Blocks in Optimizely Content Management System (CMS) are reusable smaller content parts, that editors can add to pages. Blocks can also be used in code as a property. 

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Note

The examples here are based on MVC.

Blocks, block types and block templates are linked together in the following way:

• A _block type_ defines a set of properties, for example a heading and a page listing.

• A _block_ is an instance of the .NET class defining the block type.

• As for pages, associated _controllers_, _views_ and _templates_ are used to render the block in some context.

Blocks can only be rendered in the context of other content, such as a page. A block instance is either part of a page instance if a `PageType` or `BlockType` contains a property of the block type, or a shared instance.

• For a _page instance_, the block is stored, loaded, and versioned as part of the page.

• For a _shared block_, the block is stored, loaded and versioned individually as an own entity, and can be referenced from multiple pages or blocks.

## Block types

Block types are usually defined in code as classes based on a model inheriting from `EPiServer.Core.BlockData`, in a similar fashion as for page types. During initialization, the bin folder is scanned for .NET classes inheriting `BlockData`. The `BlockData` object is the programmatic representation of a block, containing the properties defined in your .NET class. The value of `currentBlock` is automatically set to the `BlockData` object that is requested by the client. 

For each of the classes found a block type is created. For all public properties on the class, a corresponding property on the block type is created.

### Create a block type

Using the Visual Studio extension, you create a block type by adding the _Block type_ item to the **Blocks** subfolder under **Models** in your project. See [Get started with Optimizely Content Management System (CMS 11)](🔗) .

**Example:** A simple block type with properties for a heading and a link to an image. `TeaserBlock` inherits from `BlockData`, which inherits from `EPiServer.Core.BlockData`.

As for page types, a unique GUID for the block type is automatically generated when creating block types using the Episerver Visual Studio extension.

Blocks will only be editable from the All Properties edit view, and can only be previewed in context of some other content like a page. However, you can add specific preview rendering for blocks, for editors to be able to edit and preview them in the On-Page edit view.

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Note

Why are the properties declared as _virtual_ here? What happens in the background is that a proxy class is created for the block type, and data is loaded from the database to a property carrier (`Property`), receiving the data. Through `Castle` (Inversion of Control tool), the properties in the proxy block type will be set, and this only works if properties are declared as virtual. If the properties are not declared virtual, you need to implement get/set so that these will read/write data to the underlying property collection instead.

## Block controllers and views

In MVC, rendering of blocks is done by using controllers, views and associated templates, similar to the way you render pages. 

• Create a **controller** that inherits from `EPiServer.Web.Mvc.BlockController<TBlockData>`, where `TBlockData` is your block type. The system calls this controller for the block type, if it is chosen as the renderer of the block type. `EPiServer.Web.Mvc.BlockController<TBlockData>` has an implementation of the action Index, which calls a **partial view** with the same name as the block type.

• Create a **partial view without a controller**, naming the view the same as the block type. If the view is chosen as the renderer of the block type, the view is called with the page data object directly, without controller involvement. This approach is the recommended way to render blocks.

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Note

For performance reasons, it is recommended to **use partial views** **directly**, and not controllers, for block types. You can create a view to be used without a controller through naming convention in MVC.

### Create a partial view

In Visual Studio, add a partial view with the same name as your block type and based on your block model class, to the _Views/Shared_ folder of your project.

**Example:** The partial view for the `TeaserBlock` block type, displaying a heading and an image.

## Use templates

As for page types, templates can also be used to specify how blocks will be rendered in a specific context, for example a content area or a display channel. Note that if you are using partial views and no controllers, you cannot implement the `TemplateDescriptor`. Instead you can use the `ViewTemplateModelRegistrator` interface and an initalization module, to register templates. See [Rendering](🔗).

## Shared blocks folders

As previously mentioned, shared blocks are stored, loaded and versioned individually as an own entity in the database. Shared blocks are structured using folders, and a **Folder** is an instance of `EPiServer.Core.ContentFolder`. Content folders do not have associated rendering, and therefore no visual appearance on the website.

A folder in the shared blocks structure can have other folders or shared blocks as children, and a shared block cannot have any children.

You set editorial access on folders to specify which folders that are available for an editor. The global folder root `EPiServer.Core.SiteSettings.Current.GlobalBlocksRoot`, is the root folder for shared blocks that are available for sites in an enterprise scenario. There can be a site-specific folder `EPiServer.Core.SiteSettings.Current.SiteBlocksRoot`, containing the folder structure for shared blocks. In a single-site scenario, `GlobalBlocksRoot` and `SiteBlocksRoot` typically point to the same folder.