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A Brief Tour On .NET Core View Components



  • View Components are small pieces or chunks of a reusable block of code.  
  • A View Component is made of the class(Component Class) and a view that looks like Controller. But View Component can't be triggered or initialized by a user request like MVC Controller, it will be invoked from code manually.
  • A View or Razor can be completely or partially build using View Components.

Create A .Net Core MVC Sample:

Let's understand View Components by doing simple examples on it. So let's create a sample .Net Core MVC application(another choice to use like Razor Pages sample application). Use either Visual Studio 2019(IDE supports .Net Core 3.0 plus version) or Visual Studio Code.

Implement The View Component Class:

View Component Class is the logic container where we can prepare data that need to be bind to view on the render. Create a folder like ViewComponents where will create all our application components.

A POCO class that inherits Microsoft.AspNetCore.Mvc.ViewComponent can be called a View Component class. As of good coding standards, it is recommended to suffix the POCO class name as 'ViewComponent' like eg:- 'YourComponentNameViewComponent'.
ViewComponents/StudentViewComponent.ts:
using Microsoft.AspNetCore.Mvc;
using System.Threading.Tasks;

namespace SampleViewComponent.App.ViewComponents
{
    public class StudentViewComponent : ViewComponent
    {

    }
}

Invoke Or InvokeAsync Methods Of View Component:

On Calling or invoking View Component the only method that executed automatically is Invoke or InvokeAsync. Any logic or data accessing calls are implemented in this method. Invoke or InvokeAsync method return type is Microsoft.AspNetCore.Mvc.IViewComponentResult. An IVewComponentResult can accept either Html(View or Razor Page) or Plain String.
ViewComponents/StudentViewComponent.ts:
using Microsoft.AspNetCore.Mvc;
using System.Threading.Tasks;

namespace SampleViewComponent.App.ViewComponents
{
    public class StudentViewComponent : ViewComponent
    {
        public async Task<IViewComponentResult> InvokeAsync()
        {
            return Content("Hi I'm Student Component");
        }
    }
}
#L10 at this line number retuning method ViewComponent.Content() which returns a plain string as View Component output. In real-time scenarios, most cases return a razor view using ViewComponent.View() which is an overloaded method.

Calling View Component From *.cshtml:

View Component is like building blocks of a page and most appropriate way to call or invoke them from an MVC View or from a Razor Page View. IViewComponentHelper of razor view helper helps to invoke any View Component within the application by passing component name as an input parameter.

Let's test our sample View Component by calling it on Index.cshtml(HomeController View) as follows.
Views/Home/Index.cshtml:
@{
    ViewData["Title"] = "Home Page";
}

<div class="text-center">
   @await Component.InvokeAsync("Student")
</div>
#L6 - asynchronous call for View Component, if we want we can do synchronous call but it is recommended to use async call. If we observe name of the component is passed as an input parameter so that particular View Component result gets rendered. The original name of our View Component class is 'StudentViewComponent', but while calling we removed suffix('ViewComponent') because .Net will able to identify the component without suffix only, so if you try to pass full name like 'StudentViewComponent' we will get runtime error notifying that no View Component identified with that name. So the verdict is to trim the suffix while calling component.

Run the application and test the results below.
Based on your requirement View Component can be called on n-number of different pages or n-number of location with on the same page. On every call, a separate instance or output gets rendered. To test let's update the Index.cshtml(HomeController view) to call component multiple times as follows.
@{
    ViewData["Title"] = "Home Page";
}

<div class="text-center">
   @await Component.InvokeAsync("Student")
   <br/>
   @await Component.InvokeAsync("Student")
   <br/>
   @await Component.InvokeAsync("Student")
</div>

View Component Returning Razor View(*.cshtml) As Result:

Using overloaded method like Microsoft.AspNetCore.Mvc.ViewComponent.View() renders the razor HTML as a View Component output.

By default framework searches for the component view path in the following areas
  • /Views/{Controller Name Folder}/Components/{View Component Name Folder}/{View Name}
  • /Views/Shared/Components/{View Component Name Folder}/{View Name}
Let's update View Component that searches for the Default.cshtml razor page in the specified path as we discussed above.
ViewComponents/StudentViewComponent.cs:
using Microsoft.AspNetCore.Mvc;
using System.Threading.Tasks;

namespace SampleViewComponent.App.ViewComponents
{
    public class StudentViewComponent : ViewComponent
    {
        public async Task<iviewcomponentresult> InvokeAsync()
        {
            return View();
        }
    }
}
#L10 on returning empty view from View Component, on runtime framework will automatically search for Default.cshtml as per the paths specified above.

So let's run the application and test the view component output.
On running application, we will get an error because we haven't added Default.cshtml view either in the controller or shared folder.

So let's add Default.cshtml file as below.
Views/Shared/Components/Student/Default.cshtml:
<h1>My Path: /Views/Shared/Components/Student/Default.cshtml</h1>
<h3>Hi i'm Student componen</h3>
Now run the application and view component results output as below.


Microsoft.AspNetCore.Mvc.ViewComponent.View("YourViewName") using this overloaded method we can specify view name explicitly for a view component.
ViewComponents/StudentViewComponent.cs:
using Microsoft.AspNetCore.Mvc;
using System.Threading.Tasks;

namespace SampleViewComponent.App.ViewComponents
{
    public class StudentViewComponent : ViewComponent
    {
        public async Task<IViewComponentResult> InvokeAsync()
        {
            return View("Student2");
        }
    }
}
#L10 at this line we passed view name explicitly.

Let's add a new view with the name specified above.
Views/Shared/Components/Student/Student2.cshml:
<h1>My Path: /Views/Shared/Components/Student/Student2.cshtml</h1>
<h3>Hi i'm Student component</h3>

Similar to the MVC controller in View Component also we can pass a model to the output view(razor view).

Let's first create a sample model that will be used by View Component.
Models/Profile.cs:
namespace SampleViewComponent.App.Models
{
    public class Profile
    {
        public string AvatarLink { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
    }
}
Let's update the view component below.
ViewComponents/StudentViewComponent.cs:
using Microsoft.AspNetCore.Mvc;
using SampleViewComponent.App.Models;
using System.Threading.Tasks;

namespace SampleViewComponent.App.ViewComponents
{
    public class StudentViewComponent : ViewComponent
    {
        public async Task<IViewComponentResult> InvokeAsync()
        {
            var profile = new Profile
            {
                AvatarLink = "img/profile.webp",
                FirstName = "Naveen",
                LastName = "Bommidi"
            };
            return View("Student2",profile);
        }
    }
}
Let's update the razor view with the model binding below.
Views/Shared/Components/Student/Student2.cshml:
@model SampleViewComponent.App.Models.Profile


<div class="row">
    <div class="col-sm-4">
        <div class="card">
            <img src="@Model.AvatarLink" class="card-img-top">
            <div class="card-body">
                <h4 Card-title>@Model.FirstName @Model.LastName</h4>
            </div>
        </div>
    </div>
</div>
On running application component renders output below

Dependency Injection Support:

Similar to the MVC controller, View Component also supports Dependency Injection, where we inject all service or data access layers that can be injected into the component constructor.

An example that shows IHttpContextAccessor injects to View Components:
using Microsoft.AspNetCore.Mvc;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Http;

namespace SampleViewComponent.App.ViewComponents
{
    public class StudentViewComponent : ViewComponent
    {
        private readonly IHttpContextAccessor _httpContextAccessor;
        public StudentViewComponent(IHttpContextAccessor httpContextAccessor)
        {
            _httpContextAccessor = httpContextAccessor;
        }
        public async Task<IViewComponentResult> InvokeAsync()
        {
            
            return Content("Example to show ViewComponent supports Dependency Injection")
        }
    }
}

Invoking ViewComponent From MVC Controller:

In the previous examples, we have invoked components from the *.cshtml view(Controller View), but there is an option to call or return ViewComponent as output from the MVC Controller.

Example MVC action method that returns View Component as output.
public IActionResult Index()
{
    return ViewComponent("Student2");
}

Passing Params To View Component:

On calling or invoking view component we can pass n-number of variables as below.

Example of passing values to ViewComponent
 @await Component.InvokeAsync("Student", new { id =1, firstName ="naveen", lastName ="Bommidi" })
The above variable names should be matched with input parameter names of the InvokeAsync method in View Component. If the name did not match component will ignore the input params.

Example of parameters in InvokeAsync method of View Component
using Microsoft.AspNetCore.Mvc;
using System.Threading.Tasks;

namespace SampleViewComponent.App.ViewComponents
{
    public class StudentViewComponent : ViewComponent
    {
        public StudentViewComponent()
        {
        }
        public async Task<IViewComponentResult> InvokeAsync(int id, string firstName, string lastName)
        {
            return Content("");
        }
    }
}

Wrapping Up:

Hopefully, I think this article delivered some useful information about View Components in .Net Core MVC or Razor Page Applications. I love to have your feedback, suggestions, and better techniques in the comment section below.

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