Understand and Extend

Models are generated as partial classes. In its most basic form, a model for content type TextPage ends up in a TextPage.generated.cs file and looks like:

/// <summary>TextPage</summary>
[PublishedModel("textPage")]
public partial class TextPage : PublishedContentModel
{
  // helpers
  public new const string ModelTypeAlias = "textPage";

  public new const PublishedItemType ModelItemType = PublishedItemType.Content;

  public new static IPublishedContentType GetModelContentType(IPublishedSnapshotAccessor publishedSnapshotAccessor)
    => PublishedModelUtility.GetModelContentType(publishedSnapshotAccessor, ModelItemType, ModelTypeAlias);

  public static IPublishedPropertyType GetModelPropertyType<TValue>(IPublishedSnapshotAccessor publishedSnapshotAccessor, Expression<Func<TextPage, TValue>> selector)
    => PublishedModelUtility.GetModelPropertyType(GetModelContentType(publishedSnapshotAccessor), selector);

  private IPublishedValueFallback _publishedValueFallback;

  // ctor
  public TextPage(IPublishedContent content, IPublishedValueFallback publishedValueFallback)
    : base(content, publishedValueFallback)
  {
    _publishedValueFallback = publishedValueFallback;
  }

  // properties

  ///<summary>
  /// Header
  ///</summary>
  [ImplementPropertyType("header")]
  public virtual string Header => this.Value<string>(_publishedValueFallback, "header");
}

What is important is the Header property. The rest is (a) a constructor and (b) some static helpers to get the PublishedContentType and the PublishedPropertyType objects:

var contentType = TextPage.GetModelContentType(); // is a PublishedContentType
var propertyType = TextPage.GetModelPropertyType(x => x.Header); // is a PublishedPropertyType

Composition and Inheritance

Content type composition consists in having content types "inherit" properties from other content types. Contrary to C#, where a class can only inherit from one other class, Umbraco content types can be composed of several other content types.

The TextPage content type could be composed of the MetaInfo content type (and thus inherit properties Author and Keywords) and of the PageInfo content type (and thus inherit properties Title and MainImage).

Each content type that is involved in a composition is generated both as a class and as an interface, and so the MetaInfo content type would be generated as (some code has been removed and altered for simplicity's sake):

// The composition interface
public partial interface IMetaInfo : IPublishedContent
{
  public string Author { get; }
  public IEnumerable<string> Keywords { get; }
}

// The composition class
public partial class MetaInfo : PublishedContentModel
{
  // the "static mixin getter" for the property
  public static string GetAuthor(IMetaInfo that)
  {
    return that.GetPropertyValue<string>("author");
  }

  public string Author { get { return MetaInfo.GetAuthor(this, _publishedValueFallback); } }
}

And the TextPage model would be generated as (some code has been removed and altered for simplicity's sake):

public partial class TextPage : PublishedContentModel, IMetaInfo
{
  // get the property value from the "static mixin getter"
  public string Author { get { return MetaInfo.GetAuthor(this, _publishedValueFallback); } }
}

A content type parent is a tree-related concept: In the Umbraco backoffice, a content type appears underneath its parent, if any. By convention, a content type is always composed of its parent and therefore inherits its properties. However, the parent content type is treated differently, and the child content type directly inherits (as in C# inheritance) from the parent class.

Therefore, assuming that the AboutPage content type is a direct child of TextPage, it would be generated as:

// Note: Inherits from TextPage
public partial class AboutPage : TextPage
{
  ...
}

Extending

Because a model is generated as a partial class, it is possible to extend it. That could be by adding a property, by dropping the following code in a TextPage.cs file:

public partial class TextPage
{
    public string WrappedHeader => $"[{Header}]";
}

Models builder does not take a custom partial class into account when generating the models. This means that if a custom partial class, inherits from a base class, tries to provide a constructor with the same signature, or implements a generated property, it will cause compilation errors.

Furthermore a generated model will always be instantiated with its default constructor, so if an overloading constructor is created it will never be used.

For more complex partial classes, you'll have to use the full version of the Models Builder.

Best Practices

Extending models should be used to add stateless, local features to models, and not to transform content models into view models or manage trees of content.

Example of good practice

A customer has "posts" that has two "release date" properties. One is a true date picker property and is used to specify an actual date and to order the posts. The other is a string that is used to specify dates such as "Summer 2015" or "Q1 2016". Alongside the title of the post, the customer wants to display the text date, if present, else the actual date. If none of those are present, the Umbraco update date should be used. Keep in mind that each view can contain code to deal with the situation, but it is much more efficient to extend the Post model:

    public partial class Post
    {
        public string DisplayDate
        {
            get
            {
                if(!TextDate.IsNullOrWhiteSpace())
                {
                    return TextDate;
                }

                if (ActualDate != default)
                {
                    return ActualDate.ToString();
                }

                return UpdateDate.ToString();
            }
        }
    }

And to simplify the view as:

<div class="title">
  <div class="title-text">@Model.Title</div>
  <div class="title-date">@Model.DisplayDate</div>
</div>

Example of bad practice

Because, by default, the content object is passed to views, one can be tempted to add view-related properties to the model. Some properties that do not belong to a content model would be:

• A HomePage property that would walk up the tree and return the "home page" content item

• A Menu property that would list the content items to display in a top menu

• Etc.

Generally speaking, anything that is tied to the current request, or that depends on more than the modeled content, is a bad idea. There are much cleaner solutions, such as using true view model classes that would be populated by a true controller and look like:

public class TextPageViewModel
{
  public TextPage Content; // The content model
  public HomePage HomePage; // The home page content model
  public IEnumerable<MenuItem> Menu; // The menu content models
}

One can also extend Umbraco's views to provide a special view helper that would give access to important elements of the website, so that views could contain code such as:

<a href="@MySite.HomePage.Url">@MySite.HomePage.Title</a>

Example of ugly practice

The scope and life-cycle of a model is not specified. In other words, you don't know whether the model exists only for you and for the context of the current request, or if it is cached by Umbraco and shared by all requests.

As a consequence, the following code has a major issue: the TextPage model "caches" an instance of the HomePageDocument model that will never be updated if the home page is re-published.

private HomePageDocument _homePage;
public HomePageDocument HomePage
{
    get
    {
        if (_homePage is null)
        {
            _homePage = this.AncestorOrSelf<HomePageDocument>(1);
        }
        return _homePage;
    }
}

As a rule of thumb, models should never, ever reference and cache other models.