asp.net-coreView Components

Create a View Component

View components encapsulate reusable pieces of logic and views. They are defined by:

  • A ViewComponent class containing the logic for fetching and preparing the data for the view and deciding which view to render.
  • One or more views

Since they contain logic, they are more flexible than partial views while still promoting a good separation of concerns.

A simple custom view component is defined as:

public class MyCustomViewComponent : ViewComponent
{        
    public async Task<IViewComponentResult> InvokeAsync(string param1, int param2)
    {
        //some business logic

        //renders ~/Views/Shared/Components/MyCustom/Default.cshtml
        return View(new MyCustomModel{ ... });
    }
}

@*View file located in ~/Views/Shared/Components/MyCustom/Default.cshtml*@
@model WebApplication1.Models.MyCustomModel
<p>Hello @Model.UserName!</p>

They can be invoked from any view (or even a controller by returning a ViewComponentResult)

@await Component.InvokeAsync("MyCustom", new {param1 = "foo", param2 = 42})

Login View Component

The default project template creates a partial view _LoginPartial.cshtml which contains a bit of logic for finding out whether the user is logged in or not and find out its user name.

Since a view component might be a better fit (as there is logic involved and even 2 services injected) the following example shows how to convert the LoginPartial into a view component.

View Component class

public class LoginViewComponent : ViewComponent
{
    private readonly SignInManager<ApplicationUser> signInManager;
    private readonly UserManager<ApplicationUser> userManager;

    public LoginViewComponent(SignInManager<ApplicationUser> signInManager, UserManager<ApplicationUser> userManager)
    {
        this.signInManager = signInManager;
        this.userManager = userManager;
    }

    public async Task<IViewComponentResult> InvokeAsync()
    {
        if (signInManager.IsSignedIn(this.User as ClaimsPrincipal))
        {                
            return View("SignedIn", await userManager.GetUserAsync(this.User as ClaimsPrincipal));
        }
        return View("SignedOut");
    }
}

SignedIn view (in ~/Views/Shared/Components/Login/SignedIn.cshtml)

@model WebApplication1.Models.ApplicationUser

<form asp-area="" asp-controller="Account" asp-action="LogOff" method="post" id="logoutForm" class="navbar-right">
    <ul class="nav navbar-nav navbar-right">
        <li>
            <a asp-area="" asp-controller="Manage" asp-action="Index" title="Manage">Hello @Model.UserName!</a>
        </li>
        <li>
            <button type="submit" class="btn btn-link navbar-btn navbar-link">Log off</button>
        </li>
    </ul>
</form>

SignedOut view (in ~/Views/Shared/Components/Login/SignedOut.cshtml)

<ul class="nav navbar-nav navbar-right">
    <li><a asp-area="" asp-controller="Account" asp-action="Register">Register</a></li>
    <li><a asp-area="" asp-controller="Account" asp-action="Login">Log in</a></li>
</ul>

Invocation from _Layout.cshtml

@await Component.InvokeAsync("Login")

Return from Controller Action

When inheriting from base Controller class provided by the framework, you can use the convenience method ViewComponent() to return a view component from the action:

public IActionResult GetMyComponent()
{
    return ViewComponent("Login", new { param1 = "foo", param2 = 42 });
}

If using a POCO class as a controller, you can manually create an instance of the ViewComponentResult class. This would be equivalent to the code above:

public IActionResult GetMyComponent()
{
    return new ViewComponentResult 
    { 
        ViewComponentName = "Login",
        Arguments = new { param1 = "foo", param2 = 42 } 
    };
}