ASP.NET Core Blazor data binding

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:

@page "/bind"

<p>
    <input @bind="inputValue" />
</p>

<p>
    <input @bind="InputValue" />
</p>

<ul>
    <li><code>inputValue</code>: @inputValue</li>
    <li><code>InputValue</code>: @InputValue</li>
</ul>

@code {
    private string inputValue;

    private string InputValue { get; set; }
}

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. 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:

@page "/bind-theory"

<p>
    <label>
        Normal Blazor binding: 
        <input @bind="InputValue" />
    </label>
</p>

<p>
    <label>
        Demonstration of equivalent HTML binding: 
        <input value="@InputValue"
            @onchange="@((ChangeEventArgs __e) => InputValue = __e.Value.ToString())" />
    </label>
</p>

<p>
    <code>InputValue</code>: @InputValue
</p>

@code {
    private string InputValue { get; set; }
}

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 is triggered. Unlike the onchange event, which fires when the element loses focus, oninput fires when the value of the text box changes.

Page/BindEvent.razor:

@page "/bind-event"

<p>
    <input @bind="InputValue" @bind:event="oninput" />
</p>

<p>
    <code>InputValue</code>: @InputValue
</p>

@code {
    private string InputValue { get; set; }
}

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:

@page "/unparseable-values"

<p>
    <input @bind="inputValue" />
</p>

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

@code {
    private int inputValue = 123;
}

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 InputNumber<TValue> or InputDate<TValue>. 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 EditContext.
    • Display validation errors in the UI without interfering with the user entering additional webform data.

Format strings

Data binding works with a single 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:

@page "/date-binding"

<p>
    <label>
        <code>yyyy-MM-dd</code> format:
        <input @bind="startDate" @bind:format="yyyy-MM-dd" />
    </label>
</p>

<p>
    <code>startDate</code>: @startDate
</p>

@code {
    private DateTime startDate = new(2020, 1, 1);
}

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

Nullable System.DateTime and System.DateTimeOffset 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:

@page "/decimal-binding"
@using System.Globalization

<p>
    <label>
        Decimal value (&plusmn;0.000 format):
        <input @bind="DecimalValue" />
    </label>
</p>

<p>
    <code>decimalValue</code>: @decimalValue
</p>

@code {
    private decimal decimalValue = 1.1M;
    private NumberStyles style = 
        NumberStyles.AllowDecimalPoint | NumberStyles.AllowLeadingSign;
    private CultureInfo culture = CultureInfo.CreateSpecificCulture("en-US");

    private string DecimalValue
    {
        get => decimalValue.ToString("0.000", culture);
        set
        {
            if (Decimal.TryParse(value, style, culture, out var number))
            {
                decimalValue = Math.Round(number, 3);
            }
        }
    }
}

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 EventCallback<TValue>. By convention, the EventCallback<TValue> 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 EventCallback<TValue> is named YearChanged.

EventCallback.InvokeAsync invokes the delegate associated with the binding with the provided argument and dispatches an event notification for the changed property.

Shared/ChildBind.razor:

<div class="card bg-light mt-3" style="width:18rem ">
    <div class="card-body">
        <h3 class="card-title">ChildBind Component</h3>
        <p class="card-text">
            Child <code>Year</code>: @Year
        </p>
        <button @onclick="UpdateYearFromChild">Update Year from Child</button>
    </div>
</div>

@code {
    private Random r = new();

    [Parameter]
    public int Year { get; set; }

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

    private async Task UpdateYearFromChild()
    {
        await YearChanged.InvokeAsync(r.Next(1950, 2021));
    }
}

For more information on events and EventCallback<TValue>, see the EventCallback section of the ASP.NET Core Blazor event handling article.

In the following Parent 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/Parent.razor:

@page "/Parent"

<h1>Parent Component</h1>

<p>Parent <code>year</code>: @year</p>

<button @onclick="UpdateYear">Update Parent <code>year</code></button>

<ChildBind @bind-Year="year" />

@code {
    private Random r = new();
    private int year = 1979;

    private void UpdateYear()
    {
        year = r.Next(1950, 2021);
    }
}

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 ASP.NET Core Blazor event handling.

Shared/PasswordEntry.razor:

<div class="card bg-light mt-3" style="width:22rem ">
    <div class="card-body">
        <h3 class="card-title">Password Component</h3>
        <p class="card-text">
            <label>
                Password:
                <input @oninput="OnPasswordChanged"
                       required
                       type="@(showPassword ? "text" : "password")"
                       value="@password" />
            </label>
        </p>
        <button class="btn btn-primary" @onclick="ToggleShowPassword">
            Show password
        </button>
    </div>
</div>

@code {
    private bool showPassword;
    private string password;

    [Parameter]
    public string Password { get; set; }

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

    private async Task OnPasswordChanged(ChangeEventArgs e)
    {
        password = e.Value.ToString();

        await PasswordChanged.InvokeAsync(password);
    }

    private void ToggleShowPassword()
    {
        showPassword = !showPassword;
    }
}

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

Pages/PasswordBinding.razor:

@page "/password-binding"

<h1>Password Binding</h1>

<PasswordEntry @bind-Password="password" />

<p>
    <code>password</code>: @password
</p>

@code {
    private string password = "Not set";
}

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:

<div class="card bg-light mt-3" style="width:22rem ">
    <div class="card-body">
        <h3 class="card-title">Password Component</h3>
        <p class="card-text">
            <label>
                Password:
                <input @oninput="OnPasswordChanged"
                       required
                       type="@(showPassword ? "text" : "password")"
                       value="@password" />
            </label>
            <span class="text-danger">@validationMessage</span>
        </p>
        <button class="btn btn-primary" @onclick="ToggleShowPassword">
            Show password
        </button>
    </div>
</div>

@code {
    private bool showPassword;
    private string password;
    private string validationMessage;

    [Parameter]
    public string Password { get; set; }

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

    private Task OnPasswordChanged(ChangeEventArgs e)
    {
        password = e.Value.ToString();

        if (password.Contains(' '))
        {
            validationMessage = "Spaces not allowed!";

            return Task.CompletedTask;
        }
        else
        {
            validationMessage = string.Empty;

            return PasswordChanged.InvokeAsync(password);
        }
    }

    private void ToggleShowPassword()
    {
        showPassword = !showPassword;
    }
}

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/Parent.razor:

@page "/parent"

<h1>Parent Component</h1>

<p>Parent Message: <b>@parentMessage</b></p>

<p>
    <button @onclick="ChangeValue">Change from Parent</button>
</p>

<NestedChild @bind-ChildMessage="parentMessage" />

@code {
    private string parentMessage = "Initial value set in Parent";

    private void ChangeValue()
    {
        parentMessage = $"Set in Parent {DateTime.Now}";
    }
}

Shared/NestedChild.razor:

<div class="border rounded m-1 p-1">
    <h2>Child Component</h2>

    <p>Child Message: <b>@ChildMessage</b></p>

    <p>
        <button @onclick="ChangeValue">Change from Child</button>
    </p>

    <NestedGrandchild @bind-GrandchildMessage="BoundValue" />
</div>

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

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

    private string BoundValue
    {
        get => ChildMessage;
        set => ChildMessageChanged.InvokeAsync(value);
    }

    private async Task ChangeValue()
    {
        await ChildMessageChanged.InvokeAsync(
            $"Set in Child {DateTime.Now}");
    }
}

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 ASP.NET Core Razor components.

Shared/NestedGrandchild.razor:

<div class="border rounded m-1 p-1">
    <h3>Grandchild Component</h3>

    <p>Grandchild Message: <b>@GrandchildMessage</b></p>

    <p>
        <button @onclick="ChangeValue">Change from Grandchild</button>
    </p>
</div>

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

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

    private async Task ChangeValue()
    {
        await GrandchildMessageChanged.InvokeAsync(
            $"Set in Grandchild {DateTime.Now}");
    }
}

For an alternative approach suited to sharing data in memory and across components that aren't necessarily nested, see ASP.NET Core Blazor state management.

Additional resources