> For the complete documentation index, see [llms.txt](https://docs.umbraco.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.umbraco.com/umbraco-cms/13.latest/reference/notifications/sendingallowedchildrennotifications.md).

# Sending Allowed Children Notification

The `SendingAllowedChildrenNotification` enables you to manipulate the Document Types that will be shown in the create menu when adding new content in the backoffice.

## Usage

With the example below we can ensure that a Document Type cannot be selected if the type already exists in the Content tree.

```csharp
using System.Web;
using Umbraco.Cms.Core.Events;
using Umbraco.Cms.Core.Notifications;

namespace Umbraco.Docs.Samples.Web.Notifications;

public class SendingAllowedChildrenNotificationHandler : INotificationHandler<SendingAllowedChildrenNotification>
{
    public void Handle(SendingAllowedChildrenNotification notification)
    {
        const string contentIdKey = "contentId";

            // Try get the id from the content item in the backoffice
            var queryStringCollection = HttpUtility.ParseQueryString(notification.UmbracoContext.OriginalRequestUrl.Query);

        if (!queryStringCollection.ContainsKey(contentIdKey))
        {
            return;
        }

        var contentId = queryStringCollection[contentIdKey].TryConvertTo<int>().ResultOr(-1);

        if (contentId == -1)
        {
            return;
        }

        var content = notification.UmbracoContext.Content?.GetById(true, contentId);

        if (content is null)
        {
            return;
        }

        // Allowed children as configured in the backoffice
        var allowedChildren = notification.Children.ToList();

        if (content.ChildrenForAllCultures is not null)
        {
            // Get all children of current page
            var childNodes = content.ChildrenForAllCultures.ToList();

            // If there is a Settings page already created, then don't allow it anymore
            // You can also use the ModelTypeAlias property from your PublishedModel for comparison,
            // like Settings.ModelTypeAlias if you have set models builder to generate SourceCode models
            if (childNodes.Any(x => x.ContentType.Alias == "settings"))
            {
                allowedChildren.RemoveAll(x => x.Alias == "settings");
            }
        }

        // Update the allowed children
        notification.Children = allowedChildren;
    }
}
```

You also need to register this notification handler. You can achieve this by updating the `Program` class like:

```csharp
builder.CreateUmbracoBuilder()
    .AddBackOffice()
    .AddWebsite()
    .AddDeliveryApi()
    .AddComposers()
    .AddNotificationHandler<SendingAllowedChildrenNotification, SendingAllowedChildrenNotificationHandler>()
    .Build();
```

{% hint style="info" %}
For more information about registering notifications read the [Registering notification handlers](/umbraco-cms/13.latest/reference/notifications.md) article.
{% endhint %}


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