How to: Use Read-Only Persistent Properties

Sep 15, 2020
3 minutes to read

In this document, you will learn how to define a read-only persistent property of a persistent object. The value of this property is set only once when the object is created. Thereafter, it is never changed. Generally, XPO stores only writable properties, but in certain scenarios, you may need to store the values of read-only properties in the database.

First, a property without a “setter” method in a persistent class must be created. By default, this property is non-persistent and its value can be stored in a private field. Private fields are also non-persistent members of an XPObject. To mark it as persistent, add a PersistentAttribute to the field with the name of the read-only property. This name will be used as the column’s name in the database table that corresponds to the persistent object. To avoid data store overhead, associate the public property with the persistent field using the PersistentAliasAttribute, as shown in the following code snippet.

C#
VB.NET

csharp

public class Client : XPObject {
    [Persistent("ClientID")]
    private string clientID;

    [PersistentAlias(nameof(clientID))]
    public string ClientID {
        get { return clientID; }
    }

    public Client(string clientID) {
        // The clientID is passed as a parameter in the persistent object's constructor.
        // Its value is specified only for a new object and cannot be changed afterwards.
        this.clientID = clientID;
    }

    public Client(Session session) : base(session) {}
}

Public Class Client
    Inherits XPObject
    <Persistent("ClientID")> _
    Private _clientID As String

    <PersistentAlias(NameOf(_clientID))> _
   Public ReadOnly Property ClientID() As String
        Get
            Return _clientID
        End Get
    End Property

    Public Sub New(ByVal clientID As String)
        ' The _clientID is passed as a parameter in the persistent object's constructor.
        ' Its value is specified only for a new object and cannot be changed afterwards.
        Me._clientID = clientID
    End Sub

    Public Sub New(ByVal session As Session)
        MyBase.New(session)
    End Sub
End Class

Note

The property is marked with the PersistentAliasAttribute, to be able to use it in filtering, searching, and inclusion in XPBaseCollection.DisplayableProperties.

Another useful example of using read-only persistent properties is saving the object’s creation date.

C#
VB.NET

csharp

public class Company: XPObject {
    // ...
    [Persistent("CreatedOn")]
    DateTime createdOn = DateTime.Today;
    // Marked as persistent to use the CreatedOn property in selection criteria.
    [PersistentAlias(nameof(createdOn))]
    public DateTime CreatedOn { get { return createdOn; }}
}

Public Class Company : Inherits XPObject
    ' ...
    <Persistent("CreatedOn")> _
    Private _createdOn As DateTime = DateTime.Today
    ' Marked as persistent to use the CreatedOn property in selection criteria
    <PersistentAlias(NameOf(createdOn))> _
    Public ReadOnly Property CreatedOn() As DateTime
        Get
            Return _createdOn
        End Get
    End Property
End Class