TitleUnassignedId Field

A special default value for the id property to indicate that it has not been assigned a value. Null can't be used because this project is compiled with nullable=true and an attempt to pass a null id into the web service generates a validation error. There is a [ValidateNever] attribute to workaround this problem, but it exists in the Microsoft.AspNetCore.App package which is too heavy for this shared library.

public const string UnassignedId = "_unassigned_id"