AspNetCore.Docs/aspnetcore/mvc/views/view-components.md

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View components in ASP.NET Core rick-anderson Learn how view components are used in ASP.NET Core and how to add them to apps. riande 12/03/2018 mvc/views/view-components

View components in ASP.NET Core

By Rick Anderson

View or download sample code (how to download)

View components

View components are similar to partial views, but they're much more powerful. View components don't use model binding, and only depend on the data provided when calling into it. This article was written using ASP.NET Core MVC, but view components also work with Razor Pages.

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.
  • Can have parameters and business logic.
  • Is typically invoked from a layout page.

View components are intended anywhere you have reusable rendering logic that's too complex for a partial view, such as:

  • Dynamic navigation menus
  • Tag cloud (where it queries the database)
  • Login panel
  • Shopping cart
  • Recently published articles
  • Sidebar content on a typical blog
  • A login panel that would be rendered on every page and show either the links to log out or log in, depending on the log in state of the user

A view component consists of two parts: the class (typically derived from 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.

Creating a view component

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.

The view component class

A view component class can be created by any of the following:

  • Deriving from ViewComponent
  • Decorating a class with the [ViewComponent] attribute, or deriving from a class with the [ViewComponent] attribute
  • Creating a class where the name ends with the suffix ViewComponent

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 class:

  • Fully supports constructor dependency injection

  • Doesn't take part in the controller lifecycle, which means you can't use filters in a view component

View component methods

A view component defines its logic in an InvokeAsync method that returns a Task<IViewComponentResult> or in a synchronous Invoke 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:

  • Define an InvokeAsync method that returns a Task<IViewComponentResult> or a synchronous Invoke method that returns an IViewComponentResult.
  • Typically initializes a model and passes it to a view by calling the ViewComponent View method.
  • Parameters come from the calling method, not HTTP. There's no model binding.
  • Are not reachable directly as an HTTP endpoint. They're invoked from your code (usually in a view). A view component never handles a request.
  • Are overloaded on the signature rather than any details from the current HTTP request.

View search path

The runtime searches for the view in the following paths:

  • /Pages/Components/{View Component Name}/{View Name}
  • /Views/{Controller Name}/Components/{View Component Name}/{View Name}
  • /Views/Shared/Components/{View Component Name}/{View Name}

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.

Invoking a view component

To use the view component, call the following inside a view:

@await Component.InvokeAsync("Name of view component", {Anonymous Type Containing Parameters})

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:

[!code-cshtml]

::: moniker range=">= aspnetcore-1.1"

Invoking a view component as a Tag Helper

For ASP.NET Core 1.1 and higher, you can invoke a view component as a Tag Helper:

[!code-cshtml]

Pascal-cased class and method parameters for Tag Helpers are translated into their lower kebab case. The Tag Helper to invoke a view component uses the <vc></vc> element. The view component is specified as follows:

<vc:[view-component-name]
  parameter1="parameter1 value"
  parameter2="parameter2 value">
</vc:[view-component-name]>

To use a view component as a Tag Helper, register the assembly containing the view component using the @addTagHelper directive. If your view component is in an assembly called MyWebApp, add the following directive to the _ViewImports.cshtml file:

@addTagHelper *, MyWebApp

You can register a view component as a Tag Helper to any file that references the view component. See Managing Tag Helper Scope for more information on how to register Tag Helpers.

The InvokeAsync method used in this tutorial:

[!code-cshtml]

In Tag Helper markup:

[!code-cshtml]

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.

::: moniker-end

Invoking a view component directly from a controller

View components are typically invoked from a view, but you can invoke them directly from a controller method. While view components don't define endpoints like controllers, you can easily implement a controller action that returns the content of a ViewComponentResult.

In this example, the view component is called directly from the controller:

[!code-csharp]

Walkthrough: Creating a simple view component

Download, build and test the starter code. It's a simple project with a Todo controller that displays a list of Todo items.

List of ToDos

Add a ViewComponent class

Create a ViewComponents folder and add the following PriorityListViewComponent class:

[!code-csharp]

Notes on the code:

  • View component classes can be contained in any folder in the project.

  • Because the class name PriorityListViewComponent 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've named the class XYZ and applied the ViewComponent attribute:

    [ViewComponent(Name = "PriorityList")]
       public class XYZ : ViewComponent
    
  • 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 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.

  • The InvokeAsync method returns the set of ToDo items that satisfy the isDone and maxPriority parameters.

Create the view component Razor view

  • Create the Views/Shared/Components folder. This folder must be named Components.

  • 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.

  • Create a Views/Shared/Components/PriorityList/Default.cshtml Razor view: [!code-cshtml]

    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).

    If the view component is controller-specific, you can add it to the controller-specific folder (Views/Todo/Components/PriorityList/Default.cshtml).

  • Add a div containing a call to the priority list component to the bottom of the Views/Todo/index.cshtml file:

    [!code-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.

Test the app. The following image shows the ToDo list and the priority items:

todo list and priority items

You can also call the view component directly from the controller:

[!code-csharp]

priority items from IndexVC action

Specifying a view name

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.

[!code-csharp]

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.

[!code-cshtml]

Update Views/TodoList/Index.cshtml:

[!code-cshtml]

Run the app and verify PVC view.

Priority View Component

If the PVC view isn't rendered, verify you are calling the view component with a priority of 4 or higher.

Examine the view path

  • Change the priority parameter to three or less so the priority view isn't returned.

  • Temporarily rename the Views/Todo/Components/PriorityList/Default.cshtml to 1Default.cshtml.

  • Test the app, you'll get the following error:

    An unhandled exception occurred while processing the request.
    InvalidOperationException: The view 'Components/PriorityList/Default' wasn't found. The following locations were searched:
    /Views/ToDo/Components/PriorityList/Default.cshtml
    /Views/Shared/Components/PriorityList/Default.cshtml
    EnsureSuccessful
    
  • Copy Views/Todo/Components/PriorityList/1Default.cshtml to Views/Shared/Components/PriorityList/Default.cshtml.

  • Add some markup to the Shared Todo view component view to indicate the view is from the Shared folder.

  • Test the Shared component view.

ToDo output with Shared component view

Avoiding magic strings

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:

[!code-csharp]

Add a using statement to your Razor view file, and use the nameof operator:

[!code-cshtml]

Perform synchronous work

The framework handles invoking a synchronous Invoke method if you don't need to perform asynchronous work. The following method creates a synchronous Invoke view component:

public class PriorityList : ViewComponent
{
    public IViewComponentResult Invoke(int maxPriority, bool isDone)
    {
        var items = new List<string> { $"maxPriority: {maxPriority}", $"isDone: {isDone}" };
        return View(items);
    }
}

The view component's Razor file lists the strings passed to the Invoke method (Views/Home/Components/PriorityList/Default.cshtml):

@model List<string>

<h3>Priority Items</h3>
<ul>
    @foreach (var item in Model)
    {
        <li>@item</li>
    }
</ul>

::: moniker range=">= aspnetcore-1.1"

The view component is invoked in a Razor file (for example, Views/Home/Index.cshtml) using one of the following approaches:

To use the xref:Microsoft.AspNetCore.Mvc.IViewComponentHelper approach, call Component.InvokeAsync:

::: moniker-end

::: moniker range="< aspnetcore-1.1"

The view component is invoked in a Razor file (for example, Views/Home/Index.cshtml) with xref:Microsoft.AspNetCore.Mvc.IViewComponentHelper.

Call Component.InvokeAsync:

::: moniker-end

@await Component.InvokeAsync(nameof(PriorityList), new { maxPriority = 4, isDone = true })

::: moniker range=">= aspnetcore-1.1"

To use the Tag Helper, register the assembly containing the View Component using the @addTagHelper directive (the view component is in an assembly called MyWebApp):

@addTagHelper *, MyWebApp

Use the view component Tag Helper in the Razor markup file:

<vc:priority-list max-priority="999" is-done="false">
</vc:priority-list>

::: moniker-end

The method signature of PriorityList.Invoke is synchronous, but Razor finds and calls the method with Component.InvokeAsync in the markup file.

Additional resources