IObjectSpaceAsync.EvaluateAsync(Type, CriteriaOperator, CriteriaOperator, CancellationToken) Method

Asynchronously evaluates the specified criteria for business objects of the given type.

Namespace: DevExpress.ExpressApp

Assembly: DevExpress.ExpressApp.v24.1.dll

NuGet Package: DevExpress.ExpressApp

Declaration

C#

Task<object> EvaluateAsync(
    Type objectType,
    CriteriaOperator expression,
    CriteriaOperator criteria,
    CancellationToken cancellationToken = default(CancellationToken)
)

VB.NET

Function EvaluateAsync(
    objectType As Type,
    expression As CriteriaOperator,
    criteria As CriteriaOperator,
    cancellationToken As CancellationToken = Nothing
) As Task(Of Object)

Parameters

Name	Type	Description
objectType	Type	A Type object that identifies the type of objects against which the expression is evaluated.
expression	CriteriaOperator	A CriteriaOperator object that specifies the expression to evaluate.
criteria	CriteriaOperator	A CriteriaOperator object that specifies the filter criteria. The objects that match this criteria are used to evaluate the expression.

Optional Parameters

Name	Type	Default	Description
cancellationToken	CancellationToken	null	A CancellationToken object that delivers a cancellation notice to the running operation.

Returns

Type	Description
Task<Object>	A Task that returns an object. This object represents the evaluated value. null (Nothing in Visual Basic) if no persistent object is found that matches the criteria.

Remarks

You can use the static CriteriaOperator.Parse method to construct the expression and criteria. This method takes a string representation of the required expression and creates the CriteriaOperator object that corresponds to this expression.

The following code demonstrates how to use this method in a View Controller to evaluate contact’s assigned tasks.

C#

using DevExpress.Data.Filtering;
using DevExpress.ExpressApp;
using DevExpress.ExpressApp.Actions;
using DevExpress.ExpressApp.Xpo;
using System;
using System.Threading;
// ...
public class AsyncTasksCountController : ObjectViewController<DetailView, Contact> {
    public AsyncTasksCountController() : base() {
        SimpleAction EvaluateTasksCountAction = new SimpleAction(this, "Assigned tasks count", "Edit");
        EvaluateTasksCountAction.SelectionDependencyType = SelectionDependencyType.RequireSingleObject;
        EvaluateTasksCountAction.Execute += EvaluateTasksCountAction_Execute;
    }
    async private void EvaluateTasksCountAction_Execute(object sender, SimpleActionExecuteEventArgs e) {
        CancellationTokenSource cancellationTokenSource = new CancellationTokenSource();
        Guid currentContactOid = ViewCurrentObject.Oid;
        XPObjectSpace taskObjectSpace = (XPObjectSpace)Application.CreateObjectSpace(typeof(DemoTask));
        object tasksCount =
            await taskObjectSpace.EvaluateAsync(
            typeof(DemoTask), CriteriaOperator.Parse("Count()"),
            CriteriaOperator.Parse(string.Format("[AssignedTo.Oid] = '{0}'", currentContactOid)),
            cancellationTokenSource.Token);
        if (tasksCount != null) {
            ViewCurrentObject.AssignedTasksCount = (int)tasksCount;
        }
    }
}

VB.NET

Imports DevExpress.Data.Filtering
Imports DevExpress.ExpressApp
Imports DevExpress.ExpressApp.Actions
Imports DevExpress.ExpressApp.Xpo
Imports System
Imports System.Threading
' ...
Public Class AsyncTasksCountController
    Inherits ObjectViewController(Of DetailView, Contact)
    Public Sub New()
        MyBase.New()
        Dim EvaluateTasksCountAction As New SimpleAction(Me, "Assigned tasks count", "Edit")
        EvaluateTasksCountAction.SelectionDependencyType = SelectionDependencyType.RequireSingleObject
        AddHandler EvaluateTasksCountAction.Execute, AddressOf EvaluateTasksCountAction_Execute
    End Sub
    Async Private Sub EvaluateTasksCountAction_Execute(ByVal sender As Object, ByVal e As SimpleActionExecuteEventArgs)
        Dim cancellationTokenSource As New CancellationTokenSource()
        Dim currentContactOid As Guid = ViewCurrentObject.Oid
        Dim taskObjectSpace As XPObjectSpace = CType(Application.CreateObjectSpace(GetType(DemoTask)), XPObjectSpace)
        Dim tasksCount As Object = Await taskObjectSpace.EvaluateAsync(GetType(DemoTask), _
            CriteriaOperator.Parse("Count()"), _
            CriteriaOperator.Parse(String.Format("[AssignedTo.Oid] = '{0}'", currentContactOid)), _
            cancellationTokenSource.Token)
        If tasksCount IsNot Nothing Then
            ViewCurrentObject.AssignedTasksCount = DirectCast(tasksCount, Integer)
        End If
    End Sub
End Class

In the current example, the cancellationToken parameter is used for demonstration purposes. You can skip it or use to cancel an asynchronous operation as shown in the CancellationToken topic.