CSHARP-1542: support TypeAs expressions.

Author: craiggwilson

src/MongoDB.Driver.Tests/UpdateDefinitionBuilderTests.cs

@@ -521,6 +521,18 @@ public void Set_Typed_with_cast()
             Assert(subject2.Set(x => ((SmsMessage)x.Message).PhoneNumber, "1234567890"), "{$set: {'m.pn': '1234567890'}}");
         }
 
+        [Test]
+        public void Set_Typed_with_type_as()
+        {
+            var subject = CreateSubject<Message>();
+
+            Assert(subject.Set(x => (x as SmsMessage).PhoneNumber, "1234567890"), "{$set: {pn: '1234567890'}}");
+
+            var subject2 = CreateSubject<Person>();
+
+            Assert(subject2.Set(x => (x.Message as SmsMessage).PhoneNumber, "1234567890"), "{$set: {'m.pn': '1234567890'}}");
+        }
+
         [Test]
         public void SetOnInsert()
         {

src/MongoDB.Driver/Linq/Processors/SerializationBinder.cs

@@ -185,7 +185,7 @@ protected override Expression VisitMethodCall(MethodCallExpression node)
         protected override Expression VisitUnary(UnaryExpression node)
         {
             var newNode = (UnaryExpression)base.VisitUnary(node);
-            if (newNode.NodeType == ExpressionType.Convert || newNode.NodeType == ExpressionType.ConvertChecked)
+            if (newNode.NodeType == ExpressionType.Convert || newNode.NodeType == ExpressionType.ConvertChecked || newNode.NodeType == ExpressionType.TypeAs)
             {
                 // handle unnecessary convert expressions
                 if (newNode.Method == null && !newNode.IsLiftedToNull && newNode.Type.IsAssignableFrom(newNode.Operand.Type))