Use XPO Upcasting in XAF

• Oct 18, 2023
• 4 minutes to read

In this topic, you will learn how to use the Upcasting feature of XPO in XAF. It is useful when you need to combine base and derived classes in a single query.

The following business model will be implemented to demonstrate the UpCasting.

The code below describes the persistent class hierarchy:

C#

[DefaultClassOptions]
[System.ComponentModel.DefaultProperty(nameof(Title))]
   public class Department : BaseObject {
   private string title;
   private string office;
   public Department(Session session) : base(session) {}
   public string Title {
      get {return title;}
      set {
         SetPropertyValue(nameof(Title), ref title, value);
      }
   }
   public string Office {
      get {return office;}
      set {
         SetPropertyValue(nameof(Office), ref office, value);
      }
   }
   [Association("Department-Employees")]
   public XPCollection<EmployeeBase> Employees {
      get {return GetCollection<EmployeeBase>(nameof(Employees));}
   }
}

public class EmployeeBase : BaseObject {
   public EmployeeBase(Session session) : base(session) {}
   private string name;
   private string email;
   public string Name {
      get {return name;}
      set {
         SetPropertyValue(nameof(Name), ref name, value);
      }
   }
   public string Email {
      get {return email;}
      set {
         SetPropertyValue(nameof(Email), ref email, value);
      }
   }
   private Department department;
   [Association("Department-Employees")]
   public Department Department {
      get {return department;}
      set {
         SetPropertyValue(nameof(Department), ref department, value);
      }
   }
}

[DefaultClassOptions]
public class LocalEmployee : EmployeeBase {
   public LocalEmployee(Session session): base(session) {}
   private string insurancePolicyNumber;
   public string InsurancePolicyNumber {
      get {return insurancePolicyNumber;}
      set {
         SetPropertyValue(nameof(InsurancePolicyNumber), ref insurancePolicyNumber, value);
      }
   }
}

[DefaultClassOptions]
public class ForeignEmployee : EmployeeBase {
   public ForeignEmployee(Session session): base(session) {}    
   private DateTime visaExpirationDate;
   public DateTime VisaExpirationDate {
      get {return visaExpirationDate;}
      set {
         SetPropertyValue(nameof(VisaExpirationDate), ref visaExpirationDate, value);
      }
   }
}

VB.NET

<DefaultClassOptions, System.ComponentModel.DefaultProperty(NameOf(Title))> _
Public Class Department
      Inherits BaseObject
   Private fTitle As String
   Private fOffice As String
   Public Sub New(ByVal session As Session)
      MyBase.New(session)
   End Sub
   Public Property Title() As String
      Get
         Return fTitle
      End Get
      Set
         SetPropertyValue(NameOf(Title), fTitle, value)
      End Set
   End Property
   Public Property Office() As String
      Get
         Return fOffice
      End Get
      Set
         SetPropertyValue(NameOf(Office), fOffice, value)
      End Set
   End Property
   <Association("Department-Employees"))> _
   Public ReadOnly Property Employees() As XPCollection(Of EmployeeBase)
       Get
          Return GetCollection(Of EmployeeBase)(NameOf(Employees))
      End Get
   End Property
End Class

Public Class EmployeeBase
   Inherits BaseObject
   Public Sub New(ByVal session As Session)
      MyBase.New(session)
   End Sub
   Private fName As String
   Private fEmail As String
   Public Property Name() As String
      Get
         Return fName
      End Get
      Set(ByVal value As String)
         SetPropertyValue(NameOf(Name), fName, value)
      End Set
   End Property
   Public Property Email() As String
      Get
         Return fEmail
      End Get
      Set(ByVal value As String)
         SetPropertyValue(NameOf(Email), fEmail, value)
      End Set
   End Property
   Private fDepartment As Department
   <Association("Department-Employees", ImmediatePostData()> _
   Public Property Department() As Department
      Get
         Return fDepartment
      End Get
      Set(ByVal value As Department)
         SetPropertyValue(NameOf(Department), fDepartment, value)
      End Set
   End Property
End Class

<DefaultClassOptions()> _
Public Class LocalEmployee
   Inherits EmployeeBase
   Public Sub New(ByVal session As Session)
      MyBase.New(session)
   End Sub
   Private fInsurancePolicyNumber As String
   Public Property InsurancePolicyNumber() As String
      Get
         Return fInsurancePolicyNumber
      End Get
      Set(ByVal value As String)
         SetPropertyValue(NameOf(InsurancePolicyNumber), fInsurancePolicyNumber, value)
      End Set
   End Property
End Class

<DefaultClassOptions()> _
Public Class ForeignEmployee
   Inherits EmployeeBase
   Public Sub New(ByVal session As Session)
      MyBase.New(session)
   End Sub
   Private fVisaExpirationDate As DateTime
   Public Property VisaExpirationDate() As DateTime
      Get
         Return fVisaExpirationDate
      End Get
      Set(ByVal value As DateTime)
         SetPropertyValue(NameOf(VisaExpirationDate), fVisaExpirationDate, value)
      End Set
   End Property
End Class

In the code above, the Department, LocalEmployee and ForeignEmployee classes use the DefaultClassOptions attribute. To learn more, refer to the Data Annotations in Data Model topic.

Now, run the Windows Forms, ASP.NET Web, or ASP.NET Core Blazor application. Invoke a Department Detail View:

The nested Employees List View displays the properties of the EmployeeBase class only. This behavior is by design. However, it is better to display Employee class descendant specific properties in the Employees List View. With the UpCasting feature, this is accomplished with ease.

To add required columns to the List View that represents the Department.Employees collection, invoke the Model Editor. Locate the Views | Department_Employees_ListView | Columns node. Add two additional child nodes via the context menu. Specify their PropertyName property by the following values:

• <LocalEmployee>InsurancePolicyNumber
• <ForeignEmployee>VisaExpirationDate

These values will be recognized by XPO, and the LocalEmployee.InsurancePolicyNumber and ForeignEmployee.VisaExpirationDate properties will be displayed for objects retrieved from the database to the Department.Employees collection.

In addition, set the “Insurance Policy Number” and “Visa Expiration Date” values to the Caption property of the newly added columns.

Run the application and invoke the Department Detail View once again:

You can see that the properties of the EmployeeBase class descendants are displayed together with the EmployeeBase class properties.