Querying external data sources in XAF

Querying external data sources in XAF

I will explain what XAF is just for the sake of the consistency of this article, XAF is a low code application framework for line of business applications that runs on NET framework (windows forms and web forms) and in dotnet (windows forms, Blazor and Web API)

XAF is laser focus on productivity, DevExpress team has created several modules that encapsulate design patterns and common tasks needed on L.O.B apps.

The starting point in XAF is to provide a domain model using an ORMs like XPO or Entity framework and then XAF will create an application for you using the target platform of choice.

It’s a common misunderstanding that you need to use and ORM in order to provide a domain model to XAF

DevExpress team has created ObjectSpace abstraction so XAF can be extended to use different data access technologies ( you can read more about it here https://docs.devexpress.com/eXpressAppFramework/DevExpress.ExpressApp.BaseObjectSpace)

Out of the box XAF provide 3 branches of object spaces as show is the graph below.

XPObjectSpace: this is the object space that allows you to use XPO as a data access technology.

EfCoreObjectSpace: this is the object space that allows you to use Microsoft Entity Framework as a data access technology.

NonPersistenObjectSpace: this object space is interesting as it provides the domain model needed for XAF to generate the views and interact with the data is not attached to an ORM technology so it’s up to us to provide the data, also this type of object space can be used in combination with XPObjectSpace and EfCoreObjectSpace


When querying external data sources, you also need to solve the problem of filtering and sorting data in order to provide a full solution, for that reason DevExpress team provide us with the DynamicCollection class, that is a proxy collection that allows you to filter and sort an original collection without changing it.

Now that we know the parts involved in presenting data in a XAF application, we can define the required flow.


    [DevExpress.ExpressApp.ConditionalAppearance.Appearance("", Enabled = false, TargetItems = "*")]
    public class Article : NonPersistentObjectBase {
        internal Article() { }


    public override void Setup(XafApplication application) {
        // Manage various aspects of the application UI and behavior at the module level.
        application.SetupComplete += Application_SetupComplete;


  • Wire the application object space created event.
private void Application_SetupComplete(object sender, EventArgs e) {
    Application.ObjectSpaceCreated += Application_ObjectSpaceCreated;
private void Application_ObjectSpaceCreated(object sender, ObjectSpaceCreatedEventArgs e) {
    var npos = e.ObjectSpace as NonPersistentObjectSpace;
    if (npos != null) {
        new ArticleAdapter(npos);
        new ContactAdapter(npos);


public ArticleAdapter(NonPersistentObjectSpace npos) {
     this.objectSpace = npos;
     objectSpace.ObjectsGetting += ObjectSpace_ObjectsGetting;


private void ObjectSpace_ObjectsGetting(object sender, ObjectsGettingEventArgs e) {
     if(e.ObjectType == typeof(Article)) {
         var collection = new DynamicCollection(objectSpace, e.ObjectType, e.Criteria, e.Sorting, e.InTransaction);
         collection.FetchObjects += DynamicCollection_FetchObjects;
         e.Objects = collection;
 private void DynamicCollection_FetchObjects(object sender, FetchObjectsEventArgs e) {
     if(e.ObjectType == typeof(Article)) {
         e.Objects = articles;
         e.ShapeData = true;

Full source code here

In conclusion the ObjectSpace abstraction ensures that different data access technologies can be employed, while the DynamicCollection class allows for seamless filtering and sorting of data from external sources. By following the outlined steps, developers can create robust, adaptable, and efficient applications with XAF, ultimately saving time and effort while maximizing application performance.

XPO and database replication

After 15 years of working with XPO, I have seen many tickets asking if XPO is capable of doing data replication and all the time DevExpress team answer those tickets saying that replication its out of the scope of XPO and that it’s actually true.

So, to start talking about database replication lets try to define what “replication” means. Here is the definition of replication according to Wikipedia

“A computational task is typically replicated in space, i.e. executed on separate devices, or it could be replicated in time, if it is executed repeatedly on a single device. Replication in space or in time is often linked to scheduling algorithms”

The keywords on the above statement are task, space and time, so to replicate a database operation we need to know the following information

  • What was the task?
  • Who performed the task?
  • When was the task performed?
  • Who needs to know about the task what was performed?

Now let’s think what are the steps that any traditional database does, in order to replicate the data between instances

  • It shares a copy of the database schema to the other instances/locations/devices
  • It logs the DML statements that happened to a main instance/location/device
  • It broadcast the statements on this log to the other instances/locations/devices
  • it processes the statements on the target instance/locations/devices

Now the question is, can we do the same with XPO?

thanks to the great architecture develop by the XPO Team, the answer is a big YES!!!

so, let’s compare how you can match traditional database replication with XPO

Traditional database replication XPO database replication
1 Share a copy of the database schema. In XPO we can re-create a database schema using the method UpdateSchema from the Session class.
2 Logs the DML statements. In XPO we can keep track of the BaseStatement at the DataLayer level.
3 Broadcast the statements



We can transport the log of statements using any of the dot net network technologies like WCF, remoting or Web API.
4 Process the statements on the target instance, device or location



To process any kind of BaseStatement we can use the XPO DataLayer on the target instance, device or location


So, it looks like DotNet and XPO provide us of all the necessary infrastructure to do a database replication so why not give it a try right?