@ -50,9 +87,9 @@ namespace Xamarin.Forms
 		public Command(Action execute, Func<bool> canExecute) : this(o => execute(), o => canExecute())
 		{
 			if (execute == null)
-				throw new ArgumentNullException("execute");
+				throw new ArgumentNullException(nameof(execute));
 			if (canExecute == null)
-				throw new ArgumentNullException("canExecute");
+				throw new ArgumentNullException(nameof(canExecute));
 		}
 
 		public bool CanExecute(object parameter)
@@ -73,8 +110,7 @@ namespace Xamarin.Forms
 		public void ChangeCanExecute()
 		{
 			EventHandler changed = CanExecuteChanged;
-			if (changed != null)
-				changed(this, EventArgs.Empty);
+			changed?.Invoke(this, EventArgs.Empty);
 		}
 	}
 }
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