ASP.NET Core Razor components

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This article explains how to create and use Razor components in Blazor apps, including guidance on Razor syntax, component naming, namespaces, and component parameters.

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Razor components

Blazor apps are built using Razor components, informally known as Blazor components or only components. A component is a self-contained portion of user interface (UI) with processing logic to enable dynamic behavior. Components can be nested, reused, shared among projects, and used in MVC and Razor Pages apps.

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Components render into an in-memory representation of the browser’s Document Object Model (DOM) called a render tree, which is used to update the UI in a flexible and efficient way.

Although “Razor components” shares some naming with other ASP.NET Core content-rendering technologies, Razor components must be distinguished from the following different features in ASP.NET Core:

Razor views, which are Razor-based markup pages for MVC apps.
View components, which are for rendering chunks of content rather than whole responses in Razor Pages and MVC apps.

Component classes

Components are implemented using a combination of C# and HTML markup in Razor component files with the .razor file extension.

ComponentBase is the base class for components described by Razor component files. ComponentBase implements the lowest abstraction of components, the IComponent interface. ComponentBase defines component properties and methods for basic functionality, for example, to process a set of built-in component lifecycle events.

ComponentBase in dotnet/aspnetcore reference source: The reference source contains additional remarks on the built-in lifecycle events. However, keep in mind that the internal implementations of component features are subject to change at any time without notice.

Developers typically create Razor components from Razor component files (.razor) or base their components on ComponentBase, but components can also be built by implementing IComponent. Developer-built components that implement IComponent can take low-level control over rendering at the cost of having to manually trigger rendering with events and lifecycle methods that the developer must create and maintain.

Additional conventions adopted by Blazor documentation example code and sample apps is found in ASP.NET Core Blazor fundamentals.

Razor syntax

Components use Razor syntax. Two Razor features are extensively used by components, directives and directive attributes. These are reserved keywords prefixed with @ that appear in Razor markup:

Directives: Change the way component markup is compiled or functions. For example, the @page directive specifies a routable component with a route template that can be reached directly by a user’s request in the browser at a specific URL.

By convention, a component’s directives at the top of a component definition (.razor file) are placed in a consistent order. For repeated directives, directives are placed alphabetically by namespace or type, except @using directives, which have special second-level ordering.

The following order is adopted by Blazor sample apps and documentation. Components provided by a Blazor project template may differ from the following order and use a different format. For example, Blazor framework Identity components include blank lines between blocks of @using directives and blocks of @inject directives. You’re free to use a custom ordering scheme and format in your own apps.

Documentation and sample app Razor directive order:
- @page
- @rendermode (.NET 8 or later)
- @using
  - System namespaces (alphabetical order)
  - Microsoft namespaces (alphabetical order)
  - Third-party API namespaces (alphabetical order)
  - App namespaces (alphabetical order)
- Other directives (alphabetical order)
No blank lines appear among the directives. One blank line appears between the directives and the first line of Razor markup.

Example:

@page “/doctor-who-episodes/{season:int}” @rendermode InteractiveWebAssembly @using System.Globalization @using System.Text.Json @using Microsoft.AspNetCore.Localization @using Mandrill @using BlazorSample.Components.Layout @attribute [Authorize] @implements IAsyncDisposable @inject IJSRuntime JS @inject ILogger<DoctorWhoEpisodes> Logger <PageTitle>Doctor Who Episode List</PageTitle> …
Directive attributes: Change the way a component element is compiled or functions.

Example:

<input @bind=”episodeId” />

You can prefix directive attribute values with the at symbol (@) for non-explicit Razor expressions (@bind=”@episodeId”), but we don’t recommend it, and the docs don’t adopt the approach in examples.

Directives and directive attributes used in components are explained further in this article and other articles of the Blazor documentation set. For general information on Razor syntax, see Razor syntax reference for ASP.NET Core.

Component name, class name, and namespace

Partial class support

Components are generated as C# partial classes and are authored using either of the following approaches:

A single file contains C# code defined in one or more @code blocks, HTML markup, and Razor markup. Blazor project templates define their components using this single-file approach.
HTML and Razor markup are placed in a Razor file (.razor). C# code is placed in a code-behind file defined as a partial class (.cs).

The following example shows the default Counter component with an @code block in an app generated from a Blazor project template. Markup and C# code are in the same file. This is the most common approach taken in component authoring.

Counter.razor:

The following Counter component splits presentation HTML and Razor markup from the C# code using a code-behind file with a partial class. Splitting the markup from the C# code is favored by some organizations and developers to organize their component code to suit how they prefer to work. For example, the organization’s UI expert can work on the presentation layer independently of another developer working on the component’s C# logic. The approach is also useful when working with automatically-generated code or source generators. For more information, see Partial Classes and Methods (C# Programming Guide).

CounterPartialClass.razor:

CounterPartialClass.razor.cs:

@using directives in the _Imports.razor file are only applied to Razor files (.razor), not C# files (.cs). Add namespaces to a partial class file as needed.

Typical namespaces used by components:

Specify a base class

The @inherits directive is used to specify a base class for a component. Unlike using partial classes, which only split markup from C# logic, using a base class allows you to inherit C# code for use across a group of components that share the base class’s properties and methods. Using base classes reduce code redundancy in apps and are useful when supplying base code from class libraries to multiple apps. For more information, see Inheritance in C# and .NET.

In the following example, the BlazorRocksBase1 base class derives from ComponentBase.

BlazorRocks1.razor:

BlazorRocksBase1.cs:

Routing

Routing in Blazor is achieved by providing a route template to each accessible component in the app with an @page directive. When a Razor file with an @page directive is compiled, the generated class is given a RouteAttribute specifying the route template. At runtime, the router searches for component classes with a RouteAttribute and renders whichever component has a route template that matches the requested URL.

The following HelloWorld component uses a route template of /hello-world, and the rendered webpage for the component is reached at the relative URL /hello-world.

HelloWorld.razor:

The preceding component loads in the browser at /hello-world regardless of whether or not you add the component to the app’s UI navigation. Optionally, components can be added to the NavMenu component so that a link to the component appears in the app’s UI-based navigation.

For the preceding HelloWorld component, you can add a NavLink component to the NavMenu component. For more information, including descriptions of the NavLink and NavMenu components, see ASP.NET Core Blazor routing and navigation.

Markup

A component’s UI is defined using Razor syntax, which consists of Razor markup, C#, and HTML. When an app is compiled, the HTML markup and C# rendering logic are converted into a component class. The name of the generated class matches the name of the file.

Members of the component class are defined in one or more @code blocks. In @code blocks, component state is specified and processed with C#:

Property and field initializers.
Parameter values from arguments passed by parent components and route parameters.
Methods for user event handling, lifecycle events, and custom component logic.

Component members are used in rendering logic using C# expressions that start with the @ symbol. For example, a C# field is rendered by prefixing @ to the field name. The following Markup component evaluates and renders:

headingFontStyle for the CSS property value font-style of the heading element.
headingText for the content of the heading element.

Markup.razor:

The Blazor framework processes a component internally as a render tree, which is the combination of a component’s DOM and Cascading Style Sheet Object Model (CSSOM). After the component is initially rendered, the component’s render tree is regenerated in response to events. Blazor compares the new render tree against the previous render tree and applies any modifications to the browser’s DOM for display. For more information, see ASP.NET Core Razor component rendering.

Razor syntax for C# control structures, directives, and directive attributes are lowercase (examples: @if, @code, @bind). Property names are uppercase (example: @Body for LayoutComponentBase.Body).

Asynchronous methods (async) don’t support returning void

The Blazor framework doesn’t track void-returning asynchronous methods (async). As a result, exceptions aren’t caught if void is returned. Always return a Task from asynchronous methods.

Nested components

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Components can include other components by declaring them using HTML syntax. The markup for using a component looks like an HTML tag where the name of the tag is the component type.

Consider the following Heading component, which can be used by other components to display a heading.

Heading.razor:

The following markup in the HeadingExample component renders the preceding Heading component at the location where the <Heading /> tag appears.

HeadingExample.razor:

If a component contains an HTML element with an uppercase first letter that doesn’t match a component name within the same namespace, a warning is emitted indicating that the element has an unexpected name. Adding an @using directive for the component’s namespace makes the component available, which resolves the warning. For more information, see the Component name, class name, and namespace section.

The Heading component example shown in this section doesn’t have an @page directive, so the Heading component isn’t directly accessible to a user via a direct request in the browser. However, any component with an @page directive can be nested in another component. If the Heading component was directly accessible by including @page “/heading” at the top of its Razor file, then the component would be rendered for browser requests at both /heading and /heading-example.

Component parameters

Component parameters pass data to components and are defined using public C# properties on the component class with the [Parameter] attribute. In the following example, a built-in reference type (System.String) and a user-defined reference type (PanelBody) are passed as component parameters.

PanelBody.cs:

ParameterChild.razor:

The Title and Body component parameters of the ParameterChild component are set by arguments in the HTML tag that renders the instance of the component. The following ParameterParent component renders two ParameterChild components:

The first ParameterChild component is rendered without supplying parameter arguments.
The second ParameterChild component receives values for Title and Body from the ParameterParent component, which uses an explicit C# expression to set the values of the PanelBody’s properties.

The following rendered HTML markup from the ParameterParent component shows ParameterChild component default values when the ParameterParent component doesn’t supply component parameter values. When the ParameterParent component provides component parameter values, they replace the ParameterChild component’s default values.

<h1>Child component (without attribute values)</h1> <div> <div>Set By Child</div> <div>Set by child.</div> </div> <h1>Child component (with attribute values)</h1> <div> <div>Set by Parent</div> <div>Set by parent.</div> </div>

Assign a C# field, property, or result of a method to a component parameter as an HTML attribute value. The value of the attribute can typically be any C# expression that matches the type of the parameter. The value of the attribute can optionally lead with a Razor reserved @ symbol, but it isn’t required.

If the component parameter is of type string, then the attribute value is instead treated as a C# string literal. If you want to specify a C# expression instead, then use the @ prefix.

The following ParameterParent2 component displays four instances of the preceding ParameterChild component and sets their Title parameter values to:

The value of the title field.
The result of the GetTitle C# method.
The current local date in long format with ToLongDateString, which uses an implicit C# expression.
The panelData object’s Title property.

Quotes around parameter attribute values are optional in most cases per the HTML5 specification. For example, Value=this is supported, instead of Value=”this”. However, we recommend using quotes because it’s easier to remember and widely adopted across web-based technologies.

Throughout the documentation, code examples:

Always use quotes. Example: Value=”this”.
Don’t use the @ prefix with nonliterals unless required. Example: Count=”ct”, where ct is a number-typed variable. Count=”@ct” is a valid stylistic approach, but the documentation and examples don’t adopt the convention.
Always avoid @ for literals, outside of Razor expressions. Example: IsFixed=”true”. This includes keywords (for example, this) and null, but you can choose to use them if you wish. For example, IsFixed=”@true” is uncommon but supported.

Unlike in Razor pages (.cshtml), Blazor can’t perform asynchronous work in a Razor expression while rendering a component. This is because Blazor is designed for rendering interactive UIs. In an interactive UI, the screen must always display something, so it doesn’t make sense to block the rendering flow. Instead, asynchronous work is performed during one of the asynchronous lifecycle events. After each asynchronous lifecycle event, the component may render again. The following Razor syntax is not supported:

The code in the preceding example generates a compiler error when the app is built:

The ‘await’ operator can only be used within an async method. Consider marking this method with the ‘async’ modifier and changing its return type to ‘Task’.

To obtain a value for the Title parameter in the preceding example asynchronously, the component can use the OnInitializedAsync lifecycle event, as the following example demonstrates:

<ParameterChild Title=”@title” /> @code { private string? title; protected override async Task OnInitializedAsync() { title = await …; } }

For more information, see ASP.NET Core Razor component lifecycle.

Use of an explicit Razor expression to concatenate text with an expression result for assignment to a parameter is not supported. The following example seeks to concatenate the text “Set by ” with an object’s property value. Although this syntax is supported in a Razor page (.cshtml), it isn’t valid for assignment to the child’s Title parameter in a component. The following Razor syntax is not supported:

The code in the preceding example generates a compiler error when the app is built:

Component attributes do not support complex content (mixed C# and markup).

To support the assignment of a composed value, use a method, field, or property. The following example performs the concatenation of “Set by ” and an object’s property value in the C# method GetTitle:

For more information, see Razor syntax reference for ASP.NET Core.

Component parameters should be declared as auto-properties, meaning that they shouldn’t contain custom logic in their get or set accessors. For example, the following StartData property is an auto-property:

[Parameter] public DateTime StartData { get; set; }

Don’t place custom logic in the get or set accessor because component parameters are purely intended for use as a channel for a parent component to flow information to a child component. If a set accessor of a child component property contains logic that causes rerendering of the parent component, an infinite rendering loop results.

To transform a received parameter value:

Leave the parameter property as an auto-property to represent the supplied raw data.
Create a different property or method to supply the transformed data based on the parameter property.

Override OnParametersSetAsync to transform a received parameter each time new data is received.

Writing an initial value to a component parameter is supported because initial value assignments don’t interfere with the Blazor’s automatic component rendering. The following assignment of the current local DateTime with DateTime.Now to StartData is valid syntax in a component:

[Parameter] public DateTime StartData { get; set; } = DateTime.Now;

After the initial assignment of DateTime.Now, do not assign a value to StartData in developer code. For more information, see Avoid overwriting parameters in ASP.NET Core Blazor.

Route parameters

Components can specify route parameters in the route template of the @page directive. The Blazor router uses route parameters to populate corresponding component parameters.

RouteParameter1.razor:

Child content render fragments

Components can set the content of another component. The assigning component provides the content between the child component’s opening and closing tags.

In the following example, the RenderFragmentChild component has a ChildContent component parameter that represents a segment of the UI to render as a RenderFragment. The position of ChildContent in the component’s Razor markup is where the content is rendered in the final HTML output.

RenderFragmentChild.razor:

The following component provides content for rendering the RenderFragmentChild by placing the content inside the child component’s opening and closing tags.

Render fragments are used to render child content throughout Blazor apps and are described with examples in the following articles and article sections:

Blazor layouts
Pass data across a component hierarchy
Templated components
Global exception handling

Render fragments for reusable rendering logic

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You can factor out child components purely as a way of reusing rendering logic. In any component’s @code block, define a RenderFragment and render the fragment from any location as many times as needed:

@RenderWelcomeInfo <p>Render the welcome info a second time:</p> @RenderWelcomeInfo @code { private RenderFragment RenderWelcomeInfo = @<p>Welcome to your new app!</p>; }

For more information, see Reuse rendering logic.

Loop variables with component parameters and child content

Rendering components inside a for loop requires a local index variable if the incrementing loop variable is used by the component’s parameters or RenderFragment child content.

Consider the following RenderFragmentChild2 component that has both a component parameter (Id) and a render fragment to display child content (ChildContent).

RenderFragmentChild2.razor:

<div class=”card w-25″ style=”margin-bottom:15px”> <div class=”card-header font-weight-bold”>Child content (@Id)</div> <div class=”card-body”>@ChildContent</div> </div> @code { [Parameter] public string? Id { get; set; } [Parameter] public RenderFragment? ChildContent { get; set; } }

When rendering the RenderFragmentChild2 component in a parent component, use a local index variable (ct in the following example) instead of the loop variable (c) when assigning the component parameter value and providing the child component’s content:

@for (int c = 1; c < 4; c++) { var ct = c; <RenderFragmentChild2 Id=”@($”Child{ct}”)”> Count: @ct </RenderFragmentChild2> }

Alternatively, use a foreach loop with Enumerable.Range instead of a for loop:

@foreach (var c in Enumerable.Range(1, 3)) { <RenderFragmentChild2 Id=”@($”Child{c}”)”> Count: @c </RenderFragmentChild2> }

Capture references to components

Component references provide a way to reference a component instance for issuing commands. To capture a component reference:

Add an @ref attribute to the child component.
Define a field with the same type as the child component.

When the component is rendered, the field is populated with the component instance. You can then invoke .NET methods on the instance.

Consider the following ReferenceChild component that logs a message when its ChildMethod is called.

ReferenceChild.razor:

A component reference is only populated after the component is rendered and its output includes ReferenceChild’s element. Until the component is rendered, there’s nothing to reference. Don’t attempt to call a referenced component method to an event handler directly (for example, @onclick=”childComponent!.ChildMethod(5)”) because the reference variable may not be assigned at the time the click event is assigned.

To manipulate component references after the component has finished rendering, use the OnAfterRender or OnAfterRenderAsync methods.

The following example uses the preceding ReferenceChild component.

ReferenceParent.razor:

While capturing component references use a similar syntax to capturing element references, capturing component references isn’t a JavaScript interop feature. Component references aren’t passed to JavaScript code. Component references are only used in .NET code.

Apply an attribute

Attributes can be applied to components with the @attribute directive. The following example applies the [Authorize] attribute to the component’s class:

@page “/” @attribute [Authorize]

Conditional HTML element attributes and DOM properties

Blazor adopts the following general behaviors:

For HTML attributes, Blazor sets or removes the attribute conditionally based on the .NET value. If the .NET value is false or null, the attribute isn’t set or is removed if it was previously set.
For DOM properties, such as checked or value, Blazor sets the DOM property based on the .NET value. If the .NET value is false or null, the DOM property is reset to a default value.

Which Razor syntax attributes correspond to HTML attributes and which ones correspond to DOM properties remains undocumented because this is a framework implementation detail that might change without notice.

Raw HTML

Strings are normally rendered using DOM text nodes, which means that any markup they may contain is ignored and treated as literal text. To render raw HTML, wrap the HTML content in a MarkupString value. The value is parsed as HTML or SVG and inserted into the DOM.

The following example shows using the MarkupString type to add a block of static HTML content to the rendered output of a component.

Razor templates

Render fragments can be defined using Razor template syntax to define a UI snippet. Razor templates use the following format:

@<{HTML tag}>…</{HTML tag}>

The following example illustrates how to specify RenderFragment and RenderFragment<TValue> values and render templates directly in a component. Render fragments can also be passed as arguments to templated components.

RazorTemplate.razor:

Rendered output of the preceding code:

<p>The time is 4/19/2021 8:54:46 AM.</p> <p>Pet: Nutty Rex</p>

Static assets

Blazor follows the convention of ASP.NET Core apps for static assets. Static assets are located in the project’s web root (wwwroot) folder or folders under the wwwroot folder.

Use a base-relative path (/) to refer to the web root for a static asset. In the following example, logo.png is physically located in the {PROJECT ROOT}/wwwroot/images folder. {PROJECT ROOT} is the app’s project root.

Components do not support tilde-slash notation (~/).

For information on setting an app’s base path, see Host and deploy ASP.NET Core Blazor.

Tag Helpers aren’t supported in components

Tag Helpers aren’t supported in components. To provide Tag Helper-like functionality in Blazor, create a component with the same functionality as the Tag Helper and use the component instead.

Scalable Vector Graphics (SVG) images

Since Blazor renders HTML, browser-supported images, including Scalable Vector Graphics (SVG) images (.svg), are supported via the <img> tag:

Similarly, SVG images are supported in the CSS rules of a stylesheet file (.css):

.element-class { background-image: url(“image.svg”); }

Whitespace rendering behavior

MVC and Razor Pages apps can also use the Component Tag Helper to register statically-rendered Blazor WebAssembly root components:

Statically-rendered components can only be added to the app. They can’t be removed or updated afterwards.

For more information, see the following resources:

Component Tag Helper in ASP.NET Core
Integrate ASP.NET Core Razor components with MVC or Razor Pages

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Razor components