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

• Aug 24, 2020
• 3 minutes to read

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:

C#

using DevExpress.Xpo;

//...

public class BaseClass : XPObject {
    public BaseClass(Session session) : base(session) { }
    public string BaseField {
        get { return fBaseField; }
        set { SetPropertyValue(nameof(BaseField), ref fBaseField, value); }
    }
    string fBaseField;

}

public class DerivedClass : BaseClass {
    public DerivedClass(Session session) : base(session) { }
    public string DerivedField {
        get { return fDerivedField; }
        set { SetPropertyValue(nameof(DerivedField), ref fDerivedField, value); }
    }
    string fDerivedField;

}

VB.NET

Imports DevExpress.Xpo

'...

Public Class BaseClass
  Inherits XPObject
    Public Sub New(ByVal session As Session)
        MyBase.New(session)
    End Sub
    Public Property BaseField() As String
        Get
            Return fBaseField
        End Get
        Set(ByVal value as String)
            SetPropertyValue(NameOf(BaseField), fBaseField, value)
        End Set
    End Property
    Private fBaseField As String

End Class

Public Class DerivedClass
  Inherits BaseClass
    Public Sub New(ByVal session As Session)
        MyBase.New(session)
    End Sub
    Public Property DerivedField() As String
        Get
            Return fDerivedField
        End Get
        Set(ByVal value as String)
            SetPropertyValue(NameOf(DerivedField), fDerivedField, value)
        End Set
    End Property
    Private fDerivedField As String

End Class

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:

C#

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;
    }
}

VB.NET

Imports DevExpress.Xpo

'...

Public Class MyXPCollection(Of BaseClass)
    Inherits XPCollection(Of BaseClass)
    Public Sub New(ByVal session As Session)
        MyBase.New(session)
    End Sub
    Protected Overrides Function CreateAddNewInstance() As Object
        Return New DerivedClass(Session)
    End Function
End Class

'...

Partial Public Class Form1
    Inherits Form
    '...
    Private Sub Form1_Load(ByVal sender As Object, ByVal e As EventArgs)
        '...
        Dim MySession As New Session()
        Dim xpCollection As MyXPCollection(Of BaseClass) = _
        New MyXPCollection(Of BaseClass)(MySession)
        gridControl1.DataSource = xpCollection
    End Sub
End Class

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