Filtering

• Oct 03, 2022
• 4 minutes to read

XPO allows you to:

• filter data items in a data store prior to retrieving data,
• filter the already retrieved persistent objects on the client side.

Filter Data on the Server Side

Using LINQ to XPO

The following example demonstrates how to use LINQ to XPO to query the data store for those customers who are older than 40.

C#

using System.Linq;
using System.Linq.Expressions;
using DevExpress.Xpo;

// ...

class Customer : XPObject {
    public string Name {
        get { return fName; }
        set { SetPropertyValue(nameof(Name), ref fName, value); }
    }
    string fName;

    public int Age {
        get { return fAge; }
        set { SetPropertyValue(nameof(Age), ref fAge, value); }
    }
    int fAge;

}

// ...

using (var uow = new UnitOfWork()){
    // Select customers older than 40
    var list = uow.Query<Customer>().Where(c => c.Age > 40);

    foreach (Customer cust in list)
        Console.WriteLine(string.Format("{0}\t{1}", cust.Name,
            cust.Age));
}

VB.NET

Imports System.Linq;
Imports System.Linq.Expressions;
Imports DevExpress.Xpo

'...

Class Customer
   Inherits XPObject
   Public Property Name() As String
       Get
           Return fName
       End Get
       Set(ByVal value as String)
           SetPropertyValue(NameOf(Name), fName, value)
       End Set
   End Property
   Private fName As String

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

End Class

'...

Using uow = New UnitOfWork()
    ' Select customers older than 40
    Dim list = uow.Query(Of Customer)().Where(Function(c) c.Age > 40)

    For Each cust As Customer In list
        Console.WriteLine(customer.Name & Constants.vbTab & _
                  customer.Age)
    Next
End Using

Using the Criteria Property

Use the Criteria property (for example, XPBaseCollection.Criteria, XPView.Criteria, or XpoDataSource.Criteria) to specify the criteria used to filter objects on the data store side.

The following example demonstrates how to define a simple criteria to query the data store for those customers who are older than 40.

C#

using DevExpress.Xpo;
using DevExpress.Data.Filtering;

class Customer : XPObject {
    public string Name {
        get { return fName; }
        set { SetPropertyValue(nameof(Name), ref fName, value); }
    }
    string fName;

    public int Age {
        get { return fAge; }
        set { SetPropertyValue(nameof(Age), ref fAge, value); }
    }
    int fAge;

}

...
// Specify the filter criteria.
BinaryOperator filterCriteria = new BinaryOperator(nameof(Customer.Age), 40, BinaryOperatorType.Greater);

// Retrieve only those customers who are older than 40.
XPCollection collection = new XPCollection(typeof(Customer), filterCriteria);

VB.NET

Imports DevExpress.Xpo
Imports DevExpress.Data.Filtering

Class Customer
   Inherits XPObject
   Public Property Name() As String
       Get
           Return fName
       End Get
       Set(ByVal value as String)
           SetPropertyValue(NameOf(Name), fName, value)
       End Set
   End Property
   Private fName As String

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

End Class
...
' Specify the filter criteria.
Dim filterCriteria As BinaryOperator = New BinaryOperator(NameOf(Customer.Age), 40, BinaryOperatorType.Greater)

' Retrieve only those customers who are older than 40.
Dim collection As XPCollection = New XPCollection(GetType(Customer), filterCriteria)

The image below shows the result:

Filter Data on the Client Side

You can filter the collections of the retrieved persistent objects. Use the Filter (for example, XPBaseCollection.Filter, XPView.Filter, or XPDataView.Filter) property to specify the criteria used to filter objects on the client side.

In the example below, the collection is filtered to return only those customers who are older than 42 but younger than 50.

C#

collection.Filter = new BetweenOperator(nameof(Customer.Age), 42, 50);

VB.NET

collection.Filter = New BetweenOperator(NameOf(Customer.Age), 42, 50)

The image below shows the result:

 

Note

Unsaved objects are not loaded into an XPCollection.

XPO queries the database and retrieves values. Then it instantiates persistent objects from the query result and adds these objects to the XPCollection. Before an object is instantiated, XPO searches for it in the cache by its Oid. If the object is found, its properties are refreshed and it is then added to the collection. Unsaved objects don’t have corresponding records in the database and therefore they don’t fall into the query result and thus aren’t added to the collection. This is the expected result. Objects must be saved to be included in the query results.

Creating Criteria

XPO supports multiple ways for writing criteria. The common ways of doing this are: creating criteria in code as CriteriaOperator objects, and using the static CriteriaOperator.Parse method.

• Simplified Criteria Syntax
• Criteria Operators
• Criteria Language Syntax
• Build Criteria - Cheat Sheet
• Free Joins
• Limitations of CriteriaOperator.Parse
• NULL Value Handling Specifics

Task-Based Help

• How to: Build Simple Criteria
• How to: Build Complex Criteria
• How to: Filter Persistent Objects by Type
• How to: Implement a Full-Text Search
• How to: Use the ContainsOperator for Objects in a Many-to-Many Relationship

Member Table

• Member Table: Filtering

Online Knowledge Base

• Related Knowledge Base Articles