Eager Loading in asp.net web api

Question

Hi community I recenty began learning how to code a web api,

I had a project in winform that I wanted to remake using web api, the thing is I can't use eager loading, I always get error 500, the code is like this:

public HttpResponseMessage GetExpediente()
    {
        db.Configuration.ProxyCreationEnabled = false;
        var expediente = db.Expediente.Include(x=>x.Documento);
        if (expediente.Any())
        {
            return Request.CreateResponse(HttpStatusCode.OK, expediente);
        }
        else
        {
            return Request.CreateErrorResponse(HttpStatusCode.NotFound,
                "No se encontraron expedientes.");
        }
    }

the error I always get is:

{"Message":"An error has occurred.","ExceptionMessage":"The 'ObjectContent`1' type failed to serialize the response body for content type 'application/json; charset=utf-8'.","ExceptionType":"System.InvalidOperationException","StackTrace":null,"InnerException":{"Message":"An error has occurred.","ExceptionMessage":"Self referencing loop detected for property 'Expediente' with type 'Modelo.Expediente'. Path '[0].Documento[0]'.","ExceptionType":"Newtonsoft.Json.JsonSerializationException","StackTrace":"   en Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.CheckForCircularReference(JsonWriter writer, Object value, JsonProperty property, JsonContract contract, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n   en Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.CalculatePropertyValues(JsonWriter writer, Object value, JsonContainerContract contract, JsonProperty member, JsonProperty property, JsonContract& memberContract, Object& memberValue)\r\n   en Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeObject(JsonWriter writer, Object value, JsonObjectContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n   en Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n   en Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer, IEnumerable values, JsonArrayContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n   en Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n   en Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeObject(JsonWriter writer, Object value, JsonObjectContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n   en Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n   en Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeList(JsonWriter writer, IEnumerable values, JsonArrayContract contract, JsonProperty member, JsonContainerContract collectionContract, JsonProperty containerProperty)\r\n   en Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.SerializeValue(JsonWriter writer, Object value, JsonContract valueContract, JsonProperty member, JsonContainerContract containerContract, JsonProperty containerProperty)\r\n   en Newtonsoft.Json.Serialization.JsonSerializerInternalWriter.Serialize(JsonWriter jsonWriter, Object value, Type objectType)\r\n   en Newtonsoft.Json.JsonSerializer.SerializeInternal(JsonWriter jsonWriter, Object value, Type objectType)\r\n   en System.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, Encoding effectiveEncoding)\r\n   en System.Net.Http.Formatting.JsonMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, Encoding effectiveEncoding)\r\n   en System.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStream(Type type, Object value, Stream writeStream, HttpContent content)\r\n   en System.Net.Http.Formatting.BaseJsonMediaTypeFormatter.WriteToStreamAsync(Type type, Object value, Stream writeStream, HttpContent content, TransportContext transportContext, CancellationToken cancellationToken)\r\n--- Fin del seguimiento de la pila de la ubicación anterior donde se produjo la excepción ---\r\n   en System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)\r\n   en System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)\r\n   en System.Web.Http.WebHost.HttpControllerHandler.<WriteBufferedResponseContentAsync>d__1b.MoveNext()"}}

I've been looking for info. that could help me but I always find that I should disable lazyloading, I did, and it didn't work, can anybody here help me?

Btw my objective is to consume this api from a windows form project, someone told me to do this:

var emp1 = response.Content.ReadAsStringAsync().Result;
        var result = JsonConvert.DeserializeObject<IEnumerable<ExpedientePOCO>>(emp1);

but what I get from the line of var emp1 = ... is an error 500. Oh and when I take the include from the context, it works just fine, but I want to use Eager loading hope you can help!

Thanks! Regards!

What I understood of DTOs:

 public HttpResponseMessage GetExpediente()
    {
        db.Configuration.ProxyCreationEnabled = false;
        var expediente = db.Expediente.Include(x=>x.Documento);
        List<ExpedientePOCO> expPOCO = new List<ExpedientePOCO>();
        foreach (var entidad in expediente)
        {
            String fecha = String.Format("{0:dd-MM-yyy}", entidad.FechaCreacion);
            ExpedientePOCO expedientePOCO = new ExpedientePOCO()
            {
                idExpediente = entidad.idExpediente,
                NombreExpediente = entidad.NombreExpediente,
                FechaCreacion = Convert.ToDateTime(fecha),
                DuenioExpediente = entidad.DuenioExpediente,
                CantidadDocumento = entidad.CantidadDocumento
            };
            foreach (var documentos in entidad.Documento)
            {
                DocumentoPOCO documento = new DocumentoPOCO()
                {
                    idDocumento = documentos.idDocumento,
                    idExpediente = documentos.idExpediente,
                    NombreDocumento = documentos.NombreDocumento,
                    FechaCreacion = documentos.FechaCreacion,
                    Expedientes = expedientePOCO
                };
                expedientePOCO.Documentos.Add(documento);
            }
            expPOCO.Add(expedientePOCO);
        }
        if (expPOCO.Any())
        {
            return Request.CreateResponse(HttpStatusCode.OK, expPOCO);
        }
        else
        {
            return Request.CreateErrorResponse(HttpStatusCode.NotFound,
                "No se encontraron expedientes.");
        }
    }

Answer 1

The problem is, as the error message states, that you have an endless loop, most likely something like this:

public class A
{
    public B B { get; set; }
}

public class B
{
    public virtual ICollection<A> A { get; set; }
}

So you get A => B => A endlessly, hence the error. You can solve this by configuring the serializer (ReferenceLoopHandling.Ignore in NewtonSoft.Json), or by using attributes ([JsonIgnore]), depending on your specific needs and tools used.

Another solution is to use Data Transfer Objects (DTOs) or similar:

public class ADTO
{
    // needed properties only
    public BDTO B { get; set; }
}

public class BDTO
{
    // needed properties only
    public List<ADTO> A { get; set; }
}

And then select into DTOs instead of entities:

var data = db.As
    .Select(a => new ADTO 
    {
        x = a.x ....
        B = new BDTO { x = a.B.x ...  }
    }
    .ToList();

Or, the other way around:

var data = context.B
    .Select(b => new BDTO
    {
        x = b.x ...
        A = b.A
            .Select(a => new ADTO
            {
                x = a.x ...
            }
            .ToList()
    }
    .ToList();