CSHARP-4681: InvalidCastException when rendering projections (#1146)

src/MongoDB.Driver/IFindFluentExtensions.cs

@@ -57,7 +57,7 @@ public static class IFindFluentExtensions
             Ensure.IsNotNull(find, nameof(find));
             Ensure.IsNotNull(projection, nameof(projection));
 
-            return find.Project<TNewProjection>(new ExpressionProjectionDefinition<TDocument, TNewProjection>(projection, null));
+            return find.Project<TNewProjection>(new FindExpressionProjectionDefinition<TDocument, TNewProjection>(projection));
         }
 
         /// <summary>

tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Jira/CSharp4681Tests.cs

@@ -0,0 +1,60 @@
+/* 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 FluentAssertions;
+using MongoDB.Bson.Serialization;
+using MongoDB.Driver.Linq;
+using MongoDB.TestHelpers.XunitExtensions;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3Implementation.Jira
+{
+    public class CSharp4681Tests : Linq3IntegrationTest
+    {
+        [Theory]
+        [ParameterAttributeData]
+        public void Find_projection_render_should_work(
+            [Values(LinqProvider.V2, LinqProvider.V3)] LinqProvider linqProvider)
+        {
+            var collection = GetCollection(linqProvider);
+
+            var fluentFind = collection.Find(a => a.Id == "1").Project(a => a.Id);
+
+            var documentSerializer = collection.DocumentSerializer;
+            var serializerRegistry = BsonSerializer.SerializerRegistry;
+            var renderedProjection = fluentFind.Options.Projection.Render(documentSerializer, serializerRegistry, linqProvider);
+
+            renderedProjection.Document.Should().Be("{ _id : 1 }");
+
+            var result = fluentFind.Single();
+            result.Should().Be("1");
+        }
+
+        private IMongoCollection<A> GetCollection(LinqProvider linqProvider)
+        {
+            var collection = GetCollection<A>("test", linqProvider);
+            CreateCollection(
+                collection,
+                new A { Id = "1" },
+                new A { Id = "2" });
+            return collection;
+        }
+
+        private class A
+        {
+            public string Id { get; set; }
+        }
+    }
+}