Internal exception has occured: Type '<>f__AnonymousType2`6[...]' cannot be serialized

Question

I have written an endpoint in ASP.net (which should give back a nice JSON) and the following error puzzles me since via my Swagger interface, everything works fine, but if I call the endpoint directly in a browser with http://localhost:63291/api/AutoUpload/, I receive:

Internal exception has occured: Type '<>f__AnonymousType2`6[System.String,System.DateTime]' cannot be serialized. Consider marking it with the DataContractAttribute attribute, and marking all of its members you want serialized with the DataMemberAttribute attribute. If the type is a collection, consider marking it with the CollectionDataContractAttribute. See the Microsoft .NET Framework documentation for other supported types.

This seems strange, because I was thinking that I did indeed implement a parameter-less (default) constructor. My code reads as follows

using AutoMapper;
using myProject.API.Filters;
using myProject.API.Models;
using myProject.Entity.DAL;
using System;
using System.Collections.Generic;
using System.Configuration;
using System.IO;
using System.Linq;
using System.Net;
using System.Net.Http;
using System.Web.Http;
using System.Web.Http.Description;
namespace myProject.API.Controllers
{
    [UserAuthenticationFilter]
    public class AutoUploadController : BaseController
    {
        public AutoUploadController() {
            System.Diagnostics.Debug.WriteLine("Parameterless default constructor only for serialization");
        }

        // GET: api/AutoUpload
        [ResponseType(typeof(IEnumerable<FilesDetailsDto>))]
        public IHttpActionResult GetAutoUpload(string OptionString = "status")
        {
            if (OptionString == "status")
            {
                var rootLocation = ConfigurationManager.AppSettings["ROOT-FOLDER-LOCATION"];
                string[] entries = Directory.GetFiles(rootLocation, "*.csv", SearchOption.AllDirectories);
                var convList = entries.Select(x => new
                {
                    FullPath = x,
                    LastModifed = System.IO.File.GetLastWriteTime(x)
                });

                return Ok(convList.AsEnumerable());
            } // end IF-clause for optional parameter
            else
            {
                return NotFound();
            }
        }
    }
}

The above piece of code uses the following data transfer object (DTO) definition:

using System;

namespace myProject.API.Models
{
    public class FilesDetailsDto
    {
        public string FileName { get; set; }
        public DateTime LastModifiedOnFilesystem { get; set; }
    }
}

References

Datacontract exception. Cannot be serialized

score 2 · Accepted Answer · answered Dec 02 '19 at 10:28

You are returning collection of anonymous type instead of your actual type defined as attribute which can be the culprit for this error.

You can try to be more explicit when using the Select here something like:

var convList = entries.Select(x => new FilesDetailsDto()
            {
                FullPath = x,
                LastModifiedOnFilesystem = System.IO.File.GetLastWriteTime(x)
            });

Internal exception has occured: Type '<>f__AnonymousType2`6[...]' cannot be serialized

References

1 Answers1