Fix PerspexProperty equality.

src/Perspex.Base/PerspexProperty.cs

@@ -288,7 +288,18 @@ namespace Perspex
         /// <returns>True if the properties are equal, otherwise false.</returns>
         public static bool operator ==(PerspexProperty a, PerspexProperty b)
         {
-            return a?.Equals(b) ?? false;
+            if (object.ReferenceEquals(a, b))
+            {
+                return true;
+            }
+            else if (((object)a == null) || ((object)b == null))
+            {
+                return false;
+            }
+            else
+            {
+                return a.Equals(b);
+            }
         }
 
         /// <summary>
@@ -299,7 +310,7 @@ namespace Perspex
         /// <returns>True if the properties are equal, otherwise false.</returns>
         public static bool operator !=(PerspexProperty a, PerspexProperty b)
         {
-            return !a?.Equals(b) ?? false;
+            return !(a == b);
         }
 
         /// <summary>

tests/Perspex.Base.UnitTests/PerspexObjectTests_Direct.cs

@@ -26,6 +26,14 @@ namespace Perspex.Base.UnitTests
             Assert.Equal("initial", target.GetValue((PerspexProperty)Class1.FooProperty));
         }
 
+        [Fact]
+        public void GetValue_On_Unregistered_Property_Throws_Exception()
+        {
+            var target = new Class2();
+
+            Assert.Throws<ArgumentException>(() => target.GetValue(Class1.BarProperty));
+        }
+
         [Fact]
         public void SetValue_Sets_Value()
         {
@@ -80,6 +88,14 @@ namespace Perspex.Base.UnitTests
             Assert.True(raised);
         }
 
+        [Fact]
+        public void SetValue_On_Unregistered_Property_Throws_Exception()
+        {
+            var target = new Class2();
+
+            Assert.Throws<ArgumentException>(() => target.SetValue(Class1.BarProperty, "value"));
+        }
+
         [Fact]
         public void GetObservable_Returns_Values()
         {

tests/Perspex.Base.UnitTests/PerspexPropertyTests.cs

@@ -137,6 +137,19 @@ namespace Perspex.Base.UnitTests
             Assert.True(target.IsDirect);
         }
 
+        [Fact]
+        public void Property_Equals_Should_Handle_Null()
+        {
+            var p1 = new PerspexProperty<string>("p1", typeof(Class1));
+
+            Assert.NotEqual(p1, null);
+            Assert.NotEqual(null, p1);
+            Assert.False(p1 == null);
+            Assert.False(null == p1);
+            Assert.False(p1.Equals(null));
+            Assert.True((PerspexProperty)null == (PerspexProperty)null);
+        }
+
         [Fact]
         public void AddOwnered_Property_Should_Equal_Original()
         {