What is the best way to obtain a collection of persistent objects for processing?

Description:
The task is: retrieve objects by particular criteria from the database, process them and save them. I don't need to display objects in data bound visual controls. It's a kind of background data processing, where the low memory footprint and performance are critical. Is there a better way to get a list of the objects other than loading them into an XPCollection?

Answer:
The XPCollection is a component with a lot of features. It implements the IBindingList and ITypedList interfaces, supports on-the-fly filtering and sorting and it can be bound to visual controls. Obviously, you don't need this functionality for background data processing. It may make sense to load objects into a simple array rather than to use the XPCollection. We advise you to use the Session.GetObjects method in your task. Below is some sample code.

C#
using DevExpress.Xpo;  
using DevExpress.Xpo.DB;  
using DevExpress.Xpo.Metadata;  
using DevExpress.Data.Filtering;  
using System.Collections;  

void ProcessDiscontinued() {  
    XPClassInfo productClass;  
    CriteriaOperator criteria;  
    SortingCollection sortProps;  
    using(UnitOfWork uow = new UnitOfWork()) {  

        // Obtain the persistent object class info required by the GetObjects method  
        productClass = uow.GetClassInfo(typeof(Product));  
        // Create criteria to get objects  
        criteria = new BinaryOperator("Discontinued", true);  
        // Create a sort list if objects must be processed in a specific order  
        sortProps = new SortingCollection(null);  
        sortProps.Add(new SortProperty("Price", SortingDirection.Ascending));  
        // Call GetObjects  
        ICollection products = uow.GetObjects(productClass, criteria, sortProps, int.MaxValue, false, true);  

        // Do processing  
        foreach(Product product in products) {  
            // do some action with the product object  
            Console.WriteLine("{0,-25} {1,8:c} discontinued: {2}", product.Name, product.Price, product.Discontinued);  
        }  

        // Save changes if any  
        //if(uow.InTransaction)  
        //    uow.CommitChanges();  
    }  
}  

Visual Basic
Sub ProcessDiscontinued()  
    Dim products As ICollection  
    Dim productClass As XPClassInfo  
    Dim criteria As CriteriaOperator  
    Dim sortProps As SortingCollection  
    Dim session As New UnitOfWork  

    ' Obtain the persistent object class info required by the GetObjects method  
    productClass = session.GetClassInfo(GetType(Product))  
    ' Create criteria to get objects  
    criteria = New BinaryOperator("Discontinued", True)  
    ' Create a sort list if objects must be processed in a specific order  
    sortProps = New SortingCollection(Nothing)  
    sortProps.Add(New SortProperty("Price", SortingDirection.Ascending))  
    ' Call GetObjects  
    products = session.GetObjects(productClass, criteria, sortProps, Integer.MaxValue, False, True)  

    ' Do processing  
    Dim product As Product  
    For Each product In products  
        ' do some action with the product object  
        Console.WriteLine("{0,-25} {1,8:c}   discontinued: {2}", product.Name, product.Price, product.Discontinued)  
    Next  

    ' Save changes if any  
    'If session.InTransaction Then  
    '    session.CommitChanges()  
    'End If  

    session.Dispose()  
End Sub  

See Also:
How to obtain a collection of persistent objects by a set of their IDs