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title	author	description	monikerRange	ms.author	ms.custom	ms.date	uid
ASP.NET Core Blazor data binding	guardrex	Learn about data binding features for Razor components and DOM elements in Blazor apps.	>= aspnetcore-3.1	riande	mvc	11/12/2024	blazor/components/data-binding

ASP.NET Core Blazor data binding

[!INCLUDE]

This article explains data binding features for Razor components and DOM elements in Blazor apps.

Binding features

Razor components provide data binding features with the @bind Razor directive attribute with a field, property, or Razor expression value.

The following example binds:

An <input> element value to the C# inputValue field.
A second <input> element value to the C# InputValue property.

When an <input> element loses focus, its bound field or property is updated.

Bind.razor:

:::moniker range=">= aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/9.0/BlazorSample_BlazorWebApp/Components/Pages/Bind.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-8.0 < aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/Pages/Bind.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-7.0 < aspnetcore-8.0"

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

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

As a demonstration of how data binding composes in HTML, the following example binds the InputValue property to the second <input> element's value and onchange attributes (change). The second <input> element in the following example is a concept demonstration and isn't meant to suggest how you should bind data in Razor components.

BindTheory.razor:

:::moniker range=">= aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/9.0/BlazorSample_BlazorWebApp/Components/Pages/BindTheory.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-8.0 < aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/Pages/BindTheory.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-7.0 < aspnetcore-8.0"

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

When the BindTheory component is rendered, the value of the HTML demonstration <input> element comes from the InputValue property. When the user enters a value in the text box and changes element focus, the onchange event is fired and the InputValue property is set to the changed value. In reality, code execution is more complex because @bind handles cases where type conversions are performed. In general, @bind associates the current value of an expression with the value attribute of the <input> and handles changes using the registered handler.

Bind a property or field on other DOM events by including an @bind:event="{EVENT}" attribute with a DOM event for the {EVENT} placeholder. The following example binds the InputValue property to the <input> element's value when the element's oninput event (input) is triggered. Unlike the onchange event (change), which fires when the element loses focus, oninput (input) fires when the value of the text box changes.

Page/BindEvent.razor:

:::moniker range=">= aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/9.0/BlazorSample_BlazorWebApp/Components/Pages/BindEvent.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-8.0 < aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/Pages/BindEvent.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-7.0 < aspnetcore-8.0"

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

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

Razor attribute binding is case-sensitive:

@bind and @bind:event are valid.
@Bind/@Bind:Event (capital letters B and E) or @BIND/@BIND:EVENT (all capital letters) are invalid.

:::moniker-end

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

To execute asynchronous logic after binding, use @bind:after="{DELEGATE}", where the {DELEGATE} placeholder is a C# delegate (method). An assigned C# delegate isn't executed until the bound value is assigned synchronously.

Using an event callback parameter (EventCallback/EventCallback<T>) with @bind:after isn't supported. Instead, pass a method that returns an xref:System.Action or xref:System.Threading.Tasks.Task to @bind:after.

In the following example:

Each <input> element's value is bound to the searchText field synchronously.
The PerformSearch method executes asynchronously:
- When the first box loses focus (onchange event) after the value is changed.
- After each keystroke (oninput event) in the second box.
PerformSearch calls a service with an asynchronous method (FetchAsync) to return search results.

@inject ISearchService SearchService

<input @bind="searchText" @bind:after="PerformSearch" />
<input @bind="searchText" @bind:event="oninput" @bind:after="PerformSearch" />

@code {
    private string? searchText;
    private string[]? searchResult;

    private async Task PerformSearch() => 
        searchResult = await SearchService.FetchAsync(searchText);
}

Additional examples

BindAfter.razor:

@page "/bind-after"
@using Microsoft.AspNetCore.Components.Forms

<h1>Bind After Examples</h1>

<h2>Elements</h2>

<input type="text" @bind="text" @bind:after="() => { }" />

<input type="text" @bind="text" @bind:after="After" />

<input type="text" @bind="text" @bind:after="AfterAsync" />

<h2>Components</h2>

<InputText @bind-Value="text" @bind-Value:after="() => { }" />

<InputText @bind-Value="text" @bind-Value:after="After" />

<InputText @bind-Value="text" @bind-Value:after="AfterAsync" />

@code {
    private string text = "";

    private void After() {}
    private Task AfterAsync() { return Task.CompletedTask; }
}

For more information on the InputText component, see xref:blazor/forms/input-components.

Components support two-way data binding by defining a pair of @bind attributes with either a :get or :set modifier . The {PARAMETER} placeholder in the following examples is used to bind a component parameter:

@bind:get/@bind-{PARAMETER}:get: Specifies the value to bind.
@bind:set/@bind-{PARAMETER}:set: Specifies a callback for when the value changes.

The :get and :set modifiers are always used together.

With :get/:set binding, you can react to a value change before it's applied to the DOM, and you can change the applied value, if necessary. Whereas with @bind:event="{EVENT}" attribute binding, where the {EVENT} placeholder is a DOM event, you receive the notification after the DOM is updated, and there's no capacity to modify the applied value while binding.

BindGetSet.razor:

@page "/bind-get-set"
@using Microsoft.AspNetCore.Components.Forms

<h1>Bind Get Set Examples</h1>

<h2>Elements</h2>

<input type="text" @bind:get="text" @bind:set="(value) => { text = value; }" />
<input type="text" @bind:get="text" @bind:set="Set" />
<input type="text" @bind:get="text" @bind:set="SetAsync" />

<h2>Components</h2>

<InputText @bind-Value:get="text" @bind-Value:set="(value) => { text = value; }" />
<InputText @bind-Value:get="text" @bind-Value:set="Set" />
<InputText @bind-Value:get="text" @bind-Value:set="SetAsync" />

@code {
    private string text = "";

    private void Set(string value)
    {
        text = value;
    }

    private Task SetAsync(string value)
    {
        text = value;
        return Task.CompletedTask;
    }
}

For more information on the InputText component, see xref:blazor/forms/input-components.

For another example use of @bind:get and @bind:set, see the Bind across more than two components section later in this article.

Razor attribute binding is case-sensitive:

@bind, @bind:event, and @bind:after are valid.
@Bind/@bind:Event/@bind:aftEr (capital letters) or @BIND/@BIND:EVENT/@BIND:AFTER (all capital letters) are invalid.

Use `@bind:get`/`@bind:set` modifiers and avoid event handlers for two-way data binding

Two-way data binding isn't possible to implement with an event handler. Use @bind:get/@bind:set modifiers for two-way data binding.

Consider the following dysfunctional approach for two-way data binding using an event handler:

<p>
    <input value="@inputValue" @oninput="OnInput" />
</p>

<p>
    <code>inputValue</code>: @inputValue
</p>

@code {
    private string? inputValue;

    private void OnInput(ChangeEventArgs args)
    {
        var newValue = args.Value?.ToString() ?? string.Empty;

        inputValue = newValue.Length > 4 ? "Long!" : newValue;
    }
}

The OnInput event handler updates the value of inputValue to Long! after a fourth character is provided. However, the user can continue adding characters to the element value in the UI. The value of inputValue isn't bound back to the element's value with each keystroke. The preceding example is only capable of one-way data binding.

The reason for this behavior is that Blazor isn't aware that your code intends to modify the value of inputValue in the event handler. Blazor doesn't try to force DOM element values and .NET variable values to match unless they're bound with @bind syntax. In earlier versions of Blazor, two-way data binding is implemented by binding the element to a property and controlling the property's value with its setter. In ASP.NET Core in .NET 7 or later, @bind:get/@bind:set modifier syntax is used to implement two-way data binding, as the next example demonstrates.

Consider the following correct approach using @bind:get/@bind:set for two-way data binding:

<p>
    <input @bind:event="oninput" @bind:get="inputValue" @bind:set="OnInput" />
</p>

<p>
    <code>inputValue</code>: @inputValue
</p>

@code {
    private string? inputValue;

    private void OnInput(string value)
    {
        var newValue = value ?? string.Empty;

        inputValue = newValue.Length > 4 ? "Long!" : newValue;
    }
}

Using @bind:get/@bind:set modifiers both controls the underlying value of inputValue via @bind:set and binds the value of inputValue to the element's value via @bind:get. The preceding example demonstrates the correct approach for implementing two-way data binding.

:::moniker-end

Binding to a property with C# `get` and `set` accessors

C# get and set accessors can be used to create custom binding format behavior, as the following DecimalBinding component demonstrates. The component binds a positive or negative decimal with up to three decimal places to an <input> element by way of a string property (DecimalValue).

DecimalBinding.razor:

:::moniker range=">= aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/9.0/BlazorSample_BlazorWebApp/Components/Pages/DecimalBinding.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-8.0 < aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/Pages/DecimalBinding.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-7.0 < aspnetcore-8.0"

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

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

[!NOTE] Across multiple components, two-way binding to a property with get/set accessors requires discarding the xref:System.Threading.Tasks.Task returned by xref:Microsoft.AspNetCore.Components.EventCallback.InvokeAsync%2A?displayProperty=nameWithType in the property's setter. For two-way data binding, we recommend using @bind:get/@bind:set modifiers. For more information, see the @bind:get/@bind:set guidance earlier in this article.

To see an example of how the xref:System.Threading.Tasks.Task returned by xref:Microsoft.AspNetCore.Components.EventCallback.InvokeAsync%2A?displayProperty=nameWithType is discarded in .NET 6 or earlier before @bind:get/@bind:set modifiers became a framework feature, see the NestedChild component of the Bind across more than two components section in the .NET 6 version of this article.

:::moniker-end

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

[!NOTE] Two-way binding to a property with get/set accessors requires discarding the xref:System.Threading.Tasks.Task returned by xref:Microsoft.AspNetCore.Components.EventCallback.InvokeAsync%2A?displayProperty=nameWithType. For an example, see the NestedChild component of the Bind across more than two components section. For two-way data binding in .NET 7 or later, we recommend using @bind:get/@bind:set modifiers, which are described in 7.0 or later versions of this article.

:::moniker-end

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

Multiple option selection with `<select>` elements

Binding supports multiple option selection with <select> elements. The @onchange event provides an array of the selected elements via event arguments (ChangeEventArgs). The value must be bound to an array type.

BindMultipleInput.razor:

@page "/bind-multiple-input"

<h1>Bind Multiple <code>input</code>Example</h1>

<p>
    <label>
        Select one or more cars: 
        <select @onchange="SelectedCarsChanged" multiple>
            <option value="audi">Audi</option>
            <option value="jeep">Jeep</option>
            <option value="opel">Opel</option>
            <option value="saab">Saab</option>
            <option value="volvo">Volvo</option>
        </select>
    </label>
</p>

<p>
    Selected Cars: @string.Join(", ", SelectedCars)
</p>

<p>
    <label>
        Select one or more cities: 
        <select @bind="SelectedCities" multiple>
            <option value="bal">Baltimore</option>
            <option value="la">Los Angeles</option>
            <option value="pdx">Portland</option>
            <option value="sf">San Francisco</option>
            <option value="sea">Seattle</option>
        </select>
    </label>
</p>

<span>
    Selected Cities: @string.Join(", ", SelectedCities)
</span>

@code {
    public string[] SelectedCars { get; set; } = new string[] { };
    public string[] SelectedCities { get; set; } = new[] { "bal", "sea" };

    private void SelectedCarsChanged(ChangeEventArgs e)
    {
        if (e.Value is not null)
        {
            SelectedCars = (string[])e.Value;
        }
    }
}

For information on how empty strings and null values are handled in data binding, see the Binding <select> element options to C# object null values section.

:::moniker-end

Binding `<select>` element options to C# object `null` values

There's no sensible way to represent a <select> element option value as a C# object null value, because:

HTML attributes can't have null values. The closest equivalent to null in HTML is absence of the HTML value attribute from the <option> element.
When selecting an <option> with no value attribute, the browser treats the value as the text content of that <option>'s element.

The Blazor framework doesn't attempt to suppress the default behavior because it would involve:

Creating a chain of special-case workarounds in the framework.
Breaking changes to current framework behavior.

The most plausible null equivalent in HTML is an empty string value. The Blazor framework handles null to empty string conversions for two-way binding to a <select>'s value.

Unparsable values

When a user provides an unparsable value to a data-bound element, the unparsable value is automatically reverted to its previous value when the bind event is triggered.

Consider the following component, where an <input> element is bound to an int type with an initial value of 123.

UnparsableValues.razor:

:::moniker range=">= aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/9.0/BlazorSample_BlazorWebApp/Components/Pages/UnparsableValues.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-8.0 < aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/Pages/UnparsableValues.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-7.0 < aspnetcore-8.0"

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

Binding applies to the element's onchange event. If the user updates the value of the text box's entry to 123.45 and changes the focus, the element's value is reverted to 123 when onchange fires. When the value 123.45 is rejected in favor of the original value of 123, the user understands that their value wasn't accepted.

For the oninput event (@bind:event="oninput"), a value reversion occurs after any keystroke that introduces an unparsable value. When targeting the oninput event with an int-bound type, a user is prevented from typing a dot (.) character. A dot (.) character is immediately removed, so the user receives immediate feedback that only whole numbers are permitted. There are scenarios where reverting the value on the oninput event isn't ideal, such as when the user should be allowed to clear an unparsable <input> value. Alternatives include:

Don't use the oninput event. Use the default onchange event, where an invalid value isn't reverted until the element loses focus.
Bind to a nullable type, such as int? or string and either use @bind:get/@bind:set modifiers (described earlier in this article) or bind to a property with custom get and set accessor logic to handle invalid entries.
Use an input component, such as xref:Microsoft.AspNetCore.Components.Forms.InputNumber%601 or xref:Microsoft.AspNetCore.Components.Forms.InputDate%601, with form validation. Input components together with form validation components provide built-in support to manage invalid inputs:
- Permit the user to provide invalid input and receive validation errors on the associated xref:Microsoft.AspNetCore.Components.Forms.EditContext.
- Display validation errors in the UI without interfering with the user entering additional webform data.

Format strings

Data binding works with a single xref:System.DateTime format string using @bind:format="{FORMAT STRING}", where the {FORMAT STRING} placeholder is the format string. Other format expressions, such as currency or number formats, aren't available at this time but might be added in a future release.

DateBinding.razor:

:::moniker range=">= aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/9.0/BlazorSample_BlazorWebApp/Components/Pages/DateBinding.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-8.0 < aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/Pages/DateBinding.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-7.0 < aspnetcore-8.0"

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

In the preceding code, the <input> element's field type (type attribute) defaults to text.

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

Nullable xref:System.DateTime?displayProperty=fullName and xref:System.DateTimeOffset?displayProperty=fullName are supported:

private DateTime? date;
private DateTimeOffset? dateOffset;

:::moniker-end

Specifying a format for the date field type isn't recommended because Blazor has built-in support to format dates. In spite of the recommendation, only use the yyyy-MM-dd date format for binding to function correctly if a format is supplied with the date field type:

<input type="date" @bind="startDate" @bind:format="yyyy-MM-dd">

Binding with component parameters

A common scenario is binding a property of a child component to a property in its parent component. This scenario is called a chained bind because multiple levels of binding occur simultaneously.

You can't implement chained binds with @bind syntax in a child component. An event handler and value must be specified separately to support updating the property in the parent from the child component. The parent component still leverages @bind syntax to set up data binding with the child component.

The following ChildBind component has a Year component parameter and an xref:Microsoft.AspNetCore.Components.EventCallback%601. By convention, the xref:Microsoft.AspNetCore.Components.EventCallback%601 for the parameter must be named as the component parameter name with a "Changed" suffix. The naming syntax is {PARAMETER NAME}Changed, where the {PARAMETER NAME} placeholder is the parameter name. In the following example, the xref:Microsoft.AspNetCore.Components.EventCallback%601 is named YearChanged.

xref:Microsoft.AspNetCore.Components.EventCallback.InvokeAsync%2A?displayProperty=nameWithType invokes the delegate associated with the binding with the provided argument and dispatches an event notification for the changed property.

ChildBind.razor:

:::moniker range=">= aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/9.0/BlazorSample_BlazorWebApp/Components/ChildBind.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-8.0 < aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/ChildBind.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-7.0 < aspnetcore-8.0"

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

For more information on events and xref:Microsoft.AspNetCore.Components.EventCallback%601, see the EventCallback section of the xref:blazor/components/event-handling#eventcallback article.

In the following Parent1 component, the year field is bound to the Year parameter of the child component. The Year parameter is bindable because it has a companion YearChanged event that matches the type of the Year parameter.

Parent1.razor:

:::moniker range=">= aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/9.0/BlazorSample_BlazorWebApp/Components/Pages/Parent1.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-8.0 < aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/Pages/Parent1.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-7.0 < aspnetcore-8.0"

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/data-binding/Parent1.razor":::

:::moniker-end

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

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/data-binding/Parent1.razor":::

:::moniker-end

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

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/data-binding/Parent1.razor":::

:::moniker-end

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

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/data-binding/Parent1.razor":::

:::moniker-end

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

Component parameter binding can also trigger @bind:after events. In the following example, the YearUpdated method executes asynchronously after binding the Year component parameter.

<ChildBind @bind-Year="year" @bind-Year:after="YearUpdated" />

@code {
    ...

    private async Task YearUpdated()
    {
        ... = await ...;
    }
}

:::moniker-end

By convention, a property can be bound to a corresponding event handler by including an @bind-{PROPERTY}:event attribute assigned to the handler, where the {PROPERTY} placeholder is the property. <ChildBind @bind-Year="year" /> is equivalent to writing:

<ChildBind @bind-Year="year" @bind-Year:event="YearChanged" />

In a more sophisticated and real-world example, the following PasswordEntry component:

Sets an <input> element's value to a password field.
Exposes changes of a Password property to a parent component with an EventCallback that passes in the current value of the child's password field as its argument.
Uses the onclick event to trigger the ToggleShowPassword method. For more information, see xref:blazor/components/event-handling.

[!INCLUDE]

PasswordEntry.razor:

:::moniker range=">= aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/9.0/BlazorSample_BlazorWebApp/Components/PasswordEntry.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-8.0 < aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/PasswordEntry.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-7.0 < aspnetcore-8.0"

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

The PasswordEntry component is used in another component, such as the following PasswordBinding component example.

PasswordBinding.razor:

:::moniker range=">= aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/9.0/BlazorSample_BlazorWebApp/Components/Pages/PasswordBinding.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-8.0 < aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/Pages/PasswordBinding.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-7.0 < aspnetcore-8.0"

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

When the PasswordBinding component is initially rendered, the password value of Not set is displayed in the UI. After initial rendering, the value of password reflects changes made to the Password component parameter value in the PasswordEntry component.

[!NOTE] The preceding example binds the password one-way from the child PasswordEntry component to the parent PasswordBinding component. Two-way binding isn't a requirement in this scenario if the goal is for the app to have a shared password entry component for reuse around the app that merely passes the password to the parent. For an approach that permits two-way binding without writing directly to the child component's parameter, see the NestedChild component example in the Bind across more than two components section of this article.

Perform checks or trap errors in the handler. The following revised PasswordEntry component provides immediate feedback to the user if a space is used in the password's value.

PasswordEntry.razor:

:::moniker range=">= aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/9.0/BlazorSample_BlazorWebApp/Components/PasswordEntry2.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-8.0 < aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/PasswordEntry2.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-7.0 < aspnetcore-8.0"

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Shared/data-binding/PasswordEntry2.razor":::

In the following example, the PasswordUpdated method executes asynchronously after binding the Password component parameter:

<PasswordEntry @bind-Password="password" @bind-Password:after="PasswordUpdated" />

:::moniker-end

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

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Shared/data-binding/PasswordEntry2.razor":::

:::moniker-end

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

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Shared/data-binding/PasswordEntry2.razor":::

:::moniker-end

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

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Shared/data-binding/PasswordEntry2.razor":::

:::moniker-end

Bind across more than two components

You can bind parameters through any number of nested components, but you must respect the one-way flow of data:

Change notifications flow up the hierarchy.
New parameter values flow down the hierarchy.

A common and recommended approach is to only store the underlying data in the parent component to avoid any confusion about what state must be updated, as shown in the following example.

Parent2.razor:

:::moniker range=">= aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/9.0/BlazorSample_BlazorWebApp/Components/Pages/Parent2.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-8.0 < aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/Pages/Parent2.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-7.0 < aspnetcore-8.0"

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/data-binding/Parent2.razor":::

:::moniker-end

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

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/data-binding/Parent2.razor":::

:::moniker-end

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

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/data-binding/Parent2.razor":::

:::moniker-end

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

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/data-binding/Parent2.razor":::

:::moniker-end

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

In the following NestedChild component, the NestedGrandchild component:

Assigns the value of ChildMessage to GrandchildMessage with @bind:get syntax.
Updates GrandchildMessage when ChildMessageChanged executes with @bind:set syntax.

Prior to the release of .NET 7, two-way binding across components uses get/set accessors with a third property that discards the xref:System.Threading.Tasks.Task returned by xref:Microsoft.AspNetCore.Components.EventCallback.InvokeAsync%2A?displayProperty=nameWithType in its setter. To see an example of this approach for .NET 6 or earlier before @bind:get/@bind:set modifiers became a framework feature, see the NestedChild component of this section in the .NET 6 version of this article.

:::moniker-end

NestedChild.razor:

:::moniker range=">= aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/9.0/BlazorSample_BlazorWebApp/Components/NestedChild.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-8.0 < aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/NestedChild.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-7.0 < aspnetcore-8.0"

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

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

[!WARNING] Generally, avoid creating components that write directly to their own component parameters. The preceding NestedChild component makes use of a BoundValue property instead of writing directly to its ChildMessage parameter. For more information, see xref:blazor/components/overwriting-parameters.

:::moniker-end

NestedGrandchild.razor:

:::moniker range=">= aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/9.0/BlazorSample_BlazorWebApp/Components/NestedGrandchild.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-8.0 < aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/NestedGrandchild.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-7.0 < aspnetcore-8.0"

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

For an alternative approach suited to sharing data in memory and across components that aren't necessarily nested, see xref:blazor/state-management.

Bound field or property expression tree

To facilitate deeper interactions with a binding, Blazor allows you to capture of the expression tree of a bound field or property. This is achieved by defining a property with the field or property name suffixed with Expression. For any given field or property named {FIELD OR PROPERTY NAME}, the corresponding expression tree property is named {FIELD OR PROPERTY NAME}Expression.

The following ChildParameterExpression component identifies the Year expression's model and field name. A xref:Microsoft.AspNetCore.Components.Forms.FieldIdentifier, which is used to obtain the model and field name, uniquely identifies a single field that can be edited. This may correspond to a property on a model object or can be any other named value. Use of a parameter's expression is useful when creating custom validation components, which isn't covered by the Microsoft Blazor documentation but is addressed by numerous third-party resources.

ChildParameterExpression.razor:

:::moniker range=">= aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/9.0/BlazorSample_BlazorWebApp/Components/ChildParameterExpression.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-8.0 < aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/ChildParameterExpression.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-7.0 < aspnetcore-8.0"

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

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

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

:::moniker-end

Parent3.razor:

:::moniker range=">= aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/9.0/BlazorSample_BlazorWebApp/Components/Pages/Parent3.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-8.0 < aspnetcore-9.0"

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/Pages/Parent3.razor":::

:::moniker-end

:::moniker range=">= aspnetcore-7.0 < aspnetcore-8.0"

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/data-binding/Parent3.razor":::

:::moniker-end

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

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/data-binding/Parent3.razor":::

:::moniker-end

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

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/data-binding/Parent3.razor":::

:::moniker-end

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

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/data-binding/Parent3.razor":::

:::moniker-end

44 KiB Raw Blame History

ASP.NET Core Blazor data binding

Binding features

Use @bind:get/@bind:set modifiers and avoid event handlers for two-way data binding

Binding to a property with C# get and set accessors

Multiple option selection with <select> elements

Binding <select> element options to C# object null values

Unparsable values

Format strings

Binding with component parameters

Bind across more than two components

Bound field or property expression tree

Additional resources

44 KiB

Raw Blame History

Use `@bind:get`/`@bind:set` modifiers and avoid event handlers for two-way data binding

Binding to a property with C# `get` and `set` accessors

Multiple option selection with `<select>` elements

Binding `<select>` element options to C# object `null` values