CSHARP-5929: Improve SerializerFinder to proper handling of IUnknowableSerializer

src/MongoDB.Driver/Linq/Linq3Implementation/SerializerFinders/SerializerFinderHelperMethods.cs

@@ -246,9 +246,10 @@ nodeSerializer is IBsonArraySerializer arraySerializer &&
         return false;
     }
 
-    private bool IsKnown(Expression node) => _nodeSerializers.IsKnown(node);
-
-    private bool IsKnown(Expression node, out IBsonSerializer nodeSerializer) => _nodeSerializers.IsKnown(node, out nodeSerializer);
+    private bool IsKnown(Expression node, out IBsonSerializer nodeSerializer)
+    {
+        return _nodeSerializers.IsKnown(node, out nodeSerializer) && nodeSerializer is not IUnknowableSerializer and not IIgnoreSubtreeSerializer;
+    }
 
     private bool IsNotKnown(Expression node) => _nodeSerializers.IsNotKnown(node);
 }

src/MongoDB.Driver/Linq/Linq3Implementation/SerializerFinders/SerializerFinderVisitUnary.cs

@@ -105,7 +105,9 @@ static IBsonSerializer GetTargetSerializer(UnaryExpression node, Type sourceType
                 if (IsConvertToNullableType(targetType, out var valueType))
                 {
                     var valueSerializer = valueType == targetType ? sourceSerializer : GetTargetSerializer(node, sourceType, valueType, sourceSerializer);
-                    return valueSerializer != null ? GetConvertToNullableTypeSerializer(node, sourceType, targetType, valueSerializer) : null;
+                    return valueSerializer is null or IUnknowableSerializer
+                        ? valueSerializer
+                        : GetConvertToNullableTypeSerializer(node, sourceType, targetType, valueSerializer);
                 }
 
                 // from here on we know there are no longer any Nullable<T> types involved
@@ -145,7 +147,7 @@ static IBsonSerializer GetTargetSerializer(UnaryExpression node, Type sourceType
                     return GetNumericConversionSerializer(node, sourceType, targetType, sourceSerializer);
                 }
 
-                return UnknowableSerializer.Create(node.Type);
+                return UnknowableSerializer.Create(targetType);
             }
 
             static IBsonSerializer GetConvertFromBsonValueSerializer(UnaryExpression expression, Type targetType)
@@ -210,7 +212,9 @@ static IBsonSerializer GetConvertFromNullableTypeSerializer(UnaryExpression expr
                 if (targetType.IsNullable(out var targetValueType))
                 {
                     var targetValueSerializer = GetTargetSerializer(expression, sourceValueType, targetValueType, sourceValueSerializer);
-                    return NullableSerializer.Create(targetValueSerializer);
+                    return targetValueSerializer is IUnknowableSerializer
+                        ? targetValueSerializer
+                        : NullableSerializer.Create(targetValueSerializer);
                 }
                 else
                 {

src/MongoDB.Driver/Linq/Linq3Implementation/SerializerFinders/SerializerFinderVisitor.cs

@@ -58,7 +58,7 @@ public void StartPass()
 
     public override Expression Visit(Expression node)
     {
-        if (IsKnown(node, out var nodeSerializer))
+        if (_nodeSerializers.IsKnown(node, out var nodeSerializer))
         {
             if (nodeSerializer is IIgnoreSubtreeSerializer or IUnknowableSerializer)
             {

src/MongoDB.Driver/Linq/Linq3Implementation/SerializerFinders/SerializerMap.cs

@@ -18,7 +18,6 @@
 using System.Linq.Expressions;
 using MongoDB.Bson.Serialization;
 using MongoDB.Bson.Serialization.Serializers;
-using MongoDB.Driver.Core.Misc;
 using MongoDB.Driver.Linq.Linq3Implementation.Misc;
 using MongoDB.Driver.Linq.Linq3Implementation.Serializers;
 
@@ -96,12 +95,7 @@ public bool HasResultSerializer(Expression node)
 
     public bool IsNotKnown(Expression node)
     {
-        return !IsKnown(node);
-    }
-
-    public bool IsKnown(Expression node)
-    {
-        return _map.ContainsKey(node);
+        return !IsKnown(node, out _);
     }
 
     public bool IsKnown(Expression node, out IBsonSerializer serializer)

tests/MongoDB.Driver.Tests/Linq/Linq3Implementation/Jira/CSharp5929Tests.cs

@@ -0,0 +1,73 @@
+/* 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 MongoDB.Driver.TestHelpers;
+using FluentAssertions;
+using MongoDB.Bson.Serialization;
+using MongoDB.Bson.Serialization.Attributes;
+using MongoDB.Driver.Linq;
+using Xunit;
+
+namespace MongoDB.Driver.Tests.Linq.Linq3Implementation.Jira;
+
+public class CSharp5929Tests : LinqIntegrationTest<CSharp5929Tests.ClassFixture>
+{
+    public CSharp5929Tests(ClassFixture fixture)
+        : base(fixture)
+    {
+    }
+
+    [Fact]
+    public void Test1()
+    {
+        var collection = Fixture.Collection;
+
+        var queryable = collection.AsQueryable()
+            .Select(x => x.Property[0] );
+
+        var exception = Record.Exception(() => queryable.ToList());
+        exception.Should().BeOfType<ExpressionNotSupportedException>();
+    }
+
+    [BsonSerializer(typeof(CSerializer))]
+    public class C
+    {
+        public long[] Property { get; set; }
+    }
+
+    public class CSerializer : IBsonDocumentSerializer, IBsonSerializer<C>
+    {
+        public Type ValueType => typeof(C);
+        public object Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args) => throw new NotImplementedException();
+        public void Serialize(BsonSerializationContext context, BsonSerializationArgs args, C value) => throw new NotImplementedException();
+
+        C IBsonSerializer<C>.Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args) => throw new NotImplementedException();
+        public void Serialize(BsonSerializationContext context, BsonSerializationArgs args, object value) => throw new NotImplementedException();
+
+        public bool TryGetMemberSerializationInfo(string memberName, out BsonSerializationInfo serializationInfo)
+        {
+            serializationInfo = null;
+            return false;
+        }
+    }
+
+    public sealed class ClassFixture : MongoCollectionFixture<C>
+    {
+        protected override IEnumerable<C> InitialData => null;
+    }
+}