使用LINQ-to-Entities将结果从业务层返回到表示层的最佳方法

Question

I have a business layer that has DTOs that are used in the presentation layer. This application uses entity framework.

Here is an example of a class called RoleDTO:

public class RoleDTO
{
    public Guid RoleId { get; set; }
    public string RoleName { get; set; }
    public string RoleDescription { get; set; }
    public int? OrganizationId { get; set; } 
}

In the BLL I want to have a method that returns a list of DTO. I would like to know which is the better approach: returning IQueryable or list of DTOs. Although I feel that returning IQueryable is not a good idea because the connection needs to be open. Here are the 2 different methods using the different approaches:

First approach

public class RoleBLL
{
    private servicedeskEntities sde;

    public RoleBLL()
    {
        sde = new servicedeskEntities();
    }

    public  IQueryable<RoleDTO> GetAllRoles()
    {
        IQueryable<RoleDTO> role = from r in sde.Roles
                        select new RoleDTO()
                        {
                            RoleId = r.RoleID,
                            RoleName = r.RoleName,
                            RoleDescription = r.RoleDescription,
                            OrganizationId = r.OrganizationId
                        };
        return role;
    }

Note: in the above method the DataContext is a private attribute and set in the constructor, so that the connection stays opened.

Second approach

public static List<RoleDTO> GetAllRoles()
{
    List<RoleDTO> roleDTO = new List<RoleDTO>();
    using (servicedeskEntities sde = new servicedeskEntities())
    {
        var roles = from pri in sde.Roles
                         select new { pri.RoleID, pri.RoleName, pri.RoleDescription };

        //Add the role entites to the DTO list and return. This is necessary as anonymous types can be returned acrosss methods
        foreach (var item in roles)
        {
            RoleDTO roleItem = new RoleDTO();
            roleItem.RoleId = item.RoleID;
            roleItem.RoleDescription = item.RoleDescription;
            roleItem.RoleName = item.RoleName;
            roleDTO.Add(roleItem);

        }
        return roleDTO;
    }
}

Please let me know, if there is a better approach.

Answer 1

Its better not to send model objects directly to the presentation layer, you can have an intermediate layer where you map these DTO object to custom made objects that the presentation layer needs.

Which comes close to your second method, but not exactly the same.