Question

After almost 20 years using Visual FoxPro, I’m suffering with strong type language. :).

I’m trying to create a generic method, using one interface, to set a DataGrid.ItemsSource property.

Here we go…

First, I have a class (POCO):

public class PersonVO
{
    public int PersonID { get; set; }
    public string Name { get; set; }
}

Then I created an interface:

public interface IBussiness
{
    List<T> GetAll<T>();
}

And then, other class that implements this interface…

public class PersonBLL : IBussiness
{
    public List<PersonVO> CreateNewList()
    {
        List<PersonVO> list = new List<PersonVO>();
        list.Add(new PersonVO() { PersonID = 1, Name = "Robert" });
        list.Add(new PersonVO() { PersonID = 2, Name = "Julie" });
        list.Add(new PersonVO() { PersonID = 3, Name = "Bernard" });
        return list;
    }

    public List<T> GetAll<T>()
    {
        return CreateNewList();
    }
}

The statement return CreateNewList() shows an error:

C#: An instance of type
‘System.Collections.Generic.List’ can
not be returned by a method of type ‘System.Collections.Generic.List’

So I changed this method to:

public List<T> GetAll<T>()
{
    return CreateNewList() as List<T>;
}

It compiles!

Now, my problem starts… I have a WPF usercontrol MyTabItemList.

My point is, I create a new WPF TabItem at runtime, and inject my BLL class (in this example PersonBLL, but in real world I have a lot of them…).

MyTabItemList myTabItem = new MyTabItemList(new PersonBLL());

MyTabItemList is something like this:

public partial class MyTabItemList : TabItem
{
    IBussiness oBLL;
    public MyTabItemList(IBussiness oBLL)
    {
        InitializeComponent();
        this.oBLL = oBLL;
        MyGrid.ItemsSource = oBLL.GetAll<object>();
    }
}

It runs ok, but doesn’t work like I would like it to.

oBLL.GetAll<object>() always returns null.

If I change <object> to <PersonBLL> (like below), it works, but in this case MyTabItemList just works with PersonBLL class:

MyGrid.ItemsSource = oBLL.GetAll<PersonBLL>();

What should I do to make it work?

Answer 1

Why do you want to return a generic list from a none generic component like this?

public interface IBussiness
{
    List<T> GetAll<T>();
}

I think your purpose is:

public interface IBussiness<T>
{
    List<T> GetAll();
}

and make your PersonBLL implement IBussiness<PersonVO>:

public class PersonBLL : IBussiness<PersonVO>
{
    public List<PersonVO> CreateNewList() { ... }

    public List<PersonVO> GetAll()
    {
        return CreateNewList();
    }
}

That’s one of the normal patterns.