AspNetCore.Docs/aspnetcore/blazor/components/cascading-values-and-parameters.md

title	author	description	monikerRange	ms.author	ms.custom	ms.date	uid
ASP.NET Core Blazor cascading values and parameters	guardrex	Learn how to flow data from an ancestor Razor component to descendent components.	>= aspnetcore-3.1	riande	mvc	11/08/2022	blazor/components/cascading-values-and-parameters

ASP.NET Core Blazor cascading values and parameters

[!INCLUDE]

This article explains how to flow data from an ancestor Razor component to descendent components.

Cascading values and parameters provide a convenient way to flow data down a component hierarchy from an ancestor component to any number of descendent components. Unlike Component parameters, cascading values and parameters don't require an attribute assignment for each descendent component where the data is consumed. Cascading values and parameters also allow components to coordinate with each other across a component hierarchy.

[!NOTE] The code examples in this article adopt nullable reference types (NRTs) and .NET compiler null-state static analysis, which are supported in ASP.NET Core 6.0 or later. When targeting ASP.NET Core 5.0 or earlier, remove the null type designation (?) from the CascadingType?, @ActiveTab?, RenderFragment?, ITab?, TabSet?, and string? types in the article's examples.

CascadingValue component

An ancestor component provides a cascading value using the Blazor framework's CascadingValue component, which wraps a subtree of a component hierarchy and supplies a single value to all of the components within its subtree.

The following example demonstrates the flow of theme information down the component hierarchy of a layout component to provide a CSS style class to buttons in child components.

The following ThemeInfo C# class is placed in a folder named UIThemeClasses and specifies the theme information.

[!NOTE] For the examples in this section, the app's namespace is BlazorSample. When experimenting with the code in your own sample app, change the app's namespace to your sample app's namespace.

UIThemeClasses/ThemeInfo.cs:

:::moniker range=">= aspnetcore-7.0"

:::code language="csharp" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/UIThemeClasses/ThemeInfo.cs":::

:::moniker-end

:::moniker range=">= aspnetcore-6.0 < aspnetcore-7.0"

:::code language="csharp" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/UIThemeClasses/ThemeInfo.cs":::

:::moniker-end

:::moniker range=">= aspnetcore-5.0 < aspnetcore-6.0"

:::code language="csharp" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/UIThemeClasses/ThemeInfo.cs":::

:::moniker-end

:::moniker range="< aspnetcore-5.0"

:::code language="csharp" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/UIThemeClasses/ThemeInfo.cs":::

:::moniker-end

The following layout component specifies theme information (ThemeInfo) as a cascading value for all components that make up the layout body of the xref:Microsoft.AspNetCore.Components.LayoutComponentBase.Body property. ButtonClass is assigned a value of btn-success, which is a Bootstrap button style. Any descendent component in the component hierarchy can use the ButtonClass property through the ThemeInfo cascading value.

Shared/MainLayout.razor:

:::moniker range=">= aspnetcore-7.0"

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Shared/MainLayout.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-6.0 < aspnetcore-7.0"

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Shared/MainLayout.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-5.0 < aspnetcore-6.0"

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Shared/MainLayout.razor":::

:::moniker-end

:::moniker range="< aspnetcore-5.0"

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Shared/MainLayout.razor":::

:::moniker-end

[CascadingParameter] attribute

To make use of cascading values, descendent components declare cascading parameters using the [CascadingParameter] attribute. Cascading values are bound to cascading parameters by type. Cascading multiple values of the same type is covered in the Cascade multiple values section later in this article.

The following component binds the ThemeInfo cascading value to a cascading parameter, optionally using the same name of ThemeInfo. The parameter is used to set the CSS class for the Increment Counter (Themed) button.

Pages/ThemedCounter.razor:

:::moniker range=">= aspnetcore-7.0"

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/ThemedCounter.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-6.0 < aspnetcore-7.0"

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/ThemedCounter.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-5.0 < aspnetcore-6.0"

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/ThemedCounter.razor":::

:::moniker-end

:::moniker range="< aspnetcore-5.0"

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/ThemedCounter.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-6.0"

Similar to a regular component parameter, components accepting a cascading parameter are rerendered when the cascading value is changed. For instance, configuring a different theme instance causes the ThemedCounter component from the CascadingValue component section to rerender:

Shared/MainLayout.razor:

<main>
    <div class="top-row px-4">
        <a href="https://docs.microsoft.com/aspnet/" target="_blank">About</a>
    </div>

    <CascadingValue Value="@theme">
        <article class="content px-4">
            @Body
        </article>
    </CascadingValue>
    <button @onclick="ChangeToDarkTheme">Dark mode</button>
</main>

@code {
    private ThemeInfo theme = new() { ButtonClass = "btn-success" };
    
    private void ChangeToDarkTheme()
    {
        theme = new() { ButtonClass = "btn-darkmode-success" };
    }
}

xref:Microsoft.AspNetCore.Components.CascadingValue%601.IsFixed%2A?displayProperty=nameWithType can be used to indicate that a cascading parameter doesn't change after initialization.

:::moniker-end

Cascade multiple values

To cascade multiple values of the same type within the same subtree, provide a unique xref:Microsoft.AspNetCore.Components.CascadingValue%601.Name%2A string to each CascadingValue component and their corresponding [CascadingParameter] attributes.

In the following example, two CascadingValue components cascade different instances of CascadingType:

<CascadingValue Value="@parentCascadeParameter1" Name="CascadeParam1">
    <CascadingValue Value="@ParentCascadeParameter2" Name="CascadeParam2">
        ...
    </CascadingValue>
</CascadingValue>

@code {
    private CascadingType? parentCascadeParameter1;

    [Parameter]
    public CascadingType? ParentCascadeParameter2 { get; set; }
}

In a descendant component, the cascaded parameters receive their cascaded values from the ancestor component by xref:Microsoft.AspNetCore.Components.CascadingValue%601.Name%2A:

@code {
    [CascadingParameter(Name = "CascadeParam1")]
    protected CascadingType? ChildCascadeParameter1 { get; set; }

    [CascadingParameter(Name = "CascadeParam2")]
    protected CascadingType? ChildCascadeParameter2 { get; set; }
}

Pass data across a component hierarchy

Cascading parameters also enable components to pass data across a component hierarchy. Consider the following UI tab set example, where a tab set component maintains a series of individual tabs.

[!NOTE] For the examples in this section, the app's namespace is BlazorSample. When experimenting with the code in your own sample app, change the namespace to your sample app's namespace.

Create an ITab interface that tabs implement in a folder named UIInterfaces.

UIInterfaces/ITab.cs:

using Microsoft.AspNetCore.Components;

namespace BlazorSample.UIInterfaces;

public interface ITab
{
    RenderFragment ChildContent { get; }
}

[!NOTE] For more information on xref:Microsoft.AspNetCore.Components.RenderFragment, see xref:blazor/components/index#child-content-render-fragments.

The following TabSet component maintains a set of tabs. The tab set's Tab components, which are created later in this section, supply the list items (<li>...</li>) for the list (<ul>...</ul>).

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

Shared/TabSet.razor:

@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>

@code {
    [Parameter]
    public RenderFragment? ChildContent { get; set; }

    public ITab? ActiveTab { get; private set; }

    public void AddTab(ITab tab)
    {
        if (ActiveTab is null)
        {
            SetActiveTab(tab);
        }
    }

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

Descendent Tab components capture the containing TabSet as a cascading parameter. The Tab components add themselves to the TabSet and coordinate to set the active tab.

Shared/Tab.razor:

@using BlazorSample.UIInterfaces
@implements ITab

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

@code {
    [CascadingParameter]
    public TabSet? ContainerTabSet { get; set; }

    [Parameter]
    public string? Title { get; set; }

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

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

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

    private void ActivateTab()
    {
        ContainerTabSet?.SetActiveTab(this);
    }
}

The following ExampleTabSet component uses the TabSet component, which contains three Tab components.

Pages/ExampleTabSet.razor:

@page "/example-tab-set"

<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>Hello from the second tab!</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>

@code {
    private bool showThirdTab;
}