Report guards as best we can if the target does not implement INPC

Fixes #214

Stylet/Xaml/CommandAction.cs

@@ -65,16 +65,11 @@ namespace Stylet.Xaml
         /// <param name="newTarget">New target</param>
         protected internal override void OnTargetChanged(object oldTarget, object newTarget)
         {
-            var oldInpc = oldTarget as INotifyPropertyChanged;
+            if (oldTarget is INotifyPropertyChanged oldInpc)
-            if (oldInpc != null)
                 PropertyChangedEventManager.RemoveHandler(oldInpc, this.PropertyChangedHandler, this.GuardName);
 
             this.guardPropertyGetter = null;
-
+            var guardPropertyInfo = newTarget?.GetType().GetProperty(this.GuardName);
-            var inpc = newTarget as INotifyPropertyChanged;
-            if (inpc != null)
-            {
-                var guardPropertyInfo = newTarget.GetType().GetProperty(this.GuardName);
             if (guardPropertyInfo != null)
             {
                 if (guardPropertyInfo.PropertyType == typeof(bool))
@@ -90,7 +85,11 @@ namespace Stylet.Xaml
             }
 
             if (this.guardPropertyGetter != null)
+            {
+                if (newTarget is INotifyPropertyChanged inpc)
                     PropertyChangedEventManager.AddHandler(inpc, this.PropertyChangedHandler, this.GuardName);
+                else
+                    logger.Warn("Found guard property {0} for action {1} on target {2}, but the target doesn't implement INotifyPropertyChanged, so changes won't be observed", this.GuardName, this.MethodName, newTarget);
             }
 
             this.UpdateCanExecute();

StyletUnitTests/CommandActionTests.cs

@@ -62,6 +62,12 @@ namespace StyletUnitTests
         {
         }
 
+        private class TargetWithoutInpc
+        {
+            public bool CanDoSomething => false;
+            public void DoSomething() { }
+        }
+
         private DependencyObject subject;
         private Target target;
 
@@ -171,6 +177,18 @@ namespace StyletUnitTests
             Assert.True(eventRaised);
         }
 
+        [Test]
+        public void FetchesGuardPropertyWhenTargetDoesNotImplementInpc()
+        {
+            var target = new TargetWithoutInpc();
+            var cmd = new CommandAction(this.subject, null, "DoSomething", ActionUnavailableBehaviour.Throw, ActionUnavailableBehaviour.Throw);
+            bool eventRaised = false;
+            cmd.CanExecuteChanged += (o, e) => eventRaised = true;
+            View.SetActionTarget(this.subject, target);
+            Assert.True(eventRaised);
+            Assert.False(cmd.CanExecute(null));
+        }
+
         [Test]
         public void RaisesEventWhenTargetChanges()
         {