CSHARP-506: Better error message when comparing a non-nullable field to a nullable value.

Author: rstam

src/MongoDB.Driver/Linq/Translators/PredicateTranslator.cs

@@ -439,6 +439,21 @@ private FilterDefinition<BsonDocument> TranslateComparison(Expression variableEx
                 return isTrueComparison ? query : __builder.Not(query);
             }
 
+            if (variableExpression.NodeType == ExpressionType.Convert)
+            {
+                var convertExpression = (UnaryExpression)variableExpression;
+                var targetTypeInfo = convertExpression.Type.GetTypeInfo();
+                if (targetTypeInfo.IsGenericType && targetTypeInfo.GetGenericTypeDefinition() == typeof(Nullable<>))
+                {
+                    var underlyingValueType = targetTypeInfo.GetGenericArguments()[0];
+                    var convertOperand = convertExpression.Operand as FieldExpression;
+                    if (convertOperand != null && convertOperand.Type == underlyingValueType)
+                    {
+                        return null;
+                    }
+                }
+            }
+
             var fieldExpression = GetFieldExpression(variableExpression);
             var serializedValue = fieldExpression.SerializeValue(value);
             switch (operatorType)

tests/MongoDB.Driver.Tests/Linq/Translators/PredicateTranslatorTests.cs

@@ -615,6 +615,18 @@ public void Equals_with_nullable_date_time()
                 "{'R': { $ne: null, $gt: ISODate('0001-01-01T00:00:00Z') } }");
         }
 
+        [Fact]
+        public void Equals_with_non_nullable_field_and_nullable_value()
+        {
+            var value = (int?)null;
+            Expression<Func<Root, bool>> filter = d => d.Id == value;
+            var serializer = BsonSerializer.SerializerRegistry.GetSerializer<Root>();
+
+            var exception = Record.Exception(() => PredicateTranslator.Translate(filter, serializer, BsonSerializer.SerializerRegistry));
+
+            exception.Should().BeOfType<ArgumentException>();
+        }
+
         [Fact]
         public void HashSetCount()
         {