Query content of inherit type or those that implements an interface

Sometimes when I do import jobs, I check if a content type have a specific property. If a page exists I update it, and if not I create a new. Other times I want to find some pages that inherits form a basepage type and get those pages that have some properties set.
To find a page if it exists with testKey I used

var items = ContentModelUsage.Service.ListContentOfContentType(
    ContentTypeRepository.Service.Load<MyPageType>())
        .Select(x =>
        x.ContentLink.CreateReferenceWithoutVersion())
    .Distinct()
    .Select(x => ContentLoader.Service.Get<MyPageType>(x))
    .OfType<MyPageType>()
    .Where(p => !p.IsDeleted)
    .Where(p => p.MyTestProperty == testKey)
    .ToList();

The problem with this code is that it is slow. I could of course grab all items, and select on that collection, but then I need to update the list with new items. The ListContentOfContentType takes in my case 350ms to execute, and the Get takes practical zero ms (after first time). Therefore, what I will do is to cache the ListContentOfType, but clear it content is added or removed.

Another problem is that the above code does not find content that inherits MyPageType, or implements an interface.

I therefore made myself a method that finds all content types that is, or inherits or implements an interface.

public static List<ContentType> GetContentTypesOfType(Type inheritTypeOrInterface)
{
    var list = ContentTypeRepository.Service.List().
        Where(
            p => p.ModelType != null &&
            (
                p.ModelType.IsSubclassOf(inheritTypeOrInterface) ||
                p.ModelType == inheritTypeOrInterface ||
                (inheritTypeOrInterface.IsInterface && inheritTypeOrInterface.IsAssignableFrom(p.ModelType))
            )
        ).
        ToList();
    return list;
}

In addition, used that to find all content reference for a type or interface

public static IList<ContentReference> GetReferenceOf(Type inheritTypeOrInterface)
{

    var list = GetContentTypesOfType(inheritTypeOrInterface);
    var result = new List<ContentReference>();
    foreach (var contentType in list)
    {
        var subResult = GetUsagesSpesificType(contentType);
        if (subResult != null)
            result.AddRange(subResult);
    }
    return result;

}
public static IList<ContentReference> GetUsagesSpesificType(ContentType contentType)
{

    var cacheKey = CacheKeyStart + contentType.ModelType;
    var result = EPiServer.CacheManager.Get(cacheKey) as CacheResult;
    if (result == null)
    {
        if (contentType != null)
        {
            var allItems = ContentModelUsage.Service.ListContentOfContentType(contentType).
                Select(x => x.ContentLink.CreateReferenceWithoutVersion()).
                Distinct().ToList();
            result = new CacheResult(allItems);

        }
        else
        {
            result = new CacheResult(null);

        }
        EPiServer.CacheManager.Add(cacheKey, result, System.Web.Caching.CacheItemPriority.BelowNormal);

    }
    return result.GetList();
}

I cache the result for the specific contentType but not for the aggregate list. This is because it is difficult to clear the cache of those aggregate lists. I can therefore easy make a method that returns all content of a specific type or interface.

public static List<T> GetContentOf<T>(ILanguageSelector selector) 
{
    var result = new List<T>();
    foreach (var item in ContentLoader.Service.GetItems(GetReferenceOf(typeof(T)), selector))
        if (item is T)
            result.Add((T)item);;
    return result;
}

The only thing I need now is to clear the cache if new content is added or deleted.

public void Initialize(EPiServer.Framework.Initialization.InitializationEngine context)
{
    DataFactory.Instance.CreatedContent += Instance_ClearCache;
    DataFactory.Instance.DeletedContent += Instance_ClearCache;
}

void Instance_ClearCache(object sender, ContentEventArgs e)
{
    if (e.Content != null)
    {
        var contentType=ContentTypeRepository.Service.Load(e.Content.GetOriginalType());
        if (contentType != null)
        {
            var cacheKey = CacheKeyStart + contentType.ModelType;
            EPiServer.CacheManager.Remove(cacheKey);
        }
    }
}

So it can be used to find pages that inherits from a base pagetype that have a BasePropertyName equals to shouldBe

var items3 = FindContent.GetContentOf<BasePageType>(LanguageSelector.MasterLanguage()).
    Where(p => !p.IsDeleted).
    Where(p => p.BasePropertyName == shouldBe);

var items4 = FindContent.GetContentOfExcludeDeleted<IHaveInterface>(LanguageSelector.MasterLanguage()).
    Where(p => p.InterfaceName==shouldBe);

The complete code is here