Limiting Page Type Instances

We had a requirement to limit the number of instances of a particular page type within parts of the page tree. I developed a simple attribute and validator which implements this.

You add the attribute to your page type class and specifiy the scope of the check. The scope options I added were :

• Site - only x instances of the page type can exist on the site, anywhere in the page tree. Probably useful for singletons
• Same parent - only x instances of the page type can exist in the same parent node, i.e. amongst siblings
• Same parent and descendants - only x instances of the page type can exist in the same parent node including descendant nodes

Obviously, you could easily add more scope variants here to suite your requirements. The code can be found below:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using EPiServer;
using EPiServer.Core;
using EPiServer.Validation;

[AttributeUsage(AttributeTargets.Class)]
public class AllowedInstancesAttribute : Attribute
{
    public AllowedInstancesAttribute(int maxInstances)
    {
        MaxInstances = maxInstances;
    }

    public enum InstanceScope
    {
        Site,
        SameParent,
        SameParentOrDescendant
    }

    public int MaxInstances { get; set; }
    public InstanceScope Scope { get; set; }
}

public class AllowedInstancesValidator : IValidate<PageData>
{
    private readonly IContentLoader contentLoader;

    public AllowedInstancesValidator(IContentLoader contentLoader)
    {
        this.contentLoader = contentLoader;
    }

    public IEnumerable<ValidationError> Validate(PageData instance)
    {
        var allowedInstanceAttribute = instance.GetType().GetCustomAttribute<AllowedInstancesAttribute>(true);

        if (allowedInstanceAttribute == null)
        {
            return Enumerable.Empty<ValidationError>();
        }

        var searchRoot = allowedInstanceAttribute.Scope == AllowedInstancesAttribute.InstanceScope.Site
                                ? ContentReference.StartPage
                                : instance.ParentLink;

        var instancesOfType = this.GetInstancesOfType(instance.GetType(), searchRoot, allowedInstanceAttribute.Scope);

        if (instance.PendingPublish)
        {
            instancesOfType++;
        }

        if (instancesOfType > allowedInstanceAttribute.MaxInstances)
        {
            return new[]
            {
                new ValidationError()
                    {
                        ErrorMessage = string.Format("Only {0} instances of this page type can exist at this level", allowedInstanceAttribute.MaxInstances),
                        PropertyName = "PageName",
                        Severity = ValidationErrorSeverity.Error,
                        ValidationType = ValidationErrorType.StorageValidation
                    }
            };
        }

        return Enumerable.Empty<ValidationError>();
    }

    private int GetInstancesOfType(Type type, ContentReference rootPage, AllowedInstancesAttribute.InstanceScope instanceScope)
    {
        var instances = 0;
        var children = contentLoader.GetChildren<IContent>(rootPage, new LoaderOptions());

        foreach (var child in children)
        {
            if (type == child.GetType())
            {
                instances++;
            }

            if (instanceScope == AllowedInstancesAttribute.InstanceScope.SameParent)
            {
                continue;
            }

            instances += this.GetInstancesOfType(type, child.ContentLink, instanceScope);    
        }

        return instances;
    }
}