How to use Func with Entity Data Context?

Answer 1

I think one of your initial problems is that your ILoad interface is non-generic, but you have specified methods without generic constraints, e.g.

List<TResult> Load<TKey, TResult>(Func<TKey, TResult> select);

Firstly, predicates must evaluate to true of false, so it should really be Func<TModel, bool> select where TModel is your entity type.

You are then trying to do this:

List<TResult> Load<TKey, TResult>(Func<TKey, TResult> select)
{
        using (Erp.DAL.ErpEntities erpEntityCtx = new Erp.DAL.ErpEntities())
        {
            return erpEntityCtx.Customer.Select(select).ToList<TResult>();
        }
}

Without specifying any constraints on your generic methods (or better yet, your generic class), you can't satisfy the Func with Func.

You need to potentially change to this:

public interface IRepository<TModel>
{
   List<TModel> Get(Func<TModel, bool> predicate);
}

And implement as:

public class CustomerRepository : IRepository<Customer>
{
    public List<Customer> Get(Func<Customer, bool> predicate)
    {
        using (var context = new ErpEntities())
        {
             return context.Customers.Where(predicate).ToList();
        }
    }
}

You should have a read up on the repository pattern.