How to use XPO in an ASP.NET (Web) application

What makes Web and Windows Forms applications different is that the former is a multi-threaded application. This is the point to be kept in mind when you create a Web ASP.NET project with eXpress Persistent Objects.

There are several basic rules to follow for XPO to work fine in your ASP.NET application.

1. Create a ThreadSafeDataLayer instance in the entry point of your application and assign it to the static XpoDefault.DataLayer property.

It's similar to Windows Forms development where it's A2944 recommended to initialize XpoDefault.DataLayer in the Main procedure. The differences include:
A) ThreadSafeDataLayer is to be used in ASP.NET, not in a SimpleDataLayer.
B) The database schema must be up-to-date and contain the XPObjectType table. An XPO connection provider must be created with the SchemaAlreadyExists parameter.
C) ThreadSafeDataLayer requires the XPO dictionary (stores persistent objects' metadata) to be initialized at the moment when ThreadSafeDataLayer is created.
D) The Web application's entry point is the Application_Start procedure in Global.asax. However, it's not recommended to use it to initialize XPO DataLayer, because website pages can be opened even if an exception has been thrown within Application_Start.

To sum it up, here is a template for an XpoHelper module, which you can import into your application:

	
[C#]
using System;

using DevExpress.Xpo;

using DevExpress.Xpo.DB;

using DevExpress.Xpo.Metadata;



public static class XpoHelper {

   public static Session GetNewSession() {

       return new Session(DataLayer);

   }



   public static UnitOfWork GetNewUnitOfWork() {

       return new UnitOfWork(DataLayer);

   }



   private readonly static object lockObject = new object();



   static volatile IDataLayer fDataLayer;

   static IDataLayer DataLayer {

       get {

           if(fDataLayer == null) {

               lock(lockObject) {

                   if(fDataLayer == null) {

                       fDataLayer = GetDataLayer();

                   }

               }

           }

           return fDataLayer;

       }

   }



   private static IDataLayer GetDataLayer() {

       XpoDefault.Session = null;

       string conn = MSSqlConnectionProvider.GetConnectionString("(local)", "XpoWebTest");

       XPDictionary dict = new ReflectionDictionary();

       IDataStore store = XpoDefault.GetConnectionProvider(conn, AutoCreateOption.SchemaAlreadyExists);

       dict.GetDataStoreSchema(typeof(PersistentObjects.Customer).Assembly);

       IDataLayer dl = new ThreadSafeDataLayer(dict, store);

       return dl;

   }

}

	
[VB.NET]
Imports System

Imports DevExpress.Xpo

Imports DevExpress.Xpo.DB

Imports DevExpress.Xpo.Metadata



Public NotInheritable Class XpoHelper

   Public Shared Function GetNewSession() As Session

       Return New Session(DataLayer)

   End Function



   Public Shared Function GetNewUnitOfWork() As UnitOfWork

       Return New UnitOfWork(DataLayer)

   End Function



   Private Shared lockObject As New Object()



   Private Shared fDataLayer As IDataLayer

   Private Shared ReadOnly Property DataLayer() As IDataLayer

       Get

           If fDataLayer Is Nothing Then

               SyncLock lockObject

                   If Thread.VolatileRead(fDataLayer) Is Nothing Then

                       Thread.VolatileWrite(fDataLayer, GetDataLayer())

                   End If

               End SyncLock

           End If

           Return fDataLayer

       End Get

   End Property



   Private Shared Function GetDataLayer() As IDataLayer

       XpoDefault.Session = Nothing



       Dim conn As String = MSSqlConnectionProvider.GetConnectionString("(local)", "XpoWebTest")

       Dim dict As XPDictionary = New ReflectionDictionary()

       Dim store As IDataStore = XpoDefault.GetConnectionProvider(conn, AutoCreateOption.SchemaAlreadyExists)

       dict.GetDataStoreSchema(GetType(PersistentObjects.Customer).Assembly)

       Dim dl As IDataLayer = New ThreadSafeDataLayer(dict, store)

       Return dl

   End Function

End Class

2. Prevent a mute XpoDefault.Session use by setting it to null (Nothing in VB.NET).

You should reset XpoDefault.Session to null, as shown in the above code snippet, to avoid a mute use of the default XPO session. XPO Session is not a thread-safe object and it mustn't be shared among pages and users in ASP.NET.

3. Acquire a UnitOfWork / Session instance from XpoHelper within the Page_Init method of your Web pages.

This requirement is almost the same as paragraph #4 in A2944. Being just a piece of advice for Windows Forms development, it's a mandatory requirement for ASP.NET: each Web page must use its own UnitOfWork or Session instance.

	
[C#]
using DevExpress.Xpo;



protected void Page_Init(object sender, EventArgs e) {

   Session session = XpoHelper.GetNewSession();

   XpoDataSource1.Session = session;

}

	
[VB.NET]
Imports DevExpress.Xpo



Protected Sub Page_Init(ByVal sender As Object, ByVal e As EventArgs)

   Dim session As Session = XpoHelper.GetNewSession()

   XpoDataSource1.Session = session

End Sub

In addition to the above 3 requirements, there are some recommendations. First of all, suggestions provided in the A2944 article are also applicable to Web development with XPO. Second, it's highly recommended that you create a separate assembly for your persistent classes (XPObject descendants). If you prefer to maintain all your code within a Web Site project, persistent classes must be placed under the App_Code subfolder. Third, it is necessary to set the XpoDefault.UseFastAccessors property to false and the XpoDefault.IdentityMapBehavior property to IdentityMapBehavior.Strong, if the web site is hosted in partial trust environment.

Please find a sample solution in the attachment. It consists of three projects:
DatabaseUpdater - an application, which creates an XpoWebTest database on your local MS SQL Server, creates schema, and fills it with some sample data. Data from this database is displayed in XpoWebApplication (see below).
PersistentObjects - a Class Library with persistent classes.
XpoWebApplication - a simple Web application which loads XPO data and displays it via a DataView. It includes the XpoHelper module and demonstrates how to implement a Page_Init method.

See Also:
A2944
Tutorial 5 - An ASP .NET Application for Data Editing
Tutorial 6 - A Simple Guestbook (ASP.NET)