Backward Compatible Mongo fields using C# and .NET

Question

I have the following class which represents an entity in a MongoDB database:

public class Profile
{
    [BsonId]
    [BsonRepresentation(BsonType.ObjectId)]
    public string Id { get; set; } = null!;

    [BsonElement("FirstName")]
    [JsonPropertyName("FirstName")]
    public string? FirstName { get; set; }

    [BsonElement("LastName")]
    [JsonPropertyName("LastName")]
    public string? LastName { get; set; }
}

I have deprecated the FirstName and LastName fields using the [Obsolete] annotation and added a FullName field:

public class Profile
{
    [BsonId]
    [BsonRepresentation(BsonType.ObjectId)]
    public string Id { get; set; } = null!;

    // Deprecate
    [Obsolete]
    [BsonElement("FirstName")]
    [JsonPropertyName("FirstName")]
    public string? FirstName { get; set; }

    // Deprecate
    [Obsolete]
    [BsonElement("LastName")]
    [JsonPropertyName("LastName")]
    public string? LastName { get; set; }

    // New field
    [BsonElement("FullName")]
    [JsonPropertyName("FullName")]
    public string? FullName { get; set; }
}

The problem is that writing new objects to the MongoDB database still writes the old fields as null. I want to prevent this from happening without removing the fields from the class, because it needs to be backward compatible with the old objects in the database. How can I solve this?

Answer 1

You can add [BsonIgnoreIfNull] attribute to both FirstName and LastName so that when the property's value is null, the property will not be serialized and included when writing in the database.

public class Profile
{
    // Deprecate
    [Obsolete]
    [BsonElement("FirstName")]
    [BsonIgnoreIfNull]
    [JsonPropertyName("FirstName")]
    public string? FirstName { get; set; }

    // Deprecate
    [Obsolete]
    [BsonElement("LastName")]
    [BsonIgnoreIfNull]
    [JsonPropertyName("LastName")]
    public string? LastName { get; set; }

    ...
}