From 71382faea66c0c6fefb7e0fe4ad5b7c813111a5a Mon Sep 17 00:00:00 2001
From: James Kovacs <jkovacs@post.harvard.edu>
Date: Mon, 27 May 2024 16:46:59 -0600
Subject: [PATCH] CSHARP-2187: Flatten CombinedProjectionDefinitions to reduce
 recursive calls to Render.

---
 .../ProjectionDefinitionBuilder.cs            | 13 ++++++++++++-
 .../ProjectionDefinitionBuilderTests.cs       | 19 +++++++++++++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/src/MongoDB.Driver/ProjectionDefinitionBuilder.cs b/src/MongoDB.Driver/ProjectionDefinitionBuilder.cs
index 5fc7d5c4cf3..34971cd803d 100644
--- a/src/MongoDB.Driver/ProjectionDefinitionBuilder.cs
+++ b/src/MongoDB.Driver/ProjectionDefinitionBuilder.cs
@@ -789,7 +789,18 @@ internal sealed class CombinedProjectionDefinition<TSource> : ProjectionDefiniti
 
         public CombinedProjectionDefinition(IEnumerable<ProjectionDefinition<TSource>> projections)
         {
-            _projections = Ensure.IsNotNull(projections, nameof(projections)).ToList();
+            // Unwind CombinedProjectionDefinitions to avoid deep recursion on Render
+            _projections = Ensure.IsNotNull(projections, nameof(projections))
+                .Aggregate(new List<ProjectionDefinition<TSource>>(), (current, projection) =>
+                {
+                    if (projection is CombinedProjectionDefinition<TSource> combinedProjection)
+                    {
+                        current.AddRange(combinedProjection._projections);
+                    } else
+                        current.Add(projection);
+                    return current;
+                })
+                .ToList();
         }
 
         public override BsonDocument Render(IBsonSerializer<TSource> sourceSerializer, IBsonSerializerRegistry serializerRegistry, LinqProvider linqProvider)
diff --git a/tests/MongoDB.Driver.Tests/ProjectionDefinitionBuilderTests.cs b/tests/MongoDB.Driver.Tests/ProjectionDefinitionBuilderTests.cs
index 5f75bb8f8af..f3320e3723c 100644
--- a/tests/MongoDB.Driver.Tests/ProjectionDefinitionBuilderTests.cs
+++ b/tests/MongoDB.Driver.Tests/ProjectionDefinitionBuilderTests.cs
@@ -14,6 +14,7 @@
 */
 
 using System.Linq;
+using System.Text;
 using FluentAssertions;
 using MongoDB.Bson;
 using MongoDB.Bson.Serialization;
@@ -61,6 +62,24 @@ public void Combine_with_redundant_fields_using_extension_method()
             Assert(projection, "{LastName: 0, fn: 1}");
         }
 
+        [Fact]
+        public void Combine_many_fields_should_not_overflow_stack_on_Render()
+        {
+            var subject = CreateSubject<Person>();
+
+            var projection = subject.Include(x => x.FirstName);
+            var expectedProjection = new StringBuilder("{fn: 1");
+            for (int i = 0; i < 10000; i++)
+            {
+                var field = $"Field{i}";
+                projection = projection.Include(field);
+                expectedProjection.Append($", {field}: 1");
+            }
+            expectedProjection.Append("}");
+
+            Assert(projection, expectedProjection.ToString());
+        }
+
         [Fact]
         public void ElemMatch()
         {