How do I resolve a situation where one API accepts an object but the other does not.

See more:

C#

WebAPI

.NET-Core

I have two .NET Core 2.2 APIs both handling the same object in the same workflow (so they have the same auth token). The first one behaves as expected, the second refuses to fire giving Error: Model bound complex types must not be abstract or value types and must have a parameterless constructor.

This is the signature

[Authorize]
 [HttpPost("UpdateRegistration")]
 public async Task<IActionResult> UpdateRegistration(CUserDetails userDetails)
 {...}

This is the offending class:

   public class CUserDetails
    {

        #region CONSTRUCTORS

        /// <summary>
        /// Primary constructor
        /// </summary>
        /// <param name="arg_person">The details of the person to register</param>
        /// <param name="arg_username">the username to be assigned when creating the user</param>
        public CUserDetails(CPersonMasterBase arg_person, string arg_username)
        {
            this.person = arg_person;
            this.username = arg_username;

        }

        #endregion


        #region PUBLIC METHODS

        public string GetUsername()
        {
            return username;
        }

        public CPersonMasterBase GetPerson()
        {
            return person;
        }

        #endregion


        #region PUBLIC PROPERTIES
        public string username { get; set; }

        public CPersonMasterBase person { get; set; }

        #endregion
    }
}

The simple answer is to give it a parameterless constructor, but then I will not learn why it works in one case but not the other.

What I have tried:

So far just stepping in with the debugger, and examining the controller constructor. Found nothing to indicate why. I've done a search for userDetails in both solutions but found nothing to suggest why they are behaving differently.

At least one online solution just adds the parameterless constructor but that as I mentioned above does not explain the difference in behaviour.