CSHARP-6223: Fix client-side projection crash on member-init projections

src/MongoDB.Driver/Linq/Linq3Implementation/Translators/ExpressionToAggregationExpressionTranslators/ClientSideProjectionRewriter.cs

@@ -89,6 +89,35 @@ public override Expression Visit(Expression node)
             return snippetReference;
         }
 
+        protected override Expression VisitMemberInit(MemberInitExpression node)
+        {
+            // don't let Visit replace the NewExpression with a snippet reference, which would
+            // break VisitAndConvert<NewExpression> in the base visitor; visit only the arguments
+            var newExpression = node.NewExpression;
+            var arguments = Visit(newExpression.Arguments);
+            if (arguments != newExpression.Arguments)
+            {
+                newExpression = newExpression.Update(arguments);
+            }
+
+            var bindings = Visit(node.Bindings, VisitMemberBinding);
+            return node.Update(newExpression, bindings);
+        }
+
+        protected override Expression VisitListInit(ListInitExpression node)
+        {
+            // same rationale as VisitMemberInit
+            var newExpression = node.NewExpression;
+            var arguments = Visit(newExpression.Arguments);
+            if (arguments != newExpression.Arguments)
+            {
+                newExpression = newExpression.Update(arguments);
+            }
+
+            var initializers = Visit(node.Initializers, VisitElementInit);
+            return node.Update(newExpression, initializers);
+        }
+
         protected override Expression VisitMethodCall(MethodCallExpression node)
         {
             // don't split OrderBy/ThenBy across the client/server boundary

tests/MongoDB.Driver.Tests/Linq/Integration/ClientSideProjectionWithMemberInitTests.cs

@@ -0,0 +1,197 @@
+/* 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;
+using System.Collections.Generic;
+using System.Linq;
+using FluentAssertions;
+using MongoDB.Bson;
+using MongoDB.Bson.Serialization.Attributes;
+using MongoDB.Driver.Linq;
+using MongoDB.Driver.TestHelpers;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Integration;
+
+// Regression tests for CSHARP-6223.
+public class ClientSideProjectionWithMemberInitTests : LinqIntegrationTest<ClientSideProjectionWithMemberInitTests.ClassFixture>
+{
+    private static readonly DateTime __createdOn = new(2026, 1, 15, 10, 30, 0, DateTimeKind.Utc);
+    private static readonly string __createdOnFormatted = __createdOn.ToLocalTime().ToString("yyyy-MM-dd");
+
+    public ClientSideProjectionWithMemberInitTests(ClassFixture fixture)
+        : base(fixture)
+    {
+    }
+
+    [Fact]
+    public void Find_with_member_init_projection_when_client_side_projections_are_disabled_should_throw()
+    {
+        var collection = Fixture.Collection;
+        var findOptions = new FindOptions { TranslationOptions = new ExpressionTranslationOptions { EnableClientSideProjections = false } };
+
+        var find = collection
+            .Find(Builders<UserDbo>.Filter.Empty, findOptions)
+            .Project(MemberInitProjectionExpression());
+
+        var exception = Record.Exception(() => TranslateFindProjection(collection, find));
+
+        exception.Should().BeOfType<ExpressionNotSupportedException>();
+        exception.Message.Should().Contain("not supported");
+    }
+
+    [Fact]
+    public void Find_with_member_init_projection_when_client_side_projections_are_enabled_should_work()
+    {
+        var collection = Fixture.Collection;
+        var findOptions = new FindOptions { TranslationOptions = new ExpressionTranslationOptions { EnableClientSideProjections = true } };
+
+        var find = collection
+            .Find(Builders<UserDbo>.Filter.Empty, findOptions)
+            .Project(MemberInitProjectionExpression());
+
+        var projection = TranslateFindProjection(collection, find);
+        projection.Should().Be("{ _snippets : ['$name', '$createdOn'], _id : 0 }");
+
+        var result = find.Single();
+        result.Name.Should().Be("Jane");
+        result.CreatedOn.Should().Be(__createdOnFormatted);
+    }
+
+    [Fact]
+    public void Find_with_nested_member_init_projection_when_client_side_projections_are_enabled_should_work()
+    {
+        var collection = Fixture.Collection;
+        var findOptions = new FindOptions { TranslationOptions = new ExpressionTranslationOptions { EnableClientSideProjections = true } };
+
+        var find = collection
+            .Find(Builders<UserDbo>.Filter.Empty, findOptions)
+            .Project(x => new OuterDto
+            {
+                Name = x.Name,
+                Inner = new InnerDto { Label = x.Name, CreatedOn = FormatCreatedOn(x.CreatedOn) }
+            });
+
+        var projection = TranslateFindProjection(collection, find);
+        projection.Should().Be("{ _snippets : ['$name', '$name', '$createdOn'], _id : 0 }");
+
+        var result = find.Single();
+        result.Name.Should().Be("Jane");
+        result.Inner.Label.Should().Be("Jane");
+        result.Inner.CreatedOn.Should().Be(__createdOnFormatted);
+    }
+
+    [Fact]
+    public void Find_with_collection_initializer_inside_member_init_when_client_side_projections_are_enabled_should_work()
+    {
+        var collection = Fixture.Collection;
+        var findOptions = new FindOptions { TranslationOptions = new ExpressionTranslationOptions { EnableClientSideProjections = true } };
+
+        var find = collection
+            .Find(Builders<UserDbo>.Filter.Empty, findOptions)
+            .Project(x => new DtoWithList
+            {
+                Name = x.Name,
+                Tags = new List<string> { x.Name, FormatCreatedOn(x.CreatedOn) }
+            });
+
+        var projection = TranslateFindProjection(collection, find);
+        projection.Should().Be("{ _snippets : ['$name', '$name', '$createdOn'], _id : 0 }");
+
+        var result = find.Single();
+        result.Name.Should().Be("Jane");
+        result.Tags.Should().Equal("Jane", __createdOnFormatted);
+    }
+
+    [Fact]
+    public void Queryable_Select_with_member_init_projection_when_client_side_projections_are_enabled_should_work()
+    {
+        var collection = Fixture.Collection;
+        var aggregateOptions = new AggregateOptions { TranslationOptions = new ExpressionTranslationOptions { EnableClientSideProjections = true } };
+
+        var queryable = collection.AsQueryable(aggregateOptions)
+            .Select(x => new UserDto
+            {
+                Name = x.Name,
+                CreatedOn = FormatCreatedOn(x.CreatedOn)
+            });
+
+        var stages = Translate(collection, queryable, out var outputSerializer);
+        AssertStages(stages, "{ $project : { _snippets : ['$name', '$createdOn'], _id : 0 } }");
+        outputSerializer.Should().BeAssignableTo<IClientSideProjectionDeserializer>();
+
+        var result = queryable.Single();
+        result.Name.Should().Be("Jane");
+        result.CreatedOn.Should().Be(__createdOnFormatted);
+    }
+
+    private static System.Linq.Expressions.Expression<Func<UserDbo, UserDto>> MemberInitProjectionExpression() =>
+        x => new UserDto
+        {
+            Name = x.Name,
+            CreatedOn = FormatCreatedOn(x.CreatedOn)
+        };
+
+    private static string FormatCreatedOn(BsonDateTime createdOn) =>
+        createdOn != null && createdOn.ToNullableLocalTime() != null
+            ? createdOn.ToNullableLocalTime().GetValueOrDefault().ToString("yyyy-MM-dd")
+            : string.Empty;
+
+    public class UserDbo
+    {
+        public ObjectId Id { get; set; }
+        [BsonElement("name")]
+        public string Name { get; set; }
+        [BsonElement("createdOn")]
+        public BsonDateTime CreatedOn { get; set; }
+    }
+
+    public class UserDto
+    {
+        public string Name { get; set; }
+        public string CreatedOn { get; set; }
+    }
+
+    public class OuterDto
+    {
+        public string Name { get; set; }
+        public InnerDto Inner { get; set; }
+    }
+
+    public class InnerDto
+    {
+        public string Label { get; set; }
+        public string CreatedOn { get; set; }
+    }
+
+    public class DtoWithList
+    {
+        public string Name { get; set; }
+        public List<string> Tags { get; set; }
+    }
+
+    public sealed class ClassFixture : MongoCollectionFixture<UserDbo>
+    {
+        protected override IEnumerable<UserDbo> InitialData =>
+        [
+            new UserDbo
+            {
+                Id = ObjectId.Parse("0102030405060708090a0b0c"),
+                Name = "Jane",
+                CreatedOn = new BsonDateTime(__createdOn)
+            }
+        ];
+    }
+}