BySteve Smith andDave Brock

This document explains views used in ASP.NET Core MVC applications. For information on Razor Pages, seeRazor Pages architecture and concepts in ASP.NET Core.

In the Model-View-Controller (MVC) pattern, theview handles the app's data presentation and user interaction. A view is an HTML template with embeddedRazor markup. Razor markup is code that interacts with HTML markup to produce a webpage that's sent to the client.

In ASP.NET Core MVC, views are.cshtml files that use theC# programming language in Razor markup. Usually, view files are grouped into folders named for each of the app'scontrollers. The folders are stored in aViews folder at the root of the app:

TheHome controller is represented by aHome folder inside theViews folder. TheHome folder contains the views for theAbout,Contact, andIndex (homepage) webpages. When a user requests one of these three webpages, controller actions in theHome controller determine which of the three views is used to build and return a webpage to the user.

Uselayouts to provide consistent webpage sections and reduce code repetition. Layouts often contain the header, navigation and menu elements, and the footer. The header and footer usually contain boilerplate markup for many metadata elements and links to script and style assets. Layouts help you avoid this boilerplate markup in your views.

Partial views reduce code duplication by managing reusable parts of views. For example, a partial view is useful for an author biography on a blog website that appears in several views. An author biography is ordinary view content and doesn't require code to execute in order to produce the content for the webpage. Author biography content is available to the view by model binding alone, so using a partial view for this type of content is ideal.

View components are similar to partial views in that they allow you to reduce repetitive code, but they're appropriate for view content that requires code to run on the server in order to render the webpage. View components are useful when the rendered content requires database interaction, such as for a website shopping cart. View components aren't limited to model binding in order to produce webpage output.

Benefits of using views

Views help to establishseparation of concerns within an MVC app by separating the user interface markup from other parts of the app. Following SoC design makes your app modular, which provides several benefits:

• The app is easier to maintain because it's better organized. Views are generally grouped by app feature. This makes it easier to find related views when working on a feature.
• The parts of the app are loosely coupled. You can build and update the app's views separately from the business logic and data access components. You can modify the views of the app without necessarily having to update other parts of the app.
• It's easier to test the user interface parts of the app because the views are separate units.
• Due to better organization, it's less likely that you'll accidentally repeat sections of the user interface.

Creating a view

Views that are specific to a controller are created in theViews/[ControllerName] folder. Views that are shared among controllers are placed in theViews/Shared folder. To create a view, add a new file and give it the same name as its associated controller action with the.cshtml file extension. To create a view that corresponds with theAbout action in theHome controller, create anAbout.cshtml file in theViews/Home folder:

@{    ViewData["Title"] = "About";}<h2>@ViewData["Title"].</h2><h3>@ViewData["Message"]</h3><p>Use this area to provide additional information.</p>

Razor markup starts with the@ symbol. Run C# statements by placing C# code withinRazor code blocks set off by curly braces ({ ... }). For example, see the assignment of "About" toViewData["Title"] shown above. You can display values within HTML by simply referencing the value with the@ symbol. See the contents of the<h2> and<h3> elements above.

The view content shown above is only part of the entire webpage that's rendered to the user. The rest of the page's layout and other common aspects of the view are specified in other view files. To learn more, see theLayout topic.

How controllers specify views

Views are typically returned from actions as aViewResult, which is a type ofActionResult. Your action method can create and return aViewResult directly, but that isn't commonly done. Since most controllers inherit fromController, you simply use theView helper method to return theViewResult:

HomeController.cs:

public IActionResult About(){    ViewData["Message"] = "Your application description page.";    return View();}

When this action returns, theAbout.cshtml view shown in the last section is rendered as the following webpage:

TheView helper method has several overloads. You can optionally specify:

• An explicit view to return:

  return View("Orders");

• Amodel to pass to the view:

  return View(Orders);

• Both a view and a model:

  return View("Orders", Orders);

View discovery

When an action returns a view, a process calledview discovery takes place. This process determines which view file is used based on the view name.

The default behavior of theView method (return View();) is to return a view with the same name as the action method from which it's called. For example, theAboutActionResult method name of the controller is used to search for a view file namedAbout.cshtml. First, the runtime looks in theViews/[ControllerName] folder for the view. If it doesn't find a matching view there, it searches theShared folder for the view.

It doesn't matter if you implicitly return theViewResult withreturn View(); or explicitly pass the view name to theView method withreturn View("<ViewName>");. In both cases, view discovery searches for a matching view file in this order:

1. Views/\[ControllerName]/\[ViewName].cshtml
2. Views/Shared/\[ViewName].cshtml

A view file path can be provided instead of a view name. If using an absolute path starting at the app root (optionally starting with "/" or "~/"), the.cshtml extension must be specified:

return View("Views/Home/About.cshtml");

You can also use a relative path to specify views in different directories without the.cshtml extension. Inside theHomeController, you can return theIndex view of yourManage views with a relative path:

return View("../Manage/Index");

Similarly, you can indicate the current controller-specific directory with the "./" prefix:

return View("./About");

Partial views andview components use similar (but not identical) discovery mechanisms.

You can customize the default convention for how views are located within the app by using a customIViewLocationExpander.

View discovery relies on finding view files by file name. If the underlying file system is case sensitive, view names are probably case sensitive. For compatibility across operating systems, match case between controller and action names and associated view folders and file names. If you encounter an error that a view file can't be found while working with a case-sensitive file system, confirm that the casing matches between the requested view file and the actual view file name.

Follow the best practice of organizing the file structure for your views to reflect the relationships among controllers, actions, and views for maintainability and clarity.

Pass data to views

Pass data to views using several approaches:

• Strongly typed data: viewmodel
• Weakly typed data
  • ViewData (ViewDataAttribute)
  • ViewBag

Strongly-typed data (viewmodel)

The most robust approach is to specify amodel type in the view. This model is commonly referred to as aviewmodel. You pass an instance of the viewmodel type to the view from the action.

Using a viewmodel to pass data to a view allows the view to take advantage ofstrong type checking.Strong typing (orstrongly typed) means that every variable and constant has an explicitly defined type (for example,string,int, orDateTime). The validity of types used in a view is checked at compile time.

Visual Studio andVisual Studio Code list strongly typed class members using a feature calledIntelliSense. When you want to see the properties of a viewmodel, type the variable name for the viewmodel followed by a period (.). This helps you write code faster with fewer errors.

Specify a model using the@model directive. Use the model with@Model:

@model WebApplication1.ViewModels.Address<h2>Contact</h2><address>    @Model.Street<br>    @Model.City, @Model.State @Model.PostalCode<br>    <abbr title="Phone">P:</abbr> 425.555.0100</address>

To provide the model to the view, the controller passes it as a parameter:

public IActionResult Contact(){    ViewData["Message"] = "Your contact page.";    var viewModel = new Address()    {        Name = "Microsoft",        Street = "One Microsoft Way",        City = "Redmond",        State = "WA",        PostalCode = "98052-6399"    };    return View(viewModel);}

There are no restrictions on the model types that you can provide to a view. We recommend using Plain Old CLR Object (POCO) viewmodels with little or no behavior (methods) defined. Usually, viewmodel classes are either stored in theModels folder or a separateViewModels folder at the root of the app. TheAddress viewmodel used in the example above is a POCO viewmodel stored in a file namedAddress.cs:

namespace WebApplication1.ViewModels{    public class Address    {        public string Name { get; set; }        public string Street { get; set; }        public string City { get; set; }        public string State { get; set; }        public string PostalCode { get; set; }    }}

Nothing prevents you from using the same classes for both your viewmodel types and your business model types. However, using separate models allows your views to vary independently from the business logic and data access parts of your app. Separation of models and viewmodels also offers security benefits when models usemodel binding andvalidation for data sent to the app by the user.

Weakly typed data (ViewData,[ViewData] attribute, andViewBag)

ViewBagisn't available by default for use in Razor PagesPageModelclasses.

In addition to strongly typed views, views have access to aweakly typed (also calledloosely typed) collection of data. Unlike strong types,weak types (orloose types) means that you don't explicitly declare the type of data you're using. You can use the collection of weakly typed data for passing small amounts of data in and out of controllers and views.

This collection can be referenced through either theViewData orViewBag properties on controllers and views. TheViewData property is a dictionary of weakly typed objects. TheViewBag property is a wrapper aroundViewData that provides dynamic properties for the underlyingViewData collection. Note: Key lookups are case-insensitive for bothViewData andViewBag.

ViewData andViewBag are dynamically resolved at runtime. Since they don't offer compile-time type checking, both are generally more error-prone than using a viewmodel. For that reason, some developers prefer to minimally or never useViewData andViewBag.

ViewData

ViewData is aViewDataDictionary object accessed throughstring keys. String data can be stored and used directly without the need for a cast, but you must cast otherViewData object values to specific types when you extract them. You can useViewData to pass data from controllers to views and within views, includingpartial views andlayouts.

The following is an example that sets values for a greeting and an address usingViewData in an action:

public IActionResult SomeAction(){    ViewData["Greeting"] = "Hello";    ViewData["Address"]  = new Address()    {        Name = "Steve",        Street = "123 Main St",        City = "Hudson",        State = "OH",        PostalCode = "44236"    };    return View();}

Work with the data in a view:

@{    // Since Address isn't a string, it requires a cast.    var address = ViewData["Address"] as Address;}@ViewData["Greeting"] World!<address>    @address.Name<br>    @address.Street<br>    @address.City, @address.State @address.PostalCode</address>

[ViewData] attribute

Another approach that uses theViewDataDictionary isViewDataAttribute. Properties on controllers or Razor Page models marked with the[ViewData] attribute have their values stored and loaded from the dictionary.

In the following example, the Home controller contains aTitle property marked with[ViewData]. TheAbout method sets the title for the About view:

public class HomeController : Controller{    [ViewData]    public string Title { get; set; }    public IActionResult About()    {        Title = "About Us";        ViewData["Message"] = "Your application description page.";        return View();    }}

In the layout, the title is read from the ViewData dictionary:

<!DOCTYPE html><html lang="en"><head>    <title>@ViewData["Title"] - WebApplication</title>    ...

ViewBag

ViewBagisn't available by default for use in Razor PagesPageModelclasses.

ViewBag is aMicrosoft.AspNetCore.Mvc.ViewFeatures.Internal.DynamicViewData object that provides dynamic access to the objects stored inViewData.ViewBag can be more convenient to work with, since it doesn't require casting. The following example shows how to useViewBag with the same result as usingViewData above:

public IActionResult SomeAction(){    ViewBag.Greeting = "Hello";    ViewBag.Address  = new Address()    {        Name = "Steve",        Street = "123 Main St",        City = "Hudson",        State = "OH",        PostalCode = "44236"    };    return View();}

@ViewBag.Greeting World!<address>    @ViewBag.Address.Name<br>    @ViewBag.Address.Street<br>    @ViewBag.Address.City, @ViewBag.Address.State @ViewBag.Address.PostalCode</address>

UsingViewData andViewBag simultaneously

ViewBagisn't available by default for use in Razor PagesPageModelclasses.

SinceViewData andViewBag refer to the same underlyingViewData collection, you can use bothViewData andViewBag and mix and match between them when reading and writing values.

Set the title usingViewBag and the description usingViewData at the top of anAbout.cshtml view:

@{    Layout = "/Views/Shared/_Layout.cshtml";    ViewBag.Title = "About Contoso";    ViewData["Description"] = "Let us tell you about Contoso's philosophy and mission.";}

Read the properties but reverse the use ofViewData andViewBag. In the_Layout.cshtml file, obtain the title usingViewData and obtain the description usingViewBag:

<!DOCTYPE html><html lang="en"><head>    <title>@ViewData["Title"]</title>    <meta name="description" content="@ViewBag.Description">    ...

Remember that strings don't require a cast forViewData. You can use@ViewData["Title"] without casting.

Using bothViewData andViewBag at the same time works, as does mixing and matching reading and writing the properties. The following markup is rendered:

<!DOCTYPE html><html lang="en"><head>    <title>About Contoso</title>    <meta name="description" content="Let us tell you about Contoso's philosophy and mission.">    ...

Summary of the differences betweenViewData andViewBag

ViewBagisn't available by default for use in Razor PagesPageModelclasses.

• ViewData
  • Derives fromViewDataDictionary, so it has dictionary properties that can be useful, such asContainsKey,Add,Remove, andClear.
  • Keys in the dictionary are strings, so whitespace is allowed. Example:ViewData["Some Key With Whitespace"]
  • Any type other than astring must be cast in the view to useViewData.
• ViewBag
  • Derives fromMicrosoft.AspNetCore.Mvc.ViewFeatures.Internal.DynamicViewData, so it allows the creation of dynamic properties using dot notation (@ViewBag.SomeKey = <value or object>), and no casting is required. The syntax ofViewBag makes it quicker to add to controllers and views.
  • Simpler to check for null values. Example:@ViewBag.Person?.Name

When to useViewData orViewBag

BothViewData andViewBag are equally valid approaches for passing small amounts of data among controllers and views. The choice of which one to use is based on preference. You can mix and matchViewData andViewBag objects, however, the code is easier to read and maintain with one approach used consistently. Both approaches are dynamically resolved at runtime and thus prone to causing runtime errors. Some development teams avoid them.

Dynamic views

Views that don't declare a model type using@model but that have a model instance passed to them (for example,return View(Address);) can reference the instance's properties dynamically:

<address>    @Model.Street<br>    @Model.City, @Model.State @Model.PostalCode<br>    <abbr title="Phone">P:</abbr> 425.555.0100</address>

This feature offers flexibility but doesn't offer compilation protection or IntelliSense. If the property doesn't exist, webpage generation fails at runtime.

More view features

Tag Helpers make it easy to add server-side behavior to existing HTML tags. Using Tag Helpers avoids the need to write custom code or helpers within your views. Tag helpers are applied as attributes to HTML elements and are ignored by editors that can't process them. This allows you to edit and render view markup in a variety of tools.

Generating custom HTML markup can be achieved with many built-in HTML Helpers. More complex user interface logic can be handled byView Components. View components provide the same SoC that controllers and views offer. They can eliminate the need for actions and views that deal with data used by common user interface elements.

Like many other aspects of ASP.NET Core, views supportdependency injection, allowing services to beinjected into views.

h1 {    color: red;}

<link rel="stylesheet" href="~/{APP ASSEMBLY}.styles.css" />

<link rel="stylesheet" href="WebApp.styles.css" />

/* /Pages/Index.cshtml.rz.scp.css */h1[b-3xxtam6d07] {    color: red;}

<h1 b-3xxtam6d07>

<ItemGroup>  <None Update="{Pages|Views}/Index.cshtml.css" CssScope="custom-scope-identifier" /></ItemGroup>

<ItemGroup>  <None Update="{Pages|Views}/BaseView.cshtml.css" CssScope="custom-scope-identifier" />  <None Update="{Pages|Views}/DerivedView.cshtml.css" CssScope="custom-scope-identifier" /></ItemGroup>

<ItemGroup>  <None Update="{Pages|Views}/*.cshtml.css" CssScope="custom-scope-identifier" /></ItemGroup>

<PropertyGroup>  <StaticWebAssetBasePath>_content/$(PackageId)</StaticWebAssetBasePath></PropertyGroup>

<PropertyGroup>  <DisableScopedCssBundling>true</DisableScopedCssBundling></PropertyGroup>

<link href="_content/ClassLib/ClassLib.bundle.scp.css" rel="stylesheet">