Question

I have the following classes:

 public interface IWaybillDocument
    {
        long DocumentId { get; set; }
        long BillingDocumentId { get; set; }

        string ShipToName { get; set; }
        byte AddressType { get; set; }
        string City { get; set; }
        string Country { get; set; }
        string PostCode { get; set; }
        string StateRegion { get; set; }
        string Street1 { get; set; }
        string Street2 { get; set; }
        string Suburb { get; set; }

        void MergeAddressing(Address address);
    }
}

    public class WaybillDocumentList : ERPListBase<IWaybillDocument>
    {
        public WaybillDocumentList() : base() { }
    }

 public partial class WaybillDocument : IWaybillDocument, INonrepudiable
    {

        public void MergeAddressing(Address address)
        {
            address.Street1 = this.Street1;
            address.Street2 = this.Street2;
            address.Suburb = this.Suburb;
            address.City = this.City;
            address.ZipCode = this.PostCode;
            address.StateRegion = this.StateRegion;
            address.Country = this.Country;
            address.AddressKind = (AddressKind)this.AddressType;
        }
    }

They all compile nicely but when trying to apply this extension method:

 public static EntitySet<T> ToEntitySetFromInterface<T, U>(this IList<U> source)
            where T : class, U
        {
            var es = new EntitySet<T>();
            IEnumerator<U> ie = source.GetEnumerator();
            while (ie.MoveNext())
            {
                es.Add((T)ie.Current);
            }
            return es;
        }

Like this:

public void InsertWaybills(WaybillDocumentList waybills)
        {
            try
            {
                ;
                this.Context.WaybillDocuments.InsertAllOnSubmit(waybills.ToEntitySetFromInterface<WaybillDocument, IWaybillDocument>());
                this.Context.SubmitChanges();
            }
            catch (Exception ex)
            {
                throw new DALException("void InsertWaybills(WaybillList waybills) failed : " + ex.Message, ex);
            }
        }

I get a compilation error

The type ‘WaybillDocument’ cannot be used as type parameter ‘T’ in the
generic type or method
‘LinqExtensions.ToEntitySetFromInterface(System.Collections.Generic.IList)’.
There is no implicit reference conversion from ‘WaybillDocument’ to
‘IWaybillDocument’

with

waybills.ToEntitySetFromInterface<WaybillDocument, IWaybillDocument>()

underlined. Why is that when it clearly inherits the interface?

UPDATE: ERPListBase (details removed)

public class ERPListBase<T> : List<T>
{
    public ERPListBase()
        : base() {}
}

Answer 1

I’ve pasted your code to new project and with small modifications like ERPListBase -> List and it compiles. So my guess would be that you have two interfaces named IWaybillDocument and you are referencing wrong one of them somewhere.