How to: Limit the Amount of Objects Created using the New Action

• Feb 25, 2022
• 2 minutes to read

This topic describes how to limit the number of objects that an end-user can create using the New Action. Assume you are using the Task business class from the Business Class Library. When creating a new Task using the New Action, the count of existing Task objects will be checked and an end-user will not be allowed to create additional objects if there are already three objects.

Note

The approach described in this topic is not supported by the Mobile platform.

Tip

A complete sample project is available in the DevExpress Code Examples database at How to Limit the Amount of Objects Created via the New Action.

To access the Task List View when the New Action is about to create a new object, handle the NewObjectViewController.ObjectCreating event of the NewObjectViewController, which contains the New Action. To do this, implement a new View Controller and override the OnActivated method in the following manner.

C#

using DevExpress.ExpressApp;
using DevExpress.Persistent.BaseImpl;
using DevExpress.ExpressApp.SystemModule;
//...
public class LimitTaskAmountController : ViewController {
    private NewObjectViewController controller;
    protected override void OnActivated() {
        base.OnActivated();
        controller = Frame.GetController<NewObjectViewController>();
        if (controller != null) {
            controller.ObjectCreating += controller_ObjectCreating;
        }
    }
    void controller_ObjectCreating(object sender, ObjectCreatingEventArgs e) {
        if ((e.ObjectType == typeof(Task)) && 
            (e.ObjectSpace.GetObjectsCount(typeof(Task), null) >= 3)) {
                e.Cancel = true;
                throw new UserFriendlyException(
                    "Cannot create a task. Maximum allowed task count exceeded.");
        }
    }
    protected override void OnDeactivated() {
        if (controller != null) {
            controller.ObjectCreating -= controller_ObjectCreating;
        }
        base.OnDeactivated();
    }
}

VB.NET

Imports DevExpress.ExpressApp
Imports DevExpress.Persistent.BaseImpl
Imports DevExpress.ExpressApp.SystemModule
'...
Public Class LimitTaskAmountController
    Inherits ViewController
    Private controller As NewObjectViewController
    Protected Overrides Sub OnActivated()
        MyBase.OnActivated()
        controller = Frame.GetController(Of NewObjectViewController)()
        If controller IsNot Nothing Then
            AddHandler controller.ObjectCreating, AddressOf controller_ObjectCreating
        End If
    End Sub
    Private Sub controller_ObjectCreating(ByVal sender As Object, ByVal e As ObjectCreatingEventArgs)
        If (e.ObjectType Is GetType(Task)) AndAlso (e.ObjectSpace.GetObjectsCount(GetType(Task), Nothing) >= 3) Then
                e.Cancel = True
                Throw New UserFriendlyException("Cannot create a task. Maximum allowed task count exceeded.")
        End If
    End Sub
    Protected Overrides Sub OnDeactivated()
        If controller IsNot Nothing Then
            RemoveHandler controller.ObjectCreating, AddressOf controller_ObjectCreating
        End If
        MyBase.OnDeactivated()
    End Sub
End Class

Note

You can disable an Action instead of interrupting its execution. See the How to: Disable an Action When the Current View Has Unsaved Changes example.