New to ASP.NET Core MVC, view components are similar to partial views, but they are much more powerful. View components don’t use model binding, and only depend on the data you provide when calling into it. A view component:
* Renders a chunk rather than a whole response
* Includes the same separation-of-concerns and testability benefits found between a controller and view
A view component consists of two parts: the class (typically derived from [ViewComponent](https://docs.microsoft.com/en-us/aspnet/core/api/microsoft.aspnetcore.mvc.viewcomponent)) and the result it returns (typically a view). Like controllers, a view component can be a POCO, but most developers will want to take advantage of the methods and properties available by deriving from `ViewComponent`.
This section contains the high-level requirements to create a view component. Later in the article, we'll examine each step in detail and create a view component.
Like controllers, view components must be public, non-nested, and non-abstract classes. The view component name is the class name with the "ViewComponent" suffix removed. It can also be explicitly specified using the `ViewComponentAttribute.Name` property.
A view component defines its logic in an `InvokeAsync` method that returns an `IViewComponentResult`. Parameters come directly from invocation of the view component, not from model binding. A view component never directly handles a request. Typically, a view component initializes a model and passes it to a view by calling the `View` method. In summary, view component methods:
The default view name for a view component is *Default*, which means your view file will typically be named *Default.cshtml*. You can specify a different view name when creating the view component result or when calling the `View` method.
We recommend you name the view file *Default.cshtml* and use the *Views/Shared/Components/\<view_component_name>/\<view_name>* path. The `PriorityList` view component used in this sample uses *Views/Shared/Components/PriorityList/Default.cshtml* for the view component view.
To use the view component, call the following inside a view:
```html
@Component.InvokeAsync("Name of view component", <anonymoustypecontainingparameters>)
```
The parameters will be passed to the `InvokeAsync` method. The `PriorityList` view component developed in the article is invoked from the *Views/Todo/Index.cshtml* view file. In the following, the `InvokeAsync` method is called with two parameters:
Pascal-cased class and method parameters for Tag Helpers are translated into their [lower kebab case](http://stackoverflow.com/questions/11273282/whats-the-name-for-dash-separated-case/12273101#12273101). The Tag Helper to invoke a view component uses the `<vc></vc>` element. The view component is specified as follows:
Note: In order to use a View Component as a Tag Helper, you must register the assembly containing the View Component using the `@addTagHelper` directive. For example, if your View Component is in an assembly called "MyWebApp", add the following directive to the `_ViewImports.cshtml` file:
You can register a View Component as a Tag Helper to any file that references the View Component. See [Managing Tag Helper Scope](xref:mvc/views/tag-helpers/intro#managing-tag-helper-scope) for more information on how to register Tag Helpers.
In the sample above, the `PriorityList` view component becomes `priority-list`. The parameters to the view component are passed as attributes in lower kebab case.
View components are typically invoked from a view, but you can invoke them directly from a controller method. While view components do not define endpoints like controllers, you can easily implement a controller action that returns the content of a `ViewComponentResult`.
[Download](https://github.com/aspnet/Docs/tree/master/aspnetcore/mvc/views/view-components/sample), build and test the starter code. It's a simple project with a `Todo` controller that displays a list of *Todo* items.
* Because the class name PriorityList**ViewComponent** ends with the suffix **ViewComponent**, the runtime will use the string "PriorityList" when referencing the class component from a view. I'll explain that in more detail later.
* The `[ViewComponent]` attribute can change the name used to reference a view component. For example, we could have named the class `XYZ` and applied the `ViewComponent` attribute:
* The `[ViewComponent]` attribute above tells the view component selector to use the name `PriorityList` when looking for the views associated with the component, and to use the string "PriorityList" when referencing the class component from a view. I'll explain that in more detail later.
* The component uses [dependency injection](../../fundamentals/dependency-injection.md) to make the data context available.
*`InvokeAsync` exposes a method which can be called from a view, and it can take an arbitrary number of arguments.
* Create the *Views/Shared/Components/PriorityList* folder. This folder name must match the name of the view component class, or the name of the class minus the suffix (if we followed convention and used the *ViewComponent* suffix in the class name). If you used the `ViewComponent` attribute, the class name would need to match the attribute designation.
The Razor view takes a list of `TodoItem` and displays them. If the view component `InvokeAsync` method doesn't pass the name of the view (as in our sample), *Default* is used for the view name by convention. Later in the tutorial, I'll show you how to pass the name of the view. To override the default styling for a specific controller, add a view to the controller-specific view folder (for example *Views/Todo/Components/PriorityList/Default.cshtml)*.
The markup `@await Component.InvokeAsync` shows the syntax for calling view components. The first argument is the name of the component we want to invoke or call. Subsequent parameters are passed to the component. `InvokeAsync` can take an arbitrary number of arguments.
A complex view component might need to specify a non-default view under some conditions. The following code shows how to specify the "PVC" view from the `InvokeAsync` method. Update the `InvokeAsync` method in the `PriorityListViewComponent` class.
Copy the *Views/Shared/Components/PriorityList/Default.cshtml* file to a view named *Views/Shared/Components/PriorityList/PVC.cshtml*. Add a heading to indicate the PVC view is being used.
If you want compile time safety, you can replace the hard-coded view component name with the class name. Create the view component without the "ViewComponent" suffix:
