Create and use Razor components

By Luke Latham, Daniel Roth, and Morné Zaayman

View or download sample code (how to download)

Blazor apps are built using components. A component is a self-contained chunk of user interface (UI), such as a page, dialog, or form. A component includes HTML markup and the processing logic required to inject data or respond to UI events. Components are flexible and lightweight. They can be nested, reused, and shared among projects.

Component classes

Components are implemented in Razor component files (.razor) using a combination of C# and HTML markup.

Components can be authored using the .cshtml file extension as long as the files are identified as Razor component files using the _RazorComponentInclude MSBuild property. For example, an app that specifies that all .cshtml files under the Pages folder should be treated as Razor components files:

<_RazorComponentInclude>Pages\**\*.cshtml</_RazorComponentInclude>

The UI for a component is defined using HTML. Dynamic rendering logic (for example, loops, conditionals, expressions) is added using an embedded C# syntax called Razor. 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 an @functions block (more than one @functions block is permissible). In the @functions block, component state (properties, fields) is specified with methods for event handling or for defining other component logic.

Component members can then be used as part of the component's rendering logic using C# expressions that start with @. For example, a C# field is rendered by prefixing @ to the field name. The following example evaluates and renders:

  • _headingFontStyle to the CSS property value for font-style.
  • _headingText to the content of the <h1> element.
<h1 style="font-style:@_headingFontStyle">@_headingText</h1>

@functions {
    private string _headingFontStyle = "italic";
    private string _headingText = "Put on your new Blazor!";
}

After the component is initially rendered, the component regenerates its render tree in response to events. Blazor then compares the new render tree against the previous one and applies any modifications to the browser's Document Object Model (DOM).

Integrate components into Razor Pages and MVC apps

Use components with existing Razor Pages and MVC apps. There's no need to rewrite existing pages or views to use Razor components. When the page or view is rendered, components are prerendered† at the same time.

Note

†Server-side prerendering is enabled for Blazor server-side apps by default. Client-side Blazor apps will support prerendering in the upcoming Preview 5 release. For more information, see Update templates/middleware to use MapFallbackToPage/File.

To render a component from a page or view, use the RenderComponentAsync<TComponent> HTML helper method:

<div id="Counter">
    @(await Html.RenderComponentAsync<Counter>(new { IncrementAmount = 10 }))
</div>

While pages and views can use components, the converse isn't true. Components can't use view- and page-specific scenarios, such as partial views and sections. To use logic from partial view in a component, factor out the partial view logic into a component.

For more information on how components are rendered and component state is managed in Blazor server-side apps, see the Blazor hosting models article.

Using components

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

The following markup in Index.razor renders a HeadingComponent instance, which exists in another file, HeadingComponent.razor:

<HeadingComponent />

Component parameters

Components can have component parameters, which are defined using non-public properties on the component class with the [Parameter] attribute. Use attributes to specify arguments for a component in markup.

In the following example, the ParentComponent sets the value of the Title property of the ChildComponent:

Parent component:

@page "/ParentComponent"

<h1>Parent-child example</h1>

<ChildComponent Title="Panel Title from Parent"
                OnClick="@ShowMessage">
    Content of the child component is supplied
    by the parent component.
</ChildComponent>

<p><b>@messageText</b></p>

@functions {
    private string messageText;

    private void ShowMessage(UIMouseEventArgs e)
    {
        messageText = "Blaze a new trail with Blazor!";
    }
}

Child component:

<div class="panel panel-default">
    <div class="panel-heading">@Title</div>
    <div class="panel-body">@ChildContent</div>

    <button class="btn btn-primary" onclick="@OnClick">
        Trigger a Parent component method
    </button>
</div>

@functions {
    [Parameter]
    private string Title { get; set; }

    [Parameter]
    private RenderFragment ChildContent { get; set; }

    [Parameter]
    private EventCallback<UIMouseEventArgs> OnClick { get; set; }
}

Child content

Components can set the content of another component. The assigning component provides the content between the tags that specify the receiving component. For example, a ParentComponent can provide content for rendering by a Child component by placing the content inside <ChildComponent> tags.

Parent component:

@page "/ParentComponent"

<h1>Parent-child example</h1>

<ChildComponent Title="Panel Title from Parent"
                OnClick="@ShowMessage">
    Content of the child component is supplied
    by the parent component.
</ChildComponent>

<p><b>@messageText</b></p>

@functions {
    private string messageText;

    private void ShowMessage(UIMouseEventArgs e)
    {
        messageText = "Blaze a new trail with Blazor!";
    }
}

The Child component has a ChildContent property that represents a RenderFragment. The value of ChildContent is positioned in the child component's markup where the content should be rendered. In the following example, the value of ChildContent is received from the parent component and rendered inside the Bootstrap panel's panel-body.

Child component:

<div class="panel panel-default">
    <div class="panel-heading">@Title</div>
    <div class="panel-body">@ChildContent</div>

    <button class="btn btn-primary" onclick="@OnClick">
        Trigger a Parent component method
    </button>
</div>

@functions {
    [Parameter]
    private string Title { get; set; }

    [Parameter]
    private RenderFragment ChildContent { get; set; }

    [Parameter]
    private EventCallback<UIMouseEventArgs> OnClick { get; set; }
}

Note

The property receiving the RenderFragment content must be named ChildContent by convention.

Data binding

Data binding to both components and DOM elements is accomplished with the bind attribute. The following example binds the _italicsCheck field to the check box's checked state:

<input type="checkbox" class="form-check-input" id="italicsCheck" 
    bind="@_italicsCheck">

When the check box is selected and cleared, the property's value is updated to true and false, respectively.

The check box is updated in the UI only when the component is rendered, not in response to changing the property's value. Since components render themselves after event handler code executes, property updates are usually reflected in the UI immediately.

Using bind with a CurrentValue property (<input bind="@CurrentValue">) is essentially equivalent to the following:

<input value="@CurrentValue" 
    onchange="@((UIChangeEventArgs __e) => CurrentValue = __e.Value)">

When the component is rendered, the value of the input element comes from the CurrentValue property. When the user types in the text box, the onchange event is fired and the CurrentValue property is set to the changed value. In reality, the code generation is a little more complex because bind handles a few cases where type conversions are performed. In principle, bind associates the current value of an expression with a value attribute and handles changes using the registered handler.

In addition to onchange, the property can be bound using other events like oninput by being more explicit about what to bind to:

<input type="text" bind-value-oninput="@CurrentValue">

Unlike onchange, oninput fires for every character that is input into the text box.

Format strings

Data binding works with DateTime format strings. Other format expressions, such as currency or number formats, aren't available at this time.

<input bind="@StartDate" format-value="yyyy-MM-dd">

@functions {
    [Parameter]
    private DateTime StartDate { get; set; } = new DateTime(2020, 1, 1);
}

The format-value attribute specifies the date format to apply to the value of the input element. The format is also used to parse the value when an onchange event occurs.

Component parameters

Binding also recognizes component parameters, where bind-{property} can bind a property value across components.

The following component uses ChildComponent and binds the ParentYear parameter from the parent to the Year parameter on the child component:

Parent component:

@page "/ParentComponent"

<h1>Parent Component</h1>

<p>ParentYear: @ParentYear</p>

<ChildComponent bind-Year="@ParentYear" />

<button class="btn btn-primary" onclick="@ChangeTheYear">
    Change Year to 1986
</button>

@functions {
    [Parameter]
    private int ParentYear { get; set; } = 1978;

    private void ChangeTheYear()
    {
        ParentYear = 1986;
    }
}

Child component:

<h2>Child Component</h2>

<p>Year: @Year</p>

@functions {
    [Parameter]
    private int Year { get; set; }

    [Parameter]
    private EventCallback<int> YearChanged { get; set; }
}

EventCallback<T> is explained in the EventCallback section.

Loading the ParentComponent produces the following markup:

<h1>Parent Component</h1>

<p>ParentYear: 1978</p>

<h2>Child Component</h2>

<p>Year: 1978</p>

If the value of the ParentYear property is changed by selecting the button in the ParentComponent, the Year property of the ChildComponent is updated. The new value of Year is rendered in the UI when the ParentComponent is rerendered:

<h1>Parent Component</h1>

<p>ParentYear: 1986</p>

<h2>Child Component</h2>

<p>Year: 1986</p>

The Year parameter is bindable because it has a companion YearChanged event that matches the type of the Year parameter.

By convention, <ChildComponent bind-Year="@ParentYear" /> is essentially equivalent to writing,

<ChildComponent bind-Year-YearChanged="@ParentYear" />

In general, a property can be bound to a corresponding event handler using bind-property-event attribute.

Event handling

Razor components provide event handling features. For an HTML element attribute named on<event> (for example, onclick, onsubmit) with a delegate-typed value, Razor components treats the attribute's value as an event handler. The attribute's name always starts with on.

The following code calls the UpdateHeading method when the button is selected in the UI:

<button class="btn btn-primary" onclick="@UpdateHeading">
    Update heading
</button>

@functions {
    private void UpdateHeading(UIMouseEventArgs e)
    {
        ...
    }
}

The following code calls the CheckboxChanged method when the check box is changed in the UI:

<input type="checkbox" class="form-check-input" onchange="@CheckboxChanged">

@functions {
    private void CheckboxChanged()
    {
        ...
    }
}

Event handlers can also be asynchronous and return a Task. There's no need to manually call StateHasChanged(). Exceptions are logged when they occur.

<button class="btn btn-primary" onclick="@UpdateHeading">
    Update heading
</button>

@functions {
    private async Task UpdateHeading(UIMouseEventArgs e)
    {
        ...
    }
}

For some events, event-specific event argument types are permitted. If access to one of these event types isn't necessary, it isn't required in the method call.

The list of supported event arguments is:

  • UIEventArgs
  • UIChangeEventArgs
  • UIKeyboardEventArgs
  • UIMouseEventArgs

Lambda expressions can also be used:

<button onclick="@(e => Console.WriteLine("Hello, world!"))">Say hello</button>

It's often convenient to close over additional values, such as when iterating over a set of elements. The following example creates three buttons, each of which calls UpdateHeading passing an event argument (UIMouseEventArgs) and its button number (buttonNumber) when selected in the UI:

<h2>@message</h2>

@for (var i = 1; i < 4; i++)
{
    var buttonNumber = i;

    <button class="btn btn-primary"
            onclick="@(e => UpdateHeading(e, buttonNumber))">
        Button #@i
    </button>
}

@functions {
    private string message = "Select a button to learn its position.";

    private void UpdateHeading(UIMouseEventArgs e, int buttonNumber)
    {
        message = $"You selected Button #{buttonNumber} at " +
            $"mouse position: {e.ClientX} X {e.ClientY}.";
    }
}

Note

Do not use the loop variable (i) in a for loop directly in a lambda expression. Otherwise the same variable is used by all lambda expressions causing i's value to be the same in all lambdas. Always capture its value in a local variable (buttonNumber in the preceding example) and then use it.

EventCallback

A common scenario with nested components is the desire to run a parent component's method when a child component event occurs—for example, when an onclick event occurs in the child. To expose events across components, use an EventCallback. A parent component can assign a callback method to a child component's EventCallback.

The Child component in the sample app demonstrates how a button's onclick handler is set up to receive an EventCallback delegate from the sample's Parent component. The EventCallback is typed with UIMouseEventArgs, which is appropriate for an onclick event from a peripheral device:

<div class="panel panel-default">
    <div class="panel-heading">@Title</div>
    <div class="panel-body">@ChildContent</div>

    <button class="btn btn-primary" onclick="@OnClick">
        Trigger a Parent component method
    </button>
</div>

@functions {
    [Parameter]
    private string Title { get; set; }

    [Parameter]
    private RenderFragment ChildContent { get; set; }

    [Parameter]
    private EventCallback<UIMouseEventArgs> OnClick { get; set; }
}

The Parent component sets the child's EventCallback<T> to its ShowMessage method:

@page "/ParentComponent"

<h1>Parent-child example</h1>

<ChildComponent Title="Panel Title from Parent"
                OnClick="@ShowMessage">
    Content of the child component is supplied
    by the parent component.
</ChildComponent>

<p><b>@messageText</b></p>

@functions {
    private string messageText;

    private void ShowMessage(UIMouseEventArgs e)
    {
        messageText = "Blaze a new trail with Blazor!";
    }
}

When the button is selected in the Child component:

  • The Parent component's ShowMessage method is called. messageText is updated and displayed in the Parent component.
  • A call to StateHasChanged isn't required in the callback's method (ShowMessage). StateHasChanged is called automatically to rerender the Parent component, just as child events trigger component rerendering in event handlers that execute within the child.

EventCallback and EventCallback<T> permit asynchronous delegates. EventCallback<T> is strongly typed and requires a specific argument type. EventCallback is weakly typed and allows any argument type.

<p><b>@messageText</b></p>

@{ var message = "Default Text"; }

<ChildComponent 
    OnClick="@(async () => { await Task.Yield(); messageText = "Blaze It!"; })" />

@functions {
    private string messageText;
}

Invoke an EventCallback or EventCallback<T> with InvokeAsync and await the Task:

await callback.InvokeAsync(arg);

Use EventCallback and EventCallback<T> for event handling and binding component parameters. Don't use EventCallback and EventCallback<T> for child content—continue to use RenderFragment and RenderFragment<T> for child content.

Prefer the strongly typed EventCallback<T>, which provides better error feedback to users of the component. Similar to other UI event handlers, specifying the event parameter is optional. Use EventCallback when there's no value passed to the callback.

Capture references to components

Component references provide a way to reference a component instance so that you can issue commands to that instance, such as Show or Reset. To capture a component reference, add a ref attribute to the child component and then define a field with the same name and the same type as the child component.

<MyLoginDialog ref="loginDialog" ... />

@functions {
    private MyLoginDialog loginDialog;

    private void OnSomething()
    {
        loginDialog.Show();
    }
}

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

Important

The loginDialog variable is only populated after the component is rendered and its output includes the MyLoginDialog element. Until that point, there's nothing to reference. To manipulate components references after the component has finished rendering, use the OnAfterRenderAsync or OnAfterRender methods.

While capturing component references use a similar syntax to capturing element references, it isn't a JavaScript interop feature. Component references aren't passed to JavaScript code—they're only used in .NET code.

Note

Do not use component references to mutate the state of child components. Instead, use normal declarative parameters to pass data to child components. This causes child components to rerender at the correct times automatically.

Lifecycle methods

OnInitAsync and OnInit execute code to initialize the component. To perform an asynchronous operation, use OnInitAsync and the await keyword on the operation:

protected override async Task OnInitAsync()
{
    await ...
}

For a synchronous operation, use OnInit:

protected override void OnInit()
{
    ...
}

OnParametersSetAsync and OnParametersSet are called when a component has received parameters from its parent and the values are assigned to properties. These methods are executed after component initialization and each time the component is rendered:

protected override async Task OnParametersSetAsync()
{
    await ...
}
protected override void OnParametersSet()
{
    ...
}

OnAfterRenderAsync and OnAfterRender are called after a component has finished rendering. Element and component references are populated at this point. Use this stage to perform additional initialization steps using the rendered content, such as activating third-party JavaScript libraries that operate on the rendered DOM elements.

protected override async Task OnAfterRenderAsync()
{
    await ...
}
protected override void OnAfterRender()
{
    ...
}

SetParameters can be overridden to execute code before parameters are set:

public override void SetParameters(ParameterCollection parameters)
{
    ...

    base.SetParameters(parameters);
}

If base.SetParameters isn't invoked, the custom code can interpret the incoming parameters value in any way required. For example, the incoming parameters aren't required to be assigned to the properties on the class.

ShouldRender can be overridden to suppress refreshing of the UI. If the implementation returns true, the UI is refreshed. Even if ShouldRender is overridden, the component is always initially rendered.

protected override bool ShouldRender()
{
    var renderUI = true;

    return renderUI;
}

Component disposal with IDisposable

If a component implements IDisposable, the Dispose method is called when the component is removed from the UI. The following component uses @implements IDisposable and the Dispose method:

@using System
@implements IDisposable

...

@functions {
    public void Dispose()
    {
        ...
    }
}

Routing

Routing in Blazor is achieved by providing a route template to each accessible component in the app.

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 looks for component classes with a RouteAttribute and renders whichever component has a route template that matches the requested URL.

Multiple route templates can be applied to a component. The following component responds to requests for /BlazorRoute and /DifferentBlazorRoute:

@page "/BlazorRoute"
@page "/DifferentBlazorRoute"

<h1>Blazor routing</h1>

Route parameters

Components can receive route parameters from the route template provided in the @page directive. The router uses route parameters to populate the corresponding component parameters.

Route Parameter component:

@page "/RouteParameter"
@page "/RouteParameter/{text}"

<h1>Blazor is @Text!</h1>

@functions {
    [Parameter]
    private string Text { get; set; } = "fantastic";
}

Optional parameters aren't supported, so two @page directives are applied in the example above. The first permits navigation to the component without a parameter. The second @page directive takes the {text} route parameter and assigns the value to the Text property.

Base class inheritance for a "code-behind" experience

Component files mix HTML markup and C# processing code in the same file. The @inherits directive can be used to provide Blazor apps with a "code-behind" experience that separates component markup from processing code.

The sample app shows how a component can inherit a base class, BlazorRocksBase, to provide the component's properties and methods.

Blazor Rocks component:

@page "/BlazorRocks"
@*
    The inherit directive provides the properties and methods
    of the BlazorRocksBase class to this component.
*@
@inherits BlazorRocksBase

<h1>@BlazorRocksText</h1> 

BlazorRocksBase.cs:

using Microsoft.AspNetCore.Components;

namespace BlazorSample
{
    public class BlazorRocksBase : ComponentBase
    {
        public string BlazorRocksText { get; set; } = 
            "Blazor rocks the browser!";
    }
}

The base class should derive from ComponentBase.

Import components

The namespace of a component authored with Razor is based on:

  • The project's RootNamespace.
  • The path from the project root to the component. For example, ComponentsSample/Pages/Index.razor is in the namespace ComponentsSample.Pages. Components follow C# name binding rules. In the case of Index.razor, all components in the same folder, Pages, and the parent folder, ComponentsSample, are in scope.

Components defined in a different namespace can be brought into scope using Razor's @using directive.

If another component, NavMenu.razor, exists in the folder ComponentsSample/Shared/, the component can be used in Index.razor with the following @using statement:

@using ComponentsSample.Shared

This is the Index page.

<NavMenu></NavMenu>

Components can also be referenced using their fully qualified names, which removes the need for the @using directive:

This is the Index page.

<ComponentsSample.Shared.NavMenu></ComponentsSample.Shared.NavMenu>

Note

The global:: qualification isn't supported.

Importing components with aliased using statements (for example, @using Foo = Bar) isn't supported.

Partially qualified names aren't supported. For example, adding @using ComponentsSample and referencing NavMenu.razor with <Shared.NavMenu></Shared.NavMenu> isn't supported.

Razor support

Razor directives

Razor directives are shown in the following table.

Directive Description
@functions Adds a C# code block to a component.
@implements Implements an interface for the generated component class.
@inherits Provides full control of the class that the component inherits.
@inject Enables service injection from the service container. For more information, see Dependency injection into views.
@layout Specifies a layout component. Layout components are used to avoid code duplication and inconsistency.
@page Specifies that the component should handle requests directly. The @page directive can be specified with a route and optional parameters. Unlike Razor Pages, the @page directive doesn't need to be the first directive at the top of the file. For more information, see Routing.
@using Adds the C# using directive to the generated component class. This also brings all the components defined in that namespace into scope.

Conditional attributes

Attributes are conditionally rendered based on the .NET value. If the value is false or null, the attribute isn't rendered. If the value is true, the attribute is rendered minimized.

In the following example, IsCompleted determines if checked is rendered in the control's markup:

<input type="checkbox" checked="@IsCompleted">

@functions {
    [Parameter]
    private bool IsCompleted { get; set; }
}

If IsCompleted is true, the check box is rendered as:

<input type="checkbox" checked>

If IsCompleted is false, the check box is rendered as:

<input type="checkbox">

Additional information on Razor

For more information on Razor, see the Razor syntax reference.

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.

Warning

Rendering raw HTML constructed from any untrusted source is a security risk and should be avoided!

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

@((MarkupString)myMarkup)

@functions {
    private string myMarkup = 
        "<p class='markup'>This is a <em>markup string</em>.</p>";
}

Templated components

Templated components are components that accept one or more UI templates as parameters, which can then be used as part of the component's rendering logic. Templated components allow you to author higher-level components that are more reusable than regular components. A couple of examples include:

  • A table component that allows a user to specify templates for the table's header, rows, and footer.
  • A list component that allows a user to specify a template for rendering items in a list.

Template parameters

A templated component is defined by specifying one or more component parameters of type RenderFragment or RenderFragment<T>. A render fragment represents a segment of UI that is rendered by the component. A render fragment optionally takes a parameter that can be specified when the render fragment is invoked.

Table Template component:

@typeparam TItem

<table class="table">
    <thead>
        <tr>@TableHeader</tr>
    </thead>
    <tbody>
        @foreach (var item in Items)
        {
            <tr>@RowTemplate(item)</tr>
        }
    </tbody>
    <tfoot>
        <tr>@TableFooter</tr>
    </tfoot>
</table>

@functions {
    [Parameter]
    private RenderFragment TableHeader { get; set; }

    [Parameter]
    private RenderFragment<TItem> RowTemplate { get; set; }

    [Parameter]
    private RenderFragment TableFooter { get; set; }

    [Parameter]
    private IReadOnlyList<TItem> Items { get; set; }
}

When using a templated component, the template parameters can be specified using child elements that match the names of the parameters (TableHeader and RowTemplate in the following example):

<TableTemplate Items="@pets">
    <TableHeader>
        <th>ID</th>
        <th>Name</th>
    </TableHeader>
    <RowTemplate>
        <td>@context.PetId</td>
        <td>@context.Name</td>
    </RowTemplate>
</TableTemplate>

Template context parameters

Component arguments of type RenderFragment<T> passed as elements have an implicit parameter named context (for example from the preceding code sample, @context.PetId), but you can change the parameter name using the Context attribute on the child element. In the following example, the RowTemplate element's Context attribute specifies the pet parameter:

<TableTemplate Items="@pets">
    <TableHeader>
        <th>ID</th>
        <th>Name</th>
    </TableHeader>
    <RowTemplate Context="pet">
        <td>@pet.PetId</td>
        <td>@pet.Name</td>
    </RowTemplate>
</TableTemplate>

Alternatively, you can specify the Context attribute on the component element. The specified Context attribute applies to all specified template parameters. This can be useful when you want to specify the content parameter name for implicit child content (without any wrapping child element). In the following example, the Context attribute appears on the TableTemplate element and applies to all template parameters:

<TableTemplate Items="@pets" Context="pet">
    <TableHeader>
        <th>ID</th>
        <th>Name</th>
    </TableHeader>
    <RowTemplate>
        <td>@pet.PetId</td>
        <td>@pet.Name</td>
    </RowTemplate>
</TableTemplate>

Generic-typed components

Templated components are often generically typed. For example, a generic List View Template component can be used to render IEnumerable<T> values. To define a generic component, use the @typeparam directive to specify type parameters.

ListView Template component:

@typeparam TItem

<ul>
    @foreach (var item in Items)
    {
        @ItemTemplate(item)
    }
</ul>

@functions {
    [Parameter]
    private RenderFragment<TItem> ItemTemplate { get; set; }

    [Parameter]
    private IReadOnlyList<TItem> Items { get; set; }
}

When using generic-typed components, the type parameter is inferred if possible:

<ListViewTemplate Items="@pets">
    <ItemTemplate Context="pet">
        <li>@pet.Name</li>
    </ItemTemplate>
</ListViewTemplate>

Otherwise, the type parameter must be explicitly specified using an attribute that matches the name of the type parameter. In the following example, TItem="Pet" specifies the type:

<ListViewTemplate Items="@pets" TItem="Pet">
    <ItemTemplate Context="pet">
        <li>@pet.Name</li>
    </ItemTemplate>
</ListViewTemplate>

Cascading values and parameters

In some scenarios, it's inconvenient to flow data from an ancestor component to a descendent component using component parameters, especially when there are several component layers. Cascading values and parameters solve this problem by providing a convenient way for an ancestor component to provide a value to all of its descendent components. Cascading values and parameters also provide an approach for components to coordinate.

Theme example

In the following Theme example from the sample app, the ThemeInfo class specifies the theme information to flow down the component hierarchy so that all of the buttons within a given part of the app share the same style.

UIThemeClasses/ThemeInfo.cs:

public class ThemeInfo
{
    public string ButtonClass { get; set; }
}

An ancestor component can provide a cascading value using the Cascading Value component. The Cascading Value component wraps a subtree of the component hierarchy and supplies a single value to all components within that subtree.

For example, the sample app specifies theme information (ThemeInfo) in one of the app's layouts as a cascading parameter for all components that make up the layout body of the @Body property. ButtonClass is assigned a value of btn-success in the layout component. Any descendent component can consume this property through the ThemeInfo cascading object.

Cascading Values Parameters Layout component:

@inherits LayoutComponentBase
@using BlazorSample.UIThemeClasses

<div class="container-fluid">
    <div class="row">
        <div class="col-sm-3">
            <NavMenu />
        </div>
        <div class="col-sm-9">
            <CascadingValue Value="@theme">
                <div class="content px-4">
                    @Body
                </div>
            </CascadingValue>
        </div>
    </div>
</div>

@functions {
    private ThemeInfo theme = new ThemeInfo { ButtonClass = "btn-success" };
}

To make use of cascading values, components declare cascading parameters using the [CascadingParameter] attribute or based on a string name value:

<CascadingValue Value=@PermInfo Name="UserPermissions">...</CascadingValue>

[CascadingParameter(Name = "UserPermissions")]
private PermInfo Permissions { get; set; }

Binding with a string name value is relevant if you have multiple cascading values of the same type and need to differentiate them within the same subtree.

Cascading values are bound to cascading parameters by type.

In the sample app, the Cascading Values Parameters Theme component binds the ThemeInfo cascading value to a cascading parameter. The parameter is used to set the CSS class for one of the buttons displayed by the component.

Cascading Values Parameters Theme component:

@page "/cascadingvaluesparameterstheme"
@layout CascadingValuesParametersLayout
@using BlazorSample.UIThemeClasses

<h1>Cascading Values & Parameters</h1>

<p>Current count: @currentCount</p>

<p>
    <button class="btn" onclick="@IncrementCount">
        Increment Counter (Unthemed)
    </button>
</p>

<p>
    <button class="btn @ThemeInfo.ButtonClass" onclick="@IncrementCount">
        Increment Counter (Themed)
    </button>
</p>

@functions {
    private int currentCount = 0;

    [CascadingParameter] protected ThemeInfo ThemeInfo { get; set; }

    private void IncrementCount()
    {
        currentCount++;
    }
}

TabSet example

Cascading parameters also enable components to collaborate across the component hierarchy. For example, consider the following TabSet example in the sample app.

The sample app has an ITab interface that tabs implement:

using Microsoft.AspNetCore.Components;

namespace BlazorSample.UIInterfaces
{
    public interface ITab
    {
        RenderFragment ChildContent { get; }
    }
}

The Cascading Values Parameters TabSet component uses the Tab Set component, which contains several Tab components:

<TabSet>
    <Tab Title="First tab">
        <h4>Greetings from the first tab!</h4>

        <label>
            <input type="checkbox" bind="@showThirdTab" />
            Toggle third tab
        </label>
    </Tab>
    <Tab Title="Second tab">
        <h4>The second tab says Hello World!</h4>
    </Tab>

    @if (showThirdTab)
    {
        <Tab Title="Third tab">
            <h4>Welcome to the disappearing third tab!</h4>
            <p>Toggle this tab from the first tab.</p>
        </Tab>
    }
</TabSet>

The child Tab components aren't explicitly passed as parameters to the Tab Set. Instead, the child Tab components are part of the child content of the Tab Set. However, the Tab Set still needs to know about each Tab component so that it can render the headers and the active tab. To enable this coordination without requiring additional code, the Tab Set component can provide itself as a cascading value that is then picked up by the descendent Tab components.

TabSet component:

@using BlazorSample.UIInterfaces

<!-- Display the tab headers -->
<CascadingValue Value=this>
    <ul class="nav nav-tabs">
        @ChildContent
    </ul>
</CascadingValue>

<!-- Display body for only the active tab -->
<div class="nav-tabs-body p-4">
    @ActiveTab?.ChildContent
</div>

@functions {
    [Parameter]
    private RenderFragment ChildContent { get; set; }

    public ITab ActiveTab { get; private set; }

    public void AddTab(ITab tab)
    {
        if (ActiveTab == null)
        {
            SetActivateTab(tab);
        }
    }

    public void RemoveTab(ITab tab)
    {
        if (ActiveTab == tab)
        {
            SetActivateTab(null);
        }
    }

    public void SetActivateTab(ITab tab)
    {
        if (ActiveTab != tab)
        {
            ActiveTab = tab;
            StateHasChanged();
        }
    }
}

The descendent Tab components capture the containing Tab Set as a cascading parameter, so the Tab components add themselves to the Tab Set and coordinate on which tab is active.

Tab component:

@using BlazorSample.UIInterfaces
@implements IDisposable
@implements ITab

<li>
    <a onclick="@Activate" class="nav-link @TitleCssClass" role="button">
        @Title
    </a>
</li>

@functions {
    [CascadingParameter]
    private TabSet ContainerTabSet { get; set; }

    [Parameter]
    private string Title { get; set; }

    [Parameter]
    public RenderFragment ChildContent { get; private set; }

    private string TitleCssClass => ContainerTabSet.ActiveTab == this ? "active" : null;

    protected override void OnInit()
    {
        ContainerTabSet.AddTab(this);
    }

    public void Dispose()
    {
        ContainerTabSet.RemoveTab(this);
    }

    private void Activate()
    {
        ContainerTabSet.SetActivateTab(this);
    }
}

Razor templates

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

@<tag>...</tag>

The following example illustrates how to specify RenderFragment and RenderFragment<T> values.

Razor Templates component:

@{
    RenderFragment template = @<p>The time is @DateTime.Now.</p>;
    RenderFragment<Pet> petTemplate = (pet) => @<p>Your pet's name is @pet.Name.</p>;
}

Render fragments defined using Razor templates can be passed as arguments to templated components or rendered directly. For example, the previous templates are directly rendered with the following Razor markup:

@template

@petTemplate(new Pet { Name = "Rex" })

Rendered output:

The time is 10/04/2018 01:26:52.

Your pet's name is Rex.

Manual RenderTreeBuilder logic

Microsoft.AspNetCore.Components.RenderTree provides methods for manipulating components and elements, including building components manually in C# code.

Note

Use of RenderTreeBuilder to create components is an advanced scenario. A malformed component (for example, an unclosed markup tag) can result in undefined behavior.

Consider the following Pet Details component, which can be manually built into another component:

<h2>Pet Details Component</h2>

<p>@PetDetailsQuote<p>

@functions
{
    [Parameter]
    string PetDetailsQuote { get; set; }
}

In the following example, the loop in the CreateComponent method generates three Pet Details components. When calling RenderTreeBuilder methods to create the components (OpenComponent and AddAttribute), sequence numbers are source code line numbers. The Blazor difference algorithm relies on the sequence numbers corresponding to distinct lines of code, not distinct call invocations. When creating a component with RenderTreeBuilder methods, hardcode the arguments for sequence numbers. Using a calculation or counter to generate the sequence number can lead to poor performance. For more information, see the Sequence numbers relate to code line numbers and not execution order section.

Built Content component:

@page "/BuiltContent"

<h1>Build a component</h1>

@CustomRender

<button type="button" onclick="@RenderComponent">
    Create three Pet Details components
</button>

@functions {
    private RenderFragment CustomRender { get; set; }
    
    private RenderFragment CreateComponent() => builder =>
    {
        for (var i = 0; i < 3; i++) 
        {
            builder.OpenComponent(0, typeof(PetDetails));
            builder.AddAttribute(1, "PetDetailsQuote", "Someone's best friend!");
            builder.CloseComponent();
        }
    };    
    
    private void RenderComponent()
    {
        CustomRender = CreateComponent();
    }
}

Sequence numbers relate to code line numbers and not execution order

Blazor .razor files are always compiled. This is potentially a great advantage for .razor because the compile step can be used to inject information that improve app performance at runtime.

A key example of these improvements involve sequence numbers. Sequence numbers indicate to the runtime which outputs came from which distinct and ordered lines of code. The runtime uses this information to generate efficient tree diffs in linear time, which is far faster than is normally possible for a general tree diff algorithm.

Consider the following simple .razor file:

@if (someFlag)
{
    <text>First</text>
}

Second

This compiles to something like the following:

if (someFlag)
{
    builder.AddContent(0, "First");
}

builder.AddContent(1, "Second");

When this code executes for the first time, if someFlag is true, the builder receives:

Sequence Type Data
0 Text node First
1 Text node Second

Now imagine that someFlag becomes false, and we render again. This time, the builder receives:

Sequence Type Data
1 Text node Second

When the runtime performs a diff, it sees that the item at sequence 0 was removed, so it generates the following trivial edit script:

  • Remove the first text node.

What goes wrong if you generate sequence numbers programmatically

Imagine instead that you wrote the following rendertree builder logic:

var seq = 0;

if (someFlag)
{
    builder.AddContent(seq++, "First");
}

builder.AddContent(seq++, "Second");

Now the first output would be:

Sequence Type Data
0 Text node First
1 Text node Second

This outcome is identical to the prior case, so no negative issues exist. On the second render when someFlag is false, the output is:

Sequence Type Data
0 Text node Second

This time, the diff algorithm sees that two changes have occurred, and the algorithm generates the following edit script:

  • Change the value of the first text node to Second.
  • Remove the second text node.

Generating the sequence numbers has lost all the useful information about where the if/else branches and loops were present in the original code. This results in a diff twice as long as before.

This is a trivial example. In more realistic cases with complex and deeply nested structures, and especially with loops, the performance cost is more severe. Instead of immediately identifying which loop blocks or branches have been inserted or removed, the diff algorithm has to recurse deeply into the render trees and usually build far longer edit scripts because it is misinformed about how the old and new structures relate to each other.

Guidance and conclusions

  • App performance suffers if sequence numbers are generated dynamically.
  • The framework can't create its own sequence numbers automatically at runtime because the necessary information doesn't exist unless it's captured at compile time.
  • Don't write long blocks of manually-implemented RenderTreeBuilder logic. Prefer .razor files and allow the compiler to deal with the sequence numbers. If you're unable to avoid manual RenderTreeBuilder logic, split long blocks of code into smaller pieces wrapped in OpenRegion/CloseRegion calls. Each region has its own separate space of sequence numbers, so you can restart from zero (or any other arbitrary number) inside each region.
  • If sequence numbers are hardcoded, the diff algorithm only requires that sequence numbers increase in value. The initial value and gaps are irrelevant. One legitimate option is to use the code line number as the sequence number, or start from zero and increase by ones or hundreds (or any preferred interval).
  • Blazor uses sequence numbers, while other tree-diffing UI frameworks don't use them. Diffing is far faster when sequence numbers are used, and Blazor has the advantage of a compile step that deals with sequence numbers automatically for developers authoring .razor files.