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How to: Make XPCollection Create Objects of Specific Type when They are Created via Bound Controls

In certain scenarios, a collection may be designed to hold objects of derived types, but must be base-typed. To simplify working in such scenarios, the XPCollection provides a way to create derived-type objects via bound controls when the collection itself is base-typed.

To make a collection create objects of a specific type, derive from the XPCollection class, and override the protected virtual XPBaseCollection.CreateAddNewInstance method, to return an object of the required type. This method is automatically called, for example, when the Append button of the Grid control's embedded data navigator is clicked.

The following code snippets illustrate this. The sample Windows Forms application has two business classes defined - the BaseClass and DerivedClass:


using DevExpress.Xpo;

//...

public class BaseClass : XPObject {
    public BaseClass(Session session) : base(session) { }
    public string BaseField ;       
}

public class DerivedClass : BaseClass {
    public DerivedClass(Session session) : base(session) { }
    public string DerivedField;
}

The application contains a single Form1 form. On the form there is the gridControl1 XtraGrid control with the enabled data navigator. The XtraGrid control is bound to a MyXPCollection<BaseClass>-type collection, which derives from the XPCollection<BaseClass>. The derived collection has the CreateAddNewInstance method overridden, to return a new DerivedClass-type object:


using DevExpress.Xpo;

//...

public class MyXPCollection<BaseClass> : XPCollection<BaseClass> {
    public MyXPCollection(Session session) : base(session) { }
    protected override object CreateAddNewInstance() {
        return new DerivedClass(Session);
    }
}

//...

public partial class Form1 : Form {
    //...
    private void Form1_Load(object sender, EventArgs e) {
        //...
        Session MySession = new Session();
        MyXPCollection<BaseClass> xpCollection = 
            new MyXPCollection<BaseClass>(MySession);
        gridControl1.DataSource = xpCollection;
    }
}

So, the collection is BaseClass-typed, but objects created via the XtraGrid control's embedded data navigator will be DerivedClass-typed.