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Simplified Criteria Syntax

  • 3 min to read

Simplified Criteria Syntax

The common ways of building criteria are:

  • Creating criteria in code as objects.

    
    CriteriaOperator op = new BinaryOperator("Name", "Winny");
    
  • Using the static CriteriaOperator.Parse method.

    
    CriteriaOperator op = CriteriaOperator.Parse("Name = 'Winny'");
    

Both approaches are far from being type-safe. In the second approach there is a bigger potential for mistakes, because the whole string that contains the query expression can't be checked, and therefore can easily bring up runtime errors.

Since the release of DXperience 6.2 you can use new syntax when creating criteria. You would now write the above criterion like this:


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

Of course, the new system allows the use of the == operator, as well as a number of others. The list of supported operators is ==, !=, >, <, >=, <=, +, -, *, /, %, !, & and |. For technical reasons, we suggest you use the single & and | operators instead of && or ||.

The new-style 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.


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

Unfortunately, the operator overload for Boolean types cannot be implemented. In this instance, you should explicitly cast Boolean literals as shown below.


// 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

XPO 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.


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


using DevExpress.Data.Filtering;

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

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

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

You can use the CodeRush XPO Fields feature to automatically generate the Fields class.

See Also