PersistentAliasAttribute Class

Indicates that a property is not persistent and its value is calculated based upon the values of a specific field(s).

Namespace: DevExpress.Xpo

Assembly: DevExpress.Xpo.v24.2.dll

Declaration

C#

[AttributeUsage(AttributeTargets.Property, Inherited = true)]
public sealed class PersistentAliasAttribute :
    Attribute

VB.NET

<AttributeUsage(AttributeTargets.Property, Inherited:=True)>
Public NotInheritable Class PersistentAliasAttribute
    Inherits Attribute

Remarks

The expression to be evaluated can be passed as the constructor’s parameter or specified via the PersistentAliasAttribute.AliasExpression property. You can use both persistent and non-persistent fields to specify the expressions. The expression is evaluated at the server side when only persistent properties are used. When a non-persistent property is referred, XPO extracts a persistent part from an expression, sends it to the database server, and processes the rest on the client. Note that this is only possible when an operation involves object loading and won’t work in XPView.

The following code shows how to define a property as non-persistent and calculate its value based upon the values of specific persistent fields. The ExtendedPrice property is marked as a non-persistent, using the PersistentAliasAttribute attribute. The expression to evaluate the property’s value is specified as the attribute’s parameter. In addition, the property’s getter evaluates this expression using the XPBaseObject.EvaluateAlias method.

C#

using DevExpress.Xpo;

public class Order : XPObject {
   public Order() {}
   public decimal UnitPrice {
       get { return fUnitPrice; }
       set { SetPropertyValue(nameof(UnitPrice), ref fUnitPrice, value); }
   }
   decimal fUnitPrice;

   public int Qty {
       get { return fQty; }
       set { SetPropertyValue(nameof(Qty), ref fQty, value); }
   }
   int fQty;

   public decimal Discount {
       get { return fDiscount; }
       set { SetPropertyValue(nameof(Discount), ref fDiscount, value); }
   }
   decimal fDiscount;


   [PersistentAlias("UnitPrice*Qty*(1-Discount)")]
   public decimal ExtendedPrice {
      get { return Convert.ToDecimal(EvaluateAlias(nameof(ExtendedPrice))); }
   }
}

VB.NET

Imports DevExpress.Xpo

Public Class Order
   Inherits XPObject
   Public Sub New()
   End Sub
   Public Property UnitPrice() As Decimal
       Get
           Return fUnitPrice
       End Get
       Set(ByVal value as Decimal)
           SetPropertyValue(NameOf(UnitPrice), fUnitPrice, value)
       End Set
   End Property
   Private fUnitPrice As Decimal

   Public Property Qty() As Integer
       Get
           Return fQty
       End Get
       Set(ByVal value as Integer)
           SetPropertyValue(NameOf(Qty), fQty, value)
       End Set
   End Property
   Private fQty As Integer

   Public Property Discount() As Decimal
       Get
           Return fDiscount
       End Get
       Set(ByVal value as Decimal)
           SetPropertyValue(NameOf(Discount), fDiscount, value)
       End Set
   End Property
   Private fDiscount As Decimal

   <PersistentAlias("UnitPrice*Qty*(1-Discount)")> _
   Public ReadOnly Property ExtendedPrice() As Decimal
      Get
         Return Convert.ToDecimal(EvaluateAlias(NameOf(ExtendedPrice)))
      End Get
   End Property
End Class

The following example shows how to use the PersistentAliasAttribute to implement a public read-only property exposing a value of the private persistent property. The expression passed in the PersistentAliasAttribute class constructor specifies the name of the field which stores the value at runtime. The field is decorated with the PersistentAttribute whose PersistentAttribute.MapTo property holds the name of the data store column where the field’s value is stored.

C#

using DevExpress.Xpo;

// ...
[Persistent("DefaultAddress")]
private string mDefaultAddress;

[PersistentAlias(nameof(mDefaultAddress))]
public string DefaultAddress {
    get {
        return mDefaultAddress;
    }
}

VB.NET

Imports DevExpress.Xpo

' ...
<Persistent("DefaultAddress")> _
Private mDefaultAddress As String

<PersistentAlias(NameOf(mDefaultAddress))> _
Public ReadOnly Property DefaultAddress() As String
   Get
      Return mDefaultAddress
   End Get
End Property

The PersistentAliasAttribute has the following limitations:

• While a criteria expression can return an object reference, this is not supported in all scenarios. You can return an object reference by directly referencing a property as follows:

  "Iif(Part is null, MyCustOrderLine.Part, Part)"

  In this snippet, the Part or MyCustOrderLine.Part property references the Part object and this object is returned correctly. However, you cannot retrieve reference properties from functions. The following expression does not work:

  "Iif(Part is null, MyCustOrderLine, MyCustOrderLine2).Part"

• The PersistentAlias expressions with more than 2 nesting levels of aggregate functions are not supported if the expression is calculated on the server, for example:

  Collection[X = ^.Collection[Y = ^.Collection[Y = M].Min(Y)].Max(X)].Single()

• In some cases, criteria that contains such expressions as SomeLogicalFunction(Type) and <Type>ObjectProperty may not work. In such cases, the evaluator handles both parts of the expression, and when it processes the <Type>ObjectProperty expression, the evaluator assumes that the object has an ObjectProperty. However, in the case of a class hierarchy, this behavior may be different. The following Criteria with upcasting does not work in PersistentCriteriaEvaluationBehavior.InTransaction mode:

  (IsInstanceOfType(This, 'TypeA') And <TypeA>BooleanPropertyA) Or (IsInstanceOfType(This, 'TypeB') And <TypeB>BooleanPropertyB)
  or
  IIf(IsInstanceOfType(This, 'TypeA'), <TypeA>BooleanPropertyA, <TypeB>BooleanPropertyB)

  Use the following equivalent Criteria instead:

  IIf(IsInstanceOfType(This, 'TypeA'), <TypeA>BooleanPropertyA, false)
  or
  IIf(IsInstanceOfType(This, 'TypeB'), <TypeB>BooleanPropertyB, false)

Note

PersistentAliasAttribute does not require database schema changes and does not create additional table columns. To use computed database columns, apply the FetchOnlyAttribute.

Inheritance

Object

Attribute

PersistentAliasAttribute