CSHARP-4572: Support casting from an interface to a concrete type in a filter expression.

Author: rstam

src/MongoDB.Driver/Linq/Linq3Implementation/Misc/TypeExtensions.cs

@@ -253,6 +253,13 @@ public static bool IsSameAsOrNullableOf(this Type type, Type valueType)
             return type == valueType || type.IsNullableOf(valueType);
         }
 
+        public static bool IsSubclassOfOrImplements(this Type type, Type baseTypeOrInterface)
+        {
+            return
+                type.IsSubclassOf(baseTypeOrInterface) ||
+                type.Implements(baseTypeOrInterface);
+        }
+
         public static bool IsTuple(this Type type)
         {
             return

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToFilterTranslators/ToFilterFieldTranslators/ConvertExpressionToFilterFieldTranslator.cs

@@ -82,12 +82,12 @@ private static bool IsConvertEnumToUnderlyingType(Type fieldType, Type targetTyp
 
         private static bool IsConvertToBaseType(Type fieldType, Type targetType)
         {
-            return fieldType.IsSubclassOf(targetType);
+            return fieldType.IsSubclassOfOrImplements(targetType);
         }
 
         private static bool IsConvertToDerivedType(Type fieldType, Type targetType)
         {
-            return targetType.IsSubclassOf(fieldType);
+            return targetType.IsSubclassOfOrImplements(fieldType);
         }
 
         private static bool IsConvertToNullable(Type fieldType, Type targetType)

tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Jira/CSharp4572Tests.cs

@@ -0,0 +1,78 @@
+/* Copyright 2010-present MongoDB Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+using System.Linq;
+using FluentAssertions;
+using MongoDB.Bson.TestHelpers;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3Implementation.Jira
+{
+    [Collection(RegisterObjectSerializerFixture.CollectionName)]
+    public class CSharp4572Tests : Linq3IntegrationTest
+    {
+        [Fact]
+        public void Builders_Filter_Eq_with_downcast_should_work()
+        {
+            var collection = GetCollection();
+            var filter = Builders<IMyInterface>.Filter.Eq(x => ((MyClass)x).TestValue, 42);
+
+            var find = collection.Find(filter);
+
+            var renderedFilter = TranslateFindFilter(collection, find);
+            renderedFilter.Should().Be("{ TestValue : 42 }");
+
+            var results = find.ToList();
+            results.Select(x => x.Id).Should().Equal(2);
+        }
+
+        [Fact]
+        public void Where_with_downcast_should_work()
+        {
+            var collection = GetCollection();
+            var filter = Builders<IMyInterface>.Filter.Eq(x => ((MyClass)x).TestValue, 42);
+
+            var queryable = collection.AsQueryable()
+                .Where(x => ((MyClass)x).TestValue == 42);
+
+            var stages = Translate(collection, queryable);
+            AssertStages(stages, "{ $match : { TestValue : 42 } }");
+
+            var results = queryable.ToList();
+            results.Select(x => x.Id).Should().Equal(2);
+        }
+
+        private IMongoCollection<IMyInterface> GetCollection()
+        {
+            var collection = GetCollection<IMyInterface>("test");
+            CreateCollection(
+                collection,
+                new MyClass { Id = 1, TestValue = 1 },
+                new MyClass { Id = 2, TestValue = 42 });
+            return collection;
+        }
+
+        public interface IMyInterface
+        {
+            int Id { get; set; }
+        }
+
+        public class MyClass : IMyInterface
+        {
+            public int Id { get; set; }
+            public int TestValue { get; set; }
+        }
+    }
+}