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System.NotSupportedException: A serializer of type ObjectSerializer is not configurable using an attribute of type BsonRepresentationAttribute

Issue Description

MongoDB C# driver gives below error,

System.NotSupportedException: A serializer of type ObjectSerializer is not configurable using an attribute of type BsonRepresentationAttribute

Resolution

This error I found to be due to a misconfiguration on ObjectId with other .NET primitives like int or object or byte etc. When using ObjectId as BsonType within BsonRepresentation, it should be defined as ObjectId or string type.

This issue can also be resolved by removing [BsonRepresentation] attributes from properties that are decorated with these attributes unnecessarily.

[BsonRepresentation] 

This attribute lets you cast internally the Mongo type vs primitive types like string, int, etc. whenever there is conversion required.

Such conversion can be done at the .NET code level.

As good practices one should use [BsonRepresentation(BsonType.ObjectId)] attribute only on “Id” attributes.

Example

 public class Book
    {
        [BsonId]
        [BsonRepresentation(BsonType.ObjectId)]
        public ObjectId Id { get; set; }

        public string Name { get; set; }
     }

If the above code is used for other types like public Object Id or public string Id or public int Id then code will throw below error respectively,

Fails : If used as ‘public Object Id ‘

Example:

public class Book
    {
        [BsonId]
        [BsonRepresentation(BsonType.ObjectId)]
        public Object Id { get; set; }

        public string Name { get; set; }
     }

Below error will be reported,

System.NotSupportedException: A serializer of type ObjectSerializer is not configurable using an attribute of type BsonRepresentationAttribute

Fails : If used as ‘public int Id’

Example:

public class Book
    {
        [BsonId]
        [BsonRepresentation(BsonType.ObjectId)]
        public int Id { get; set; }

        public string Name { get; set; }
     }

Below error will be reported,

System.ArgumentException: ‘ObjectId is not a valid representation for an Int32Serializer.’

Passes: If used as public ObjectId Id

Example:

One can fix the issue using ObjectId or string as type,

public class Book
    {
        [BsonId]
        [BsonRepresentation(BsonType.ObjectId)]
        public ObjectId Id { get; set; }

        public string Name { get; set; }
   }

Passes: If used as public string Id

Example:

public class Book
    {
        [BsonId]
        [BsonRepresentation(BsonType.ObjectId)]
        public string Id { get; set; }

        public string Name { get; set; }
    }

Passes: If used as int Id

Example:

public class Book
    {
        [BsonId]
        [BsonRepresentation(BsonType.IntId)]
        public string Id { get; set; }

        public string Name { get; set; }
    }

• This conversion passes just fine as there is the easy conversion that exists between ObjectId and String.

• When using ‘string‘ Mongo will auto-generate ObjectId by itself and will not report an error. Here String externally generates as an ObjectId.

• In such a case, the serializer will convert the ObjectId to a string (when reading data from the database) and convert the string back to an ObjectId (when writing data to the database).

Do you have any comments or ideas or any better suggestions to share?

Please sound off your comments below.

Happy Coding !!

References:

• MongoDB using _id Field – Guidelines

• MongoDB Naming Standards and Guidelines

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