Simplified Criteria Syntax

Sep 28, 2022
3 minutes to read

Regular Criteria Syntax

The common ways of building criteria are:

Creating criteria in code as objects.
Using the static CriteriaOperator.Parse method.

Both approaches are not type-safe. In the second approach, the whole string that contains the query expression can’t be checked, and therefore can bring up runtime errors.

Simplified Criteria Syntax

The DevExpress Data Library features the simplified criteria syntax. You can rewrite the criterion in the example above like this:

C#
VB.NET

csharp

CriteriaOperator op = MyPersistentClass.Fields.Name == "Winny";

Dim op As CriteriaOperator = MyPersistentClass.Fields.Name = "Winny"

The simplified criteria syntax supports the following operators: ==, !=, >, <, >=, <=, +, -, *, /, %, !, & and |. For technical reasons, we suggest you use the single & and | operators instead of && or ||.

The simplified expressions can be used in all the places where you can otherwise pass CriteriaOperator instances. For example, you can pass them to an XPCollection<T> constructor as shown below.

C#
VB.NET

csharp

XPCollection<Person> coll = new XPCollection<Person>(session, 
  Person.Fields.Name == "Winny" & Person.Fields.Age > 30);

Dim coll As XPCollection(Of Person) = New XPCollection(Of Person)(session, _
  Person.Fields.Name = "Winny" And Person.Fields.Age > 30)

The operator overload for Boolean types cannot be implemented. In this case, explicitly cast Boolean literals as shown below.

csharp

// bool isAdmin;
CriteriaOperator filter =
    (Person.Fields.IsAdmin == new OperandValue(isAdmin) & Person.Fields.Status != "INACTIVE");
// or
// CriteriaOperator filter = 
// (Person.Fields.IsAdmin == (CriteriaOperator)isAdmin & Person.Fields.Status != "INACTIVE");

How This Works

The DevExpress Data Library exploits operator overloading to allow for the == operator (= in Visual Basic) to be used in this scenario. The expression on the left hand-side has to be of the OperandProperty type for this to work. So you can use the following code snippet to get the same result.

C#
VB.NET

csharp

OperandProperty prop = new OperandProperty("Name");
CriteriaOperator op = prop == "Winny";
// CriteriaOperator op = new BinaryOperator("Name", "Winny");

Dim prop As OperandProperty = New OperandProperty("Name")
Dim op As CriteriaOperator = prop = "Winny"
' Dim op As CriteriaOperator = New BinaryOperator("Name", "Winny")

To simplify things, you can extend a persistent class with a nested class (called Fields in the code examples above) to expose persistent properties as property operands. So, the persistent class MyPersistentClass we used previously should look like this:

C#
VB.NET

csharp

using DevExpress.Data.Filtering;

// ...
public class MyPersistentClass: XPObject {
    public MyPersistentClass(Session session): base(session) { } 

    public string Name { 
        get { return GetPropertyValue<string>(nameof(Name)); } 
        set { SetPropertyValue<string>(nameof(Name), value); } 
    } 

    public new class Fields { 
        private Fields() { } 
        public static OperandProperty Name { 
            get { return new OperandProperty("Name"); } 
        }
    }
}

Imports DevExpress.Data.Filtering

' ...
Public Class MyPersistentClass
    Inherits XPObject
    Public Property Name() As String
        Get
            Return GetPropertyValue(Of String)(NameOf(Name))
        End Get
        Set(ByVal value As String)
            SetPropertyValue(Of String)(NameOf(Name), value)
        End Set
    End Property

    Public Shadows Class Fields
        Public Shared ReadOnly Property Name() As OperandProperty
            Get
                Return New OperandProperty("Name")
            End Get
        End Property
    End Class
End Class