CSHARP-4658: NullReferenceException when using certain projections.

Author: rstam

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

@@ -190,21 +190,17 @@ public override Expression Visit(Expression expression)
             protected override Expression VisitMemberInit(MemberInitExpression node)
             {
                 // Bindings must be visited before NewExpression
-                foreach (var binding in node.Bindings)
-                {
-                    switch (binding.BindingType)
-                    {
-                        case MemberBindingType.Assignment:
-                            var memberAssignment = (MemberAssignment)binding;
-                            base.Visit(memberAssignment.Expression);
-                            break;
+                Visit(node.Bindings, VisitMemberBinding);
 
-                        default:
-                            throw new InvalidOperationException($"Unexpected binding type: {binding.BindingType}.");
-                    }
+                if (_cannotBeEvaluated)
+                {
+                    // visit only the arguments if any MemberBindings cannot be partially evaluated
+                    Visit(node.NewExpression.Arguments);
+                }
+                else
+                {
+                    Visit(node.NewExpression);
                 }
-
-                base.Visit(node.NewExpression);
 
                 return node;
             }

tests/MongoDB.Driver.Tests/Linq/Linq3ImplementationTests/Jira/CSharp4658Tests.cs

@@ -0,0 +1,103 @@
+/* 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 MongoDB.Driver.Linq;
+using MongoDB.TestHelpers.XunitExtensions;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3ImplementationTests.Jira
+{
+    public class CSharp4658Tests : Linq3IntegrationTest
+    {
+        [Theory]
+        [ParameterAttributeData]
+        public void Select_new_Model_should_work(
+            [Values(LinqProvider.V2, LinqProvider.V3)] LinqProvider linqProvider)
+        {
+            var collection = GetCollection(linqProvider);
+
+            var queryable = collection.AsQueryable()
+                .GroupBy(x => x.Name)
+                .Select(x =>
+                    new Model
+                    {
+                        NotId = string.Empty,
+                        Name = x.Key
+                    });
+
+            var stages = Translate(collection, queryable);
+            AssertStages(
+                stages,
+                "{ $group : { _id : '$Name' } }",
+                "{ $project : { NotId : '', Name : '$_id', _id : 0 } }");
+        }
+
+        [Theory]
+        [ParameterAttributeData]
+        public void Project_new_ModelAggregated_should_work(
+            [Values(LinqProvider.V2, LinqProvider.V3)] LinqProvider linqProvider)
+        {
+            var collection = GetCollection(linqProvider);
+
+            var aggregate = collection.Aggregate()
+                .Group(
+                    x => "",
+                    g => new { Count = g.Count()}
+                )
+                .Project(
+                    x => new ModelAggregated
+                    {
+                        NotId = string.Empty,
+                        Count = x.Count
+                    });
+
+            var stages = Translate(collection, aggregate);
+            if (linqProvider == LinqProvider.V2)
+            {
+                AssertStages(
+                    stages,
+                    "{ $group : { _id : '', Count : { $sum : 1 } } }",
+                    "{ $project : { NotId : '', Count : '$Count', _id : 0 } }");
+            }
+            else
+            {
+                AssertStages(
+                    stages,
+                    "{ $group : { _id : '', __agg0 : { $sum : 1 } } }",
+                    "{ $project : { Count : '$__agg0', _id : 0 } }",
+                    "{ $project : { NotId : '', Count : 1, _id : 0 } }");
+            }
+        }
+
+        private IMongoCollection<Model> GetCollection(LinqProvider linqProvider)
+        {
+            var collection = GetCollection<Model>("test", linqProvider);
+            return collection;
+        }
+
+        private class Model
+        {
+            public string NotId { get; set; }
+            public string Name { get; set; }
+        }
+
+        private class ModelAggregated
+        {
+            public string NotId { get; set; }
+            public int Count { get; set; }
+        }
+    }
+}