How to Configure MongoDB C# Driver for Polymorphic Deserialization of Abstract Classes?

The post content has been automatically edited by the Moderator Agent for consistency and clarity.

I am using MongoDB with C# where I have an abstract base class, Test, and two concrete implementations (ConcreteTest1 and ConcreteTest2). The document stored in MongoDB contains a list of Test objects, but when retrieving it, I get the error:

System.FormatException: 'An error occurred while deserializing the Tests property of class TestScript: Cannot create an instance of Test because it is an abstract class.'

Here are my classes:

CSHARP
public abstract class Test { }

public class ConcreteTest1: Test
{
    public string Foo {get; set; }
    //...
}

public class ConcreteTest2: Test
{
    public int AnInt { get; set; }
    //...
}

public class TestScript
{
    public ObjectId _id { get; set; } = new();
    public string PartNumber { get; set; }
    public IList<Test> Tests { get; set; } = [];
}

The corresponding MongoDB document looks like this:

JSON
{
  "_id": {
    "$oid": "6780467598922ae42e13a832"
  },
  "PartNumber": "1234-12345-12",
  "Tests": [
    {  
      "_t": "ConcreteTest1",
      "Foo": "bar",
      ...
    },
    {
      "_t": "ConcreteTest2",
      "AnInt": 42
      ...
    }
  ]
}

My retrieval code is as follows:

CSHARP
var collection = db.GetCollection<TestScript>("TestScripts");

var testScript = await collection.Find(ts => ts.PartNumber == partNumber).SingleOrDefaultAsync(cancellationToken);
return testScript?.Tests ?? [];

The issue is that the deserializer is unaware of the concrete types for the abstract Test class. Since MongoDB inserts the type name as _t, there must be a way to instruct the deserializer to use this discriminator. How can I configure the MongoDB C# driver to correctly handle these child types during deserialization?

This is my first attempt at using MongoDB, so any advice on improving my approach is welcome.

Comments 0

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Answers 2

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2 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

You need to inform the MongoDB C# driver how to map the abstract type to its concrete implementations. The driver supports polymorphic serialization using discriminators. In your case the discriminator field is _t, so you can decorate your base class with attributes or configure class maps. Here are two common approaches:

1. Using Attributes

Decorate your abstract base class with the [BsonKnownTypes] attribute to list the derived classes. Optionally, you can also add [BsonDiscriminator] on your classes. For example:

CSHARP
using MongoDB.Bson.Serialization.Attributes;

[BsonKnownTypes(typeof(ConcreteTest1), typeof(ConcreteTest2))]
[BsonDiscriminator(Required = true)]
public abstract class Test
{
}

[BsonDiscriminator("ConcreteTest1")]
public class ConcreteTest1 : Test
{
    public string Foo { get; set; }
    // ...
}

[BsonDiscriminator("ConcreteTest2")]
public class ConcreteTest2 : Test
{
    public int AnInt { get; set; }
    // ...
}

This tells the driver that whenever it sees a document with _t set to "ConcreteTest1", it should deserialize it as a ConcreteTest1, and similarly for ConcreteTest2.

2. Using Class Map Registration

Alternatively, you can register class maps during application startup. This is useful if you prefer not to use attributes. For example:

CSHARP
using MongoDB.Bson.Serialization;

BsonClassMap.RegisterClassMap<Test>(cm =>
{
    cm.AutoMap();
    cm.SetIsRootClass(true);
    cm.AddKnownType(typeof(ConcreteTest1));
    cm.AddKnownType(typeof(ConcreteTest2));
});

BsonClassMap.RegisterClassMap<ConcreteTest1>(cm =>
{
    cm.AutoMap();
    cm.SetDiscriminator("ConcreteTest1");
});

BsonClassMap.RegisterClassMap<ConcreteTest2>(cm =>
{
    cm.AutoMap();
    cm.SetDiscriminator("ConcreteTest2");
});

Additional Note

Since your TestScript class contains an IList, when the driver reads the _t property for each document in the collection, it will now know which concrete type to instantiate. This should resolve the deserialization error about trying to create an instance of the abstract Test class.

Both methods are valid. Choose the one that best fits your project style and requirements.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

No comments yet.

Answer by EclipseObserver151 • 1 month ago

Try specifying the BsonKnownTypesAttribute:

CSHARP
[BsonKnownTypes(typeof(ConcreteTest1), typeof(ConcreteTest2))]
public abstract class Test { }

Alternatively if using attributes is not an option you can use BsonClassMap.RegisterClassMap<T>() to create the class map manually (for every class in hierarchy):

CSHARP
BsonClassMap.RegisterClassMap<Test>();
BsonClassMap.RegisterClassMap<ConcreteTest1>();
BsonClassMap.RegisterClassMap<ConcreteTest2>();

See:

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