Don't throw when binding produces invalid value.

src/Perspex.Base/PriorityValue.cs

@@ -212,25 +212,23 @@ namespace Perspex
         /// <param name="priority">The priority level that the value came from.</param>
         private void UpdateValue(object value, int priority)
         {
-            if (!TypeUtilities.TryCast(_valueType, value, out value))
+            if (TypeUtilities.TryCast(_valueType, value, out value))
             {
-                throw new InvalidOperationException(string.Format(
-                    "Invalid value for Property '{0}': {1} ({2})",
-                    _name,
-                    value,
-                    value?.GetType().FullName ?? "(null)"));
-            }
+                var old = _value;
 
-            var old = _value;
+                if (_validate != null)
+                {
+                    value = _validate(value);
+                }
 
-            if (_validate != null)
+                ValuePriority = priority;
+                _value = value;
+                _changed.OnNext(Tuple.Create(old, _value));
+            }
+            else
             {
-                value = _validate(value);
+                // TODO: Log error.
             }
-
-            ValuePriority = priority;
-            _value = value;
-            _changed.OnNext(Tuple.Create(old, _value));
         }
 
         /// <summary>

tests/Perspex.Base.UnitTests/PerspexObjectTests_Binding.cs

@@ -59,14 +59,11 @@ namespace Perspex.Base.UnitTests
         }
 
         [Fact]
-        public void Bind_Throws_Exception_For_Invalid_Value_Type()
+        public void Bind_Ignores_Invalid_Value_Type()
         {
             Class1 target = new Class1();
-
-            Assert.Throws<InvalidOperationException>(() =>
-            {
-                target.Bind((PerspexProperty)Class1.FooProperty, Observable.Return((object)123));
-            });
+            target.Bind((PerspexProperty)Class1.FooProperty, Observable.Return((object)123));
+            Assert.Equal("foodefault", target.GetValue(Class1.FooProperty));
         }
 
         [Fact]