The expression to be evaluated can be passed as the constructor‘s parameter or specified via the 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.

 Example

using DevExpress.Xpo;

public class Order : XPObject {
   public Order() {}
   public decimal UnitPrice;
   public int Qty;
   public decimal Discount;

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

Note that while a criteria expression can return an object reference, this is not supported in all scenarios. Returning an object reference by directly referencing a property, as in the following code snippet, is fully supported.

请注意, 虽然条件表达式可以返回对象引用, 但在所有方案中不支持此项。完全支持通过直接引用属性 (如下面的代码段) 返回对象引用。

[PersistentAlias("Iif(Part is null, MyCustOrderLine.Part, Part)")]
public Part CustomPart {
    get {
        return (Part)EvaluateAlias("CustomPart");
    }
}

In this code snippet, a Part object referenced by the Part or MyCustOrderLine.Part property will be returned correctly. However, retrieving reference properties from function and aggregation operators is not supported. So, the following persistent alias declaration will not work.

在此代码段中, Part对象引用Part或MyCustOrderLine.Part属性,部件属性将正确返回。

但是, 不支持从函数和聚合运算符检索引用属性。因此, 下列持久性别名声明将不起作用。

[PersistentAlias("Iif(Part is null, MyCustOrderLine, MyCustOrderLine2).Part")]
public Part CustomPart {
    get {
        return (Part)EvaluateAlias("CustomPart");
    }
}

The following example shows how to apply the PersistentAliasAttribute to the DefaultAddress property. When initializing a new instance of the PersistentAliasAttribute class, the name of the field which stores theDefaultAddress property‘s value is specified. The mDefaultAddress field has the PersistentAttribute whose PersistentAttribute.MapTo property holds the name of the column in the data store where the field‘s value is stored.

下面的示例演示如何将 PersistentAliasAttribute 应用于 DefaultAddress 属性。在初始化 PersistentAliasAttribute 类的新实例时, 将指定存储 theDefaultAddress 属性值的字段的名称。mDefaultAddress 字段具有 PersistentAttribute, 其 PersistentAttribute. MapTo 是字段名。

using DevExpress.Xpo;

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

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

二,CalculatedAttribute - [XAF提供]

using DevExpress.ExpressApp.DC;
// ... 
public class Person {    
    // ... 
    public String FirstName { get; set; }
    public String LastName { get; set; }
    [Calculated("FirstName + ‘ ‘ + LastName")]
    public String FullName {
        get {
            return FirstName + " " + LastName;
        }
    }
}

The complete sample project is available in the DevExpress Code Examples database at http://www.devexpress.com/example=E1771.

[CalculatedPersistentAlias("ProductName", "ProductNameAlias"), DefaultClassOptions]
//ProductName 这个属性的值将来从ProductNameAlias中取得公式内容.
public class Product : BaseObject {
    public Product(Session session) : base(session) { }

    private static string productNameFormat = "{Manufacturer} {Model}";
    private static string productNameAlias = "concat(Manufacturer, Model)";
        
    public string ProductName {
        get { return ObjectFormatter.Format(
            productNameFormat, this, EmptyEntriesMode.RemoveDelimiterWhenEntryIsEmpty ); }
    }
    [Browsable(false)]
    public static string ProductNameAlias {
        get { return productNameAlias; }            
    }
    public static void SetProductNameFormat(string productNameFormat, string productNameAlias) {
        if(!string.IsNullOrEmpty(productNameFormat)) {
            Product.productNameFormat = productNameFormat;
        }
        if(!string.IsNullOrEmpty(productNameAlias)) {
            Product.productNameAlias = productNameAlias;
        }
    }

    public string Manufacturer {
        get { return GetPropertyValue<string>("Manufacturer"); }
        set { SetPropertyValue<string>("Manufacturer", value); }
    }
    public string Model {
        get { return GetPropertyValue<string>("Model"); }
        set { SetPropertyValue<string>("Model", value); }
    }
    public string Revision {
        get { return GetPropertyValue<string>("Revision"); }
        set { SetPropertyValue<string>("Revision", value); }
    }
}

public sealed partial class CalculatedAliasModule : ModuleBase {
    public CalculatedAliasModule() {
        InitializeComponent();
         
        Product.SetProductNameFormat(    
            ConfigurationManager.AppSettings["ProductNameFormat"],    
            ConfigurationManager.AppSettings["ProductNameAlias"]    
            );   
    }
}

using DevExpress.ExpressApp.Xpo;
// ... 
public override void CustomizeTypesInfo(ITypesInfo typesInfo) {
    base.CustomizeTypesInfo(typesInfo);
    CalculatedPersistentAliasHelper.CustomizeTypesInfo(typesInfo);
}