Question

Model

public partial class SayacOkumalariIstatistik
{
    public string sno { get; set; }
    public Nullable<System.DateTime> okuma_tarihi { get; set; }
    public Nullable<decimal> TotalUsageValue { get; set; }
    public string UsageType { get; set; }
}

function

public IList<SayacOkumalariIstatistik> SayacOkumalariGetir()
{
    #region linq concat
    IEnumerable<SayacOkumalariIstatistik> sayac_okumalari_istatistik = entity.TblSayacOkumalari
        .Select(x => new SayacOkumalariIstatistik
        {
            sno = x.sno.ToString() + "T1",
            okuma_tarihi = x.okuma_tarihi,
            TotalUsageValue = x.toplam_kullanim_T1,
            UsageType = "T1"
        })
        .Concat(entity.TblSayacOkumalari.Select(x => new SayacOkumalariIstatistik
         {
             sno = x.sno.ToString() + "T2",
             okuma_tarihi = x.okuma_tarihi,
             TotalUsageValue = x.toplam_kullanim_T2,
             UsageType = "T2"
         }))
         .Concat(entity.TblSayacOkumalari.Select(x => new SayacOkumalariIstatistik
         {
              sno = x.sno.ToString() + "T3",
              okuma_tarihi = x.okuma_tarihi,
              TotalUsageValue = x.toplam_kullanim_T3,
              UsageType = "T3"
         }));
        #endregion

        // Error occurs in here
        return sayac_okumalari_istatistik.ToList();
    }

And Error message is my question title. Why this error occur.

Thanks.

Answer 1

The error message is self-explanatory. You can’t and shouldn’t use your entity classes as projection DTOs (I’m guessing SayacOkumalariIstatistik is such a class)

You have at least 3 alternatives:

1. Retrieve the original entities first, then do the projections using Linq to objects (i.e. after the ToList call).
2. Create a DTO class (SayacOkumalariIstatistikViewModel, SayacOkumalariIstatistikDTO, whatever) and use that in your query.
3. Use anonymous objects for the projection.