Serialization/Deserialization of System.Type in WebAPI Project with resource model PUT

Code looks fine, this issue could be on the client side; i.e. how you're serializing it from the Front end/Client while calling/sending to the backend API.

Double check thats you are using contentType:"application/json" & using JSON.stringify method to convert it to JSON string when you send it.

Look here and here similar issue

Update 2: In your blazor entity try adding a key attribute

public class BlazorEntityModel{        [Key]  // add this to force it to recognize it as key    public int Id { get; set; } // Commenting this out makes PUT work.    public string FirstName { get; set; }    public string LastName { get; set; }    public string Address { get; set; }}

Update 3: Configure/override the serialization handling

Can you try to add the following in your global.asax

GlobalConfiguration.Configuration.Formatters.JsonFormatter.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore;  GlobalConfiguration.Configuration.Formatters.Remove(GlobalConfiguration.Configuration.Formatters.XmlFormatter);

If this works, let me know we may have to move to a DTO or VM so we trace and loops/circular refs.

One way to handle that is to use two classes with inheritance.

public class MyObject {   public string FirstName { get; set; }   public string LastName { get; set; }   public string Address { get; set; }}public class CreatedVersionOfMyObject : MyObject  {    public int Id { get; set; }}

Then your controller can easily expose multiples endpoints with those entitities

    [Route("objects")]    public class MyObjectControLler : Controller    {        [Route("{id}")]        [HttpGet]        [ProducesResponseType((int)HttpStatusCode.NotFound)]        [ProducesResponseType((int)HttpStatusCode.InternalServerError)]        [ProducesResponseType(typeof(CreatedVersionOfMyObject), (int)HttpStatusCode.OK)]        public Task<IActionResult> GetByIdAsync(Guid id)        {            // Do what you have to do to retrieve the object by id        }        [Route("{id}")]        [HttpDelete]        [ProducesResponseType((int)HttpStatusCode.NotFound)]        [ProducesResponseType((int)HttpStatusCode.InternalServerError)]        [ProducesResponseType((int)HttpStatusCode.NoContent)]        public Task<IActionResult> DeleteByidAsync(Guid id)        {            // Do what you have to do to delete the object by id        }        [Route("{id}")]        [HttpPut]        [ProducesResponseType((int)HttpStatusCode.NotFound)]        [ProducesResponseType((int)HttpStatusCode.InternalServerError)]        [ProducesResponseType(typeof(MyObject), (int)HttpStatusCode.OK)]        public Task<IActionResult> PutAsync([FromRoute]Guid id, [FromBody]MyObject theModifiedObject)        {            // Do what you have to do to retrieve the object by id        }        [Route("")]        [HttpPost]        [ProducesResponseType((int)HttpStatusCode.NotFound)]        [ProducesResponseType((int)HttpStatusCode.InternalServerError)]        [ProducesResponseType(typeof(CreatedVersionOfMyObject), (int)HttpStatusCode.Created)]        public Task<IActionResult> PostAsync(MyObject theModifiedObject)        {            // Do what you have to do to create the object and return the version with the id        }    }

Finally, regarding the mapping you can also easily handle that (see original doc https://docs.automapper.org/en/stable/Mapping-inheritance.html)

CreateMap<BaseEntity, BaseDto>()   .ForMember(dest => dest.SomeMember, opt => opt.MapFrom(src => src.OtherMember));CreateMap<DerivedEntity, DerivedDto>()    .IncludeBase<BaseEntity, BaseDto>();