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, 99 insertions, 1 deletions

diff --git a/Xamarin.Forms.Core.UnitTests/CommandTests.cs b/Xamarin.Forms.Core.UnitTests/CommandTests.cs
index 1182fe40..c653b976 100644
--- a/Xamarin.Forms.Core.UnitTests/CommandTests.cs
+++ b/Xamarin.Forms.Core.UnitTests/CommandTests.cs
@@ -1,7 +1,6 @@
 using System;
 using NUnit.Framework;
 
-
 namespace Xamarin.Forms.Core.UnitTests
 {
 	[TestFixture]
@@ -151,5 +150,104 @@ namespace Xamarin.Forms.Core.UnitTests
 			Assert.AreEqual (expected, cmd.CanExecute ("Foo"));
 			Assert.AreEqual ("Foo", result);
 		}
+
+		class FakeParentContext
+		{
+		}
+
+		// ReSharper disable once ClassNeverInstantiated.Local
+		class FakeChildContext
+		{
+		}
+
+		[Test]
+		public void CanExecuteReturnsFalseIfParameterIsWrongReferenceType()
+		{
+			var command = new Command<FakeChildContext>(context => { }, context => true);
+
+			Assert.IsFalse(command.CanExecute(new FakeParentContext()), "the parameter is of the wrong type");
+		}
+
+		[Test]
+		public void CanExecuteReturnsFalseIfParameterIsWrongValueType()
+		{
+			var command = new Command<int>(context => { }, context => true);
+
+			Assert.IsFalse(command.CanExecute(10.5), "the parameter is of the wrong type");
+		}
+
+		[Test]
+		public void CanExecuteUsesParameterIfReferenceTypeAndSetToNull()
+		{
+			var command = new Command<FakeChildContext>(context => { }, context => true);
+
+			Assert.IsTrue(command.CanExecute(null), "null is a valid value for a reference type");
+		}
+
+		[Test]
+		public void CanExecuteUsesParameterIfNullableAndSetToNull()
+		{
+			var command = new Command<int?>(context => { }, context => true);
+
+			Assert.IsTrue(command.CanExecute(null), "null is a valid value for a Nullable<int> type");
+		}
+
+		[Test]
+		public void CanExecuteIgnoresParameterIfValueTypeAndSetToNull()
+		{
+			var command = new Command<int>(context => { }, context => true);
+
+			Assert.IsFalse(command.CanExecute(null), "null is not a valid valid for int");
+		}
+
+		[Test]
+		public void ExecuteDoesNotRunIfParameterIsWrongReferenceType()
+		{
+			int executions = 0;
+			var command = new Command<FakeChildContext>(context => executions += 1);
+
+			Assert.DoesNotThrow(() => command.Execute(new FakeParentContext()), "the command should not execute, so no exception should be thrown");
+			Assert.IsTrue(executions == 0, "the command should not have executed");
+		}
+
+		[Test]
+		public void ExecuteDoesNotRunIfParameterIsWrongValueType()
+		{
+			int executions = 0;
+			var command = new Command<int>(context => executions += 1);
+
+			Assert.DoesNotThrow(() => command.Execute(10.5), "the command should not execute, so no exception should be thrown");
+			Assert.IsTrue(executions == 0, "the command should not have executed");
+		}
+
+		[Test]
+		public void ExecuteRunsIfReferenceTypeAndSetToNull()
+		{
+			int executions = 0;
+			var command = new Command<FakeChildContext>(context => executions += 1);
+
+			Assert.DoesNotThrow(() => command.Execute(null), "null is a valid value for a reference type");
+			Assert.IsTrue(executions == 1, "the command should have executed");
+		}
+
+		[Test]
+		public void ExecuteRunsIfNullableAndSetToNull()
+		{
+			int executions = 0;
+			var command = new Command<int?>(context => executions += 1);
+
+			Assert.DoesNotThrow(() => command.Execute(null), "null is a valid value for a Nullable<int> type");
+			Assert.IsTrue(executions == 1, "the command should have executed");
+		}
+
+		[Test]
+		public void ExecuteDoesNotRunIfValueTypeAndSetToNull()
+		{
+			int executions = 0;
+			var command = new Command<int>(context => executions += 1);
+
+			Assert.DoesNotThrow(() => command.Execute(null), "null is not a valid value for int");
+			Assert.IsTrue(executions == 0, "the command should not have executed");
+		}
 	}
 }