我应用了这个层次:

• Entities
• Database (with Entities reference)
• Business (with database and Entities references)
• User Interface (with Business and Entities references)

  ---

我的守则就是这方面的例子:

• UserDAL class in database layer:

public class UsersDal
{
    databaseDataContext db;
    public UsersDal()
    {
        try
        {
            db = new databaseDataContext(ConnectToDatabase.GetConnectionString());
        }
        catch (Exception ex)
        {
            throw ex;
        }
    }
    public List<User> GetAllUsers()
    {
        try
        {
            return (from u in db.Users select u).ToList();
        }
        catch (Exception ex)
        {
            throw ex;
        }
    }

......

在商层用户BL类类中,如:

public class UsersBll
{
    UsersDal user;
    public UsersBll()
    {
        try
        {
            user = new UsersDal();
        }
        catch(Exception ex)
        {
            throw new ProjectException(Errors.CannotCreateObject, ex);
        }
    }
    public List<User> GetAllUsers()
    {
        try
        {
            return user.GetAllUsers();
        }
        catch(Exception ex)
        {
            throw new ProjectException(Errors.CannotReadData, ex);
        }
    }

并且,在《国际调查》一书中写:

    private void GetUsers()
    {
        try
        {
            UsersBll u = new UsersBll();
            datagrid.DataSource = u.GetAllUsers();
        }
        catch(ProjectException ex)
        {
            MessageBox(ex.UserMessage);// and also can show ex.InnerException.Message for more info
        }
    }

Well, I use a ProjectException named class to produce an error contain a BLL created message by me and an Exception message that the OS automatically manipulate. Also i create an enum of possible errors and a dictionary here is some details about it:

namespace Entities
{
    public enum Errors
    {
        CannotCreateObject,
        CannotReadData,
        CannotAdd,
        CannotEdit,
        CannotDelete,...
    }

[global::System.Serializable]
public class ProjectException : Exception
{
    public ProjectException(Errors er, Exception ex)
        : base(errors[er], ex)
    {
        currentEx = er;//er is Errors enum
    }
    static ProjectException()
    {
        errors = new Dictionary<Errors, string>();
        errors.Add(Errors.CannotCreateObject, "the application cannot connect to database!");
        errors.Add(Errors.CannotReadData, "the application cannot read data from database"); //...
    }
    public string UserMessage
    {
        get
        {
            try
            {
                return errors[currentEx];
            }
            catch
            {
                return "Unknown error!";
            }
        }
    }

Is this good? it work for me fine. what s your idea?