public class Customer

{

    public string Id { get; set; }

    public string ContactName { get; set; }

    public string CompanyName { get; set; }

}

public interface ICustomerDataAccess

{

    Customer Insert(string id, string contactName, string companyName);

    List<Customer> GetAllCustomer();

}

public class CustomerDataAccess : ICustomerDataAccess

{

public class CustomerLogic: ICustomerLogicLayer

{

    ICustomerDataAccess _dataLayer;

    public CustomerLogic(ICustomerDataAccess dataLayer)

    {

        _dataLayer = dataLayer;

    }

    public Customer Create(string customerName, string companyName)

    {          

        var id = Guid.NewGuid().ToString("N").Substring(0, 5).ToUpper();

        return _dataLayer.Insert(id, customerName, companyName);

    }

    public List<Customer> GetAllCustomers()

    {          

        return _dataLayer.GetAllCustomer().OrderBy(cust => cust.ContactName).ToList();

    }

}

public interface ICustomerLogicLayer

{

    Customer Create(string customerName, string companyName);

    List<Customer> GetAllCustomers();

}

var logic = new LogicLayer.CustomerLogic(new DataLayer.CustomerDataAccess());