c# - Mongo linq query ignores custom serialization of linq argument using linq provider v3 - Stack Overflow

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Issue

We have a custom serializer for an enum (MeasurementValueType), which stores the enum as a string in MongoDB and deserializes it from a string back into the enum when querying. The enum MeasurementValueType is declared as ushort. If it is not then everything works as expected. We are using Mongo C# driver version 2.30 and trying to upgrade to linq provider v# from V2.

Setup

A simple class with an enum property:

public class TestA
{
    public MeasurementValueType ValueType { get; set; }
}

Custom serializer for MeasurementValueType:

public class MeasurementValueTypeSerializer : StructSerializerBase<MeasurementValueType>
{
    public override void Serialize(BsonSerializationContext context, BsonSerializationArgs args, MeasurementValueType value)
    {
        BsonSerializer.Serialize(context.Writer, value.ToString());
    }

    public override MeasurementValueType Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
    {
        var measurementValueType = (MeasurementValueType)Enum.Parse(
            args.NominalType, 
            BsonSerializer.Deserialize<string>(context.Reader)
        );
        return measurementValueType;
     }
 }

Registered in startup:

BsonSerializer.RegisterSerializer(typeof(MeasurementValueType), new MeasurementValueTypeSerializer());

Expected Behavior

When saving a document, ValueType should be stored as a string in MongoDB. When querying, the deserializer should convert it back to the enum.When inserting an object the serializer is called and the enum is persisted as a string. When fetching it back as a whole document, the deserializer is called.

Problem

If I query using LINQ, the deserializer is not applied and no match is found:

database.TestACollection.Insert(new TestA { ValueType = MeasurementValueType.Angle });

var match = database.TestACollection
    .AsQueryable()
    .Single(x => x.ValueType == MeasurementValueType.Angle);

This worked fine in LINQ Provider v2, but in LINQ Provider v3, it no longer matches.

Question

Why isn't the right-hand expression (MeasurementValueType.Angle) passed to the serializer so it can be converted to a string before executing the query? I understand that we do not whant to deserialize the stored properties but my right-hand expression i.e. the enum could be serialized to the same format (string) for fast comparison on the database side.

Is there a workaround to ensure the serializer is respected in LINQ queries?

Can I register the serializer in a different way so that LINQ v3 applies it?

Issue

We have a custom serializer for an enum (MeasurementValueType), which stores the enum as a string in MongoDB and deserializes it from a string back into the enum when querying. The enum MeasurementValueType is declared as ushort. If it is not then everything works as expected. We are using Mongo C# driver version 2.30 and trying to upgrade to linq provider v# from V2.

Setup

A simple class with an enum property:

public class TestA
{
    public MeasurementValueType ValueType { get; set; }
}

Custom serializer for MeasurementValueType:

public class MeasurementValueTypeSerializer : StructSerializerBase<MeasurementValueType>
{
    public override void Serialize(BsonSerializationContext context, BsonSerializationArgs args, MeasurementValueType value)
    {
        BsonSerializer.Serialize(context.Writer, value.ToString());
    }

    public override MeasurementValueType Deserialize(BsonDeserializationContext context, BsonDeserializationArgs args)
    {
        var measurementValueType = (MeasurementValueType)Enum.Parse(
            args.NominalType, 
            BsonSerializer.Deserialize<string>(context.Reader)
        );
        return measurementValueType;
     }
 }

Registered in startup:

BsonSerializer.RegisterSerializer(typeof(MeasurementValueType), new MeasurementValueTypeSerializer());

Expected Behavior

When saving a document, ValueType should be stored as a string in MongoDB. When querying, the deserializer should convert it back to the enum.When inserting an object the serializer is called and the enum is persisted as a string. When fetching it back as a whole document, the deserializer is called.

Problem

If I query using LINQ, the deserializer is not applied and no match is found:

database.TestACollection.Insert(new TestA { ValueType = MeasurementValueType.Angle });

var match = database.TestACollection
    .AsQueryable()
    .Single(x => x.ValueType == MeasurementValueType.Angle);

This worked fine in LINQ Provider v2, but in LINQ Provider v3, it no longer matches.

Question

Why isn't the right-hand expression (MeasurementValueType.Angle) passed to the serializer so it can be converted to a string before executing the query? I understand that we do not whant to deserialize the stored properties but my right-hand expression i.e. the enum could be serialized to the same format (string) for fast comparison on the database side.

Is there a workaround to ensure the serializer is respected in LINQ queries?

Can I register the serializer in a different way so that LINQ v3 applies it?

• c#
• mongodb
• linq
• serialization
• driver

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1 Answer 1

Sorted by: Reset to default 0

If you do not use a custom serializer, but store the value as string by applying the BsonRepresentation attribute, the LINQ statement works as expected:

public enum TestEnum
{
    Value123 = 123
}

[BsonNoId]
[BsonIgnoreExtraElements]
public class TestDoc
{
    [BsonRepresentation(BsonType.String)]
    public TestEnum TestAsString { get; set; }

    [BsonElement]
    public TestEnum TestAsInt => TestAsString;
}
  
var val123 = await test  
                .AsQueryable()  
                .SingleAsync(x => x.TestAsString == TestEnum.Value123);  

This leads to the following document stored in the database:

{
  "_id": {
    "$oid": "67ef9018ea66d16ac7360313"
  },
  "TestAsString": "Value123",
  "TestAsInt": 123
}

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