Don't allow !ed expressions to be set.

src/Markup/Perspex.Markup/Binding/LogicalNotNode.cs

@@ -14,6 +14,11 @@ namespace Perspex.Markup.Binding
         {
         }
 
+        public override bool SetValue(object value)
+        {
+            throw new NotSupportedException("Cannot set a negated binding.");
+        }
+
         public override IDisposable Subscribe(IObserver<ExpressionValue> observer)
         {
             return Next.Select(x => Negate(x)).Subscribe(observer);

tests/Perspex.Markup.UnitTests/Binding/ExpressionObserverTests_Negation.cs

@@ -1,6 +1,7 @@
 // Copyright (c) The Perspex Project. All rights reserved.
 // Licensed under the MIT license. See licence.md file in the project root for full license information.
 
+using System;
 using System.Reactive.Linq;
 using Perspex.Markup.Binding;
 using Xunit;
@@ -64,6 +65,16 @@ namespace Perspex.Markup.UnitTests.Binding
             Assert.Equal(false, result.Value);
         }
 
+        [Fact]
+        public async void Should_Return_Empty_For_String_Not_Convertible_To_Boolean()
+        {
+            var data = new { Foo = "foo" };
+            var target = new ExpressionObserver(data, "!Foo");
+            var result = await target.Take(1);
+
+            Assert.False(result.HasValue);
+        }
+
         [Fact]
         public async void Should_Return_Empty_For_Value_Not_Convertible_To_Boolean()
         {
@@ -73,5 +84,14 @@ namespace Perspex.Markup.UnitTests.Binding
 
             Assert.False(result.HasValue);
         }
+
+        [Fact]
+        public void SetValue_Should_Throw()
+        {
+            var data = new { Foo = "foo" };
+            var target = new ExpressionObserver(data, "!Foo");
+
+            Assert.Throws<NotSupportedException>(() => target.SetValue("bar"));
+        }
     }
 }