C# 9 introduced return type covariance, and that allows us to override properties with more specific types. MongoDB's driver can't serialize such properties at the moment.

Example:

{}

public class Entity
{
    public virtual object SomeProperty { get; set; }
}

{{And:}}
{}

public class User : Entity
{
    public override string SomeProperty { get; set; }
}

{{The Mongo BSON serializer thinks these are two separate properties, and it throws an exception because there are two properties that map to the same element.}}

(BTW, I'm new here so if I did something wrong with this feature request please let me know){}