Work in progress.

Author: rstam

src/MongoDB.Driver/Linq/Linq3Implementation/KnownSerializerFinders/KnownSerializerFinderVisitMethodCall.cs

@@ -592,6 +592,18 @@ internal partial class KnownSerializerFinderVisitor
         MongoEnumerableMethod.StandardDeviationSampleSingleWithSelector,
     ];
 
+    private static readonly HashSet<MethodInfo> __stringConcatMethods =
+    [
+        StringMethod.ConcatWith1Object,
+        StringMethod.ConcatWith2Objects,
+        StringMethod.ConcatWith2Strings,
+        StringMethod.ConcatWith3Objects,
+        StringMethod.ConcatWith3Strings,
+        StringMethod.ConcatWith4Strings,
+        StringMethod.ConcatWithObjectArray,
+        StringMethod.ConcatWithStringArray
+    ];
+
     private static readonly HashSet<MethodInfo> __stringContainsMethods =
     [
         StringMethod.ContainsWithChar,
@@ -1307,7 +1319,7 @@ void DeduceAsQueryableMethodSerializers()
 
                 if (IsNotKnown(node) && IsItemSerializerKnown(sourceExpression, out var sourceItemSerializer))
                 {
-                    var resultSerializer = IQueryableSerializer.Create(sourceItemSerializer);
+                    var resultSerializer = NestedAsQueryableSerializer.Create(sourceItemSerializer);
                     AddKnownSerializer(node, resultSerializer);
                 }
             }
@@ -1439,6 +1451,10 @@ void DeduceConcatMethodSerializers()
                 DeduceCollectionAndCollectionSerializers(firstExpression, secondExpression);
                 DeduceCollectionAndCollectionSerializers(node, firstExpression);
             }
+            else if (method.IsOneOf(__stringConcatMethods))
+            {
+                DeduceReturnsStringSerializer();
+            }
             else
             {
                 DeduceUnknownMethodSerializer();

src/MongoDB.Driver/Linq/Linq3Implementation/KnownSerializerFinders/KnownSerializerFinderVisitNew.cs

@@ -68,7 +68,11 @@ protected override Expression VisitNew(NewExpression node)
 
         IBsonSerializer GetKnownSerializer(ConstructorInfo constructor)
         {
-            if (constructor.DeclaringType == typeof(BsonDocument))
+            if (constructor == null)
+            {
+                return null;
+            }
+            else if (constructor.DeclaringType == typeof(BsonDocument))
             {
                 return BsonDocumentSerializer.Instance;
             }