This was actually reported by MARCOSB on GitHub to me in the comments for a pull request I made here: https://github.com/mongodb/mongo-csharp-driver/commit/5a62ca86fba984803e445a1d182bc125a56c7714

public interface IMyInterface
{
string SomeField

{ get; set; }
}

public class MyClass : IMyInterface
{
[BsonElement("bla")]
public string SomeField { get; set; }

}

public class HelperClass<T>
where T : IMyInterface
{
public void DoSomething()

{ var classMapForT = new BsonClassMap<T>(); classMapForT.AutoMap(); // This will throw an exception - but seems like a reasonable use-case, right? classMapForT.MapMember(t => t.SomeField); }

}

I believe it is a regression; I will issue a fix for this.