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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 Document Object Model (DOM) elements in Blazor apps.	>= aspnetcore-3.1	riande	mvc	11/08/2022	blazor/components/data-binding

ASP.NET Core Blazor data binding

This article explains data binding features for Razor components and Document Object Model (DOM) elements in Blazor apps.

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

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.

Pages/Bind.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/data-binding/Bind.razor" highlight="4,8":::

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.

Pages/BindTheory.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/data-binding/BindTheory.razor" highlight="12-14":::

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 a value attribute and handles changes using the registered handler.

Bind a property or field on other Document Object Model (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:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/data-binding/BindEvent.razor" highlight="4":::

To execute asynchronous logic after binding, use @bind:after="{EVENT}" with a DOM event for the {EVENT} placeholder. An assigned C# method isn't executed until the bound value is assigned synchronously.

In the following example:

The <input> element's value is bound to the value of searchText synchronously.
After each keystroke (onchange event) in the field, the PerformSearch method executes asynchronously.
PerformSearch calls a service with an asynchronous method (FetchAsync) to return search results.

@inject ISearchService SearchService

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

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

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

Components support two-way data binding by defining a pair of parameters:

@bind:get: Specifies the value to bind.
@bind:set: Specifies a callback for when the value changes.

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

Example:

<input @bind:get="Value" @bind:set="ValueChanged" />

@code {
    [Parameter]
    public string? Value { get; set; }

    [Parameter]
    public EventCallback<string> ValueChanged { get; set; }
}

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.

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.

Pages/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" };

    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.

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 databound 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.

Pages/UnparsableValues.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/data-binding/UnparsableValues.razor" highlight="4,12":::

By default, 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 provide custom get and set accessor logic to handle invalid entries.
Use a form validation component, such as xref:Microsoft.AspNetCore.Components.Forms.InputNumber%601 or xref:Microsoft.AspNetCore.Components.Forms.InputDate%601. Form validation components provide built-in support to manage invalid inputs. Form validation components:
- 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.

Pages/DateBinding.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/data-binding/DateBinding.razor" highlight="6":::

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

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

private DateTime? date;
private DateTimeOffset? dateOffset;

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">

Custom binding formats

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).

Pages/DecimalBinding.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/data-binding/DecimalBinding.razor" highlight="7,21-31":::

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.

Component parameters permit binding properties of a parent component with @bind-{PROPERTY} syntax, where the {PROPERTY} placeholder is the property to bind.

You can't implement chained binds with @bind syntax in the 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 the @bind syntax to set up the databinding 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.

Shared/ChildBind.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Shared/data-binding/ChildBind.razor" highlight="14-15,17-18,22":::

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.

Pages/Parent1.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/data-binding/Parent1.razor" highlight="9":::

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 ...;
    }
}

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.

Shared/PasswordEntry.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Shared/data-binding/PasswordEntry.razor" highlight="7-10,13,23-24,26-27,36-39":::

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

Pages/PasswordBinding.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Pages/data-binding/PasswordBinding.razor" highlight="5":::

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.

Shared/PasswordEntry.razor:

:::code language="razor" source="~/../blazor-samples/7.0/BlazorSample_WebAssembly/Shared/data-binding/PasswordEntry2.razor" highlight="35-46":::

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

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

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.

Pages/Parent2.razor:

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

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.

Shared/NestedChild.razor:

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

Shared/NestedGrandchild.razor:

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

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

Additional resources

:::moniker-end

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

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.

Pages/Bind.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/data-binding/Bind.razor" highlight="4,8":::

Pages/BindTheory.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/data-binding/BindTheory.razor" highlight="12-14":::

Page/BindEvent.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/data-binding/BindEvent.razor" highlight="4":::

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.

Multiple option selection with `<select>` elements

Pages/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" };

    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.

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 databound 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.

Pages/UnparsableValues.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/data-binding/UnparsableValues.razor" highlight="4,12":::

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 provide custom get and set accessor logic to handle invalid entries.
Use a form validation component, such as xref:Microsoft.AspNetCore.Components.Forms.InputNumber%601 or xref:Microsoft.AspNetCore.Components.Forms.InputDate%601. Form validation components provide built-in support to manage invalid inputs. Form validation components:
- 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

Pages/DateBinding.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/data-binding/DateBinding.razor" highlight="6":::

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

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

private DateTime? date;
private DateTimeOffset? dateOffset;

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

Custom binding formats

Pages/DecimalBinding.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/data-binding/DecimalBinding.razor" highlight="7,21-31":::

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.

Component parameters permit binding properties of a parent component with @bind-{PROPERTY} syntax, where the {PROPERTY} placeholder is the property to bind.

The parent component still leverages the @bind syntax to set up the databinding with the child component.

Shared/ChildBind.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Shared/data-binding/ChildBind.razor" highlight="14-15,17-18,22":::

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.

Pages/Parent1.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/data-binding/Parent1.razor" highlight="9":::

<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.

Shared/PasswordEntry.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Shared/data-binding/PasswordEntry.razor" highlight="7-10,13,23-24,26-27,36-39":::

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

Pages/PasswordBinding.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Pages/data-binding/PasswordBinding.razor" highlight="5":::

[!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.

Shared/PasswordEntry.razor:

:::code language="razor" source="~/../blazor-samples/6.0/BlazorSample_WebAssembly/Shared/data-binding/PasswordEntry2.razor" highlight="35-46":::

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.

Pages/Parent2.razor:

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

Shared/NestedChild.razor:

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

[!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/index#overwritten-parameters.

Shared/NestedGrandchild.razor:

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

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

Additional resources

:::moniker-end

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

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.

Pages/Bind.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/data-binding/Bind.razor" highlight="4,8":::

Pages/BindTheory.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/data-binding/BindTheory.razor" highlight="12-14":::

Page/BindEvent.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/data-binding/BindEvent.razor" highlight="4":::

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.

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 databound 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.

Pages/UnparsableValues.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/data-binding/UnparsableValues.razor" highlight="4,12":::

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 provide custom get and set accessor logic to handle invalid entries.
Use a form validation component, such as xref:Microsoft.AspNetCore.Components.Forms.InputNumber%601 or xref:Microsoft.AspNetCore.Components.Forms.InputDate%601. Form validation components provide built-in support to manage invalid inputs. Form validation components:
- 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

Pages/DateBinding.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/data-binding/DateBinding.razor" highlight="6":::

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

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

private DateTime? date;
private DateTimeOffset? dateOffset;

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

Custom binding formats

Pages/DecimalBinding.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/data-binding/DecimalBinding.razor" highlight="7,21-31":::

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.

Component parameters permit binding properties of a parent component with @bind-{PROPERTY} syntax, where the {PROPERTY} placeholder is the property to bind.

The parent component still leverages the @bind syntax to set up the databinding with the child component.

Shared/ChildBind.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Shared/data-binding/ChildBind.razor" highlight="14-15,17-18,22":::

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.

Pages/Parent1.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/data-binding/Parent1.razor" highlight="9":::

<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.

Shared/PasswordEntry.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Shared/data-binding/PasswordEntry.razor" highlight="7-10,13,23-24,26-27,36-39":::

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

Pages/PasswordBinding.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Pages/data-binding/PasswordBinding.razor" highlight="5":::

[!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.

Shared/PasswordEntry.razor:

:::code language="razor" source="~/../blazor-samples/5.0/BlazorSample_WebAssembly/Shared/data-binding/PasswordEntry2.razor" highlight="35-46":::

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.

Pages/Parent2.razor:

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

Shared/NestedChild.razor:

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

[!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/index#overwritten-parameters.

Shared/NestedGrandchild.razor:

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

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

Additional resources

:::moniker-end

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

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.

Pages/Bind.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/data-binding/Bind.razor" highlight="4,8":::

Pages/BindTheory.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/data-binding/BindTheory.razor" highlight="12-14":::

Page/BindEvent.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/data-binding/BindEvent.razor" highlight="4":::

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.

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 Blazor framework doesn't automatically handle null to empty string conversions when attempting two-way binding to a <select>'s value. For more information, see Fix binding <select> to a null value (dotnet/aspnetcore #23221).

Unparsable values

When a user provides an unparsable value to a databound 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.

Pages/UnparsableValues.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/data-binding/UnparsableValues.razor" highlight="4,12":::

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 provide custom get and set accessor logic to handle invalid entries.
Use a form validation component, such as xref:Microsoft.AspNetCore.Components.Forms.InputNumber%601 or xref:Microsoft.AspNetCore.Components.Forms.InputDate%601. Form validation components provide built-in support to manage invalid inputs. Form validation components:
- 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

Pages/DateBinding.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/data-binding/DateBinding.razor" highlight="6":::

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

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

private DateTime? date;
private DateTimeOffset? dateOffset;

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

Custom binding formats

Pages/DecimalBinding.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/data-binding/DecimalBinding.razor" highlight="7,21-31":::

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.

Component parameters permit binding properties of a parent component with @bind-{PROPERTY} syntax, where the {PROPERTY} placeholder is the property to bind.

The parent component still leverages the @bind syntax to set up the databinding with the child component.

Shared/ChildBind.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Shared/data-binding/ChildBind.razor" highlight="14-15,17-18,22":::

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.

Pages/Parent1.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/data-binding/Parent1.razor" highlight="9":::

<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.

Shared/PasswordEntry.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Shared/data-binding/PasswordEntry.razor" highlight="7-10,13,23-24,26-27,36-39":::

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

Pages/PasswordBinding.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Pages/data-binding/PasswordBinding.razor" highlight="5":::

[!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.

Shared/PasswordEntry.razor:

:::code language="razor" source="~/../blazor-samples/3.1/BlazorSample_WebAssembly/Shared/data-binding/PasswordEntry2.razor" highlight="35-46":::

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.

Pages/Parent2.razor:

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

Shared/NestedChild.razor:

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

[!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/index#overwritten-parameters.

Shared/NestedGrandchild.razor:

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

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

Additional resources

:::moniker-end

66 KiB Raw Blame History

ASP.NET Core Blazor data binding

Multiple option selection with <select> elements

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

Unparsable values

Format strings

Custom binding formats

Binding with component parameters

Bind across more than two components

Additional resources

Multiple option selection with <select> elements

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

Unparsable values

Format strings

Custom binding formats

Binding with component parameters

Bind across more than two components

Additional resources

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

Unparsable values

Format strings

Custom binding formats

Binding with component parameters

Bind across more than two components

Additional resources

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

Unparsable values

Format strings

Custom binding formats

Binding with component parameters

Bind across more than two components

Additional resources

66 KiB

Raw Blame History

Multiple option selection with `<select>` elements

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

Multiple option selection with `<select>` elements

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

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

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