ASP.NET Core Blazor forms binding

This article explains how to use binding in Blazor forms.

EditForm/EditContext model

An EditForm creates an EditContext based on the assigned object as a cascading value for other components in the form. The EditContext tracks metadata about the edit process, including which form fields have been modified and the current validation messages. Assigning to either an EditForm.Model or an EditForm.EditContext can bind a form to data.

Model binding

Assignment to EditForm.Model:

<EditForm ... Model="Model" ...>
    ...
</EditForm>

@code {
    [SupplyParameterFromForm]
    private Starship? Model { get; set; }

    protected override void OnInitialized() => Model ??= new();
}

Context binding

Assignment to EditForm.EditContext:

<EditForm ... EditContext="editContext" ...>
    ...
</EditForm>

@code {
    private EditContext? editContext;

    [SupplyParameterFromForm]
    private Starship? Model { get; set; }

    protected override void OnInitialized()
    {
        Model ??= new();
        editContext = new(Model);
    }
}

Assign either an EditContext or a Model to an EditForm. If both are assigned, a runtime error is thrown.

Supported types

Binding supports:

• Primitive types
• Collections
• Complex types
• Recursive types
• Types with constructors
• Enums

You can also use the [DataMember] and [IgnoreDataMember] attributes to customize model binding. Use these attributes to rename properties, ignore properties, and mark properties as required.

Additional binding options

Additional model binding options are available from RazorComponentsServiceOptions when calling AddRazorComponents:

• MaxFormMappingCollectionSize: Maximum number of elements allowed in a form collection.
• MaxFormMappingRecursionDepth: Maximum depth allowed when recursively mapping form data.
• MaxFormMappingErrorCount: Maximum number of errors allowed when mapping form data.
• MaxFormMappingKeySize: Maximum size of the buffer used to read form data keys.

The following demonstrates the default values assigned by the framework:

builder.Services.AddRazorComponents(options =>
{
    options.FormMappingUseCurrentCulture = true;
    options.MaxFormMappingCollectionSize = 1024;
    options.MaxFormMappingErrorCount = 200;
    options.MaxFormMappingKeySize = 1024 * 2;
    options.MaxFormMappingRecursionDepth = 64;
}).AddInteractiveServerComponents();

Form names

Use the FormName parameter to assign a form name. Form names must be unique to bind model data. The following form is named RomulanAle:

<EditForm ... FormName="RomulanAle" ...>
    ...
</EditForm>

Supplying a form name:

• Is required for all forms that are submitted by statically-rendered server-side components.
• Isn't required for forms that are submitted by interactively-rendered components, which includes forms in Blazor WebAssembly apps and components with an interactive render mode. However, we recommend supplying a unique form name for every form to prevent runtime form posting errors if interactivity is ever dropped for a form.

The form name is only checked when the form is posted to an endpoint as a traditional HTTP POST request from a statically-rendered server-side component. The framework doesn't throw an exception at the point of rendering a form, but only at the point that an HTTP POST arrives and doesn't specify a form name.

There's an unnamed (empty string) form scope above the app's root component, which suffices when there are no form name collisions in the app. If form name collisions are possible, such as when including a form from a library and you have no control of the form name used by the library's developer, provide a form name scope with the FormMappingScope component in the Blazor Web App's main project.

In the following example, the HelloFormFromLibrary component has a form named Hello and is in a library.

HelloFormFromLibrary.razor:

<EditForm FormName="Hello" Model="this" OnSubmit="Submit">
    <InputText @bind-Value="Name" />
    <button type="submit">Submit</button>
</EditForm>

@if (submitted)
{
    <p>Hello @Name from the library's form!</p>
}

@code {
    bool submitted = false;

    [SupplyParameterFromForm]
    private string? Name { get; set; }

    private void Submit() => submitted = true;
}

The following NamedFormsWithScope component uses the library's HelloFormFromLibrary component and also has a form named Hello. The FormMappingScope component's scope name is ParentContext for any forms supplied by the HelloFormFromLibrary component. Although both of the forms in this example have the form name (Hello), the form names don't collide and events are routed to the correct form for form POST events.

NamedFormsWithScope.razor:

@page "/named-forms-with-scope"

<div>Hello form from a library</div>

<FormMappingScope Name="ParentContext">
    <HelloFormFromLibrary />
</FormMappingScope>

<div>Hello form using the same form name</div>

<EditForm FormName="Hello" Model="this" OnSubmit="Submit">
    <InputText @bind-Value="Name" />
    <button type="submit">Submit</button>
</EditForm>

@if (submitted)
{
    <p>Hello @Name from the app form!</p>
}

@code {
    bool submitted = false;

    [SupplyParameterFromForm]
    private string? Name { get; set; }

    private void Submit() => submitted = true;
}

Supply a parameter from the form ([SupplyParameterFromForm])

The [SupplyParameterFromForm] attribute indicates that the value of the associated property should be supplied from the form data for the form. Data in the request that matches the name of the property is bound to the property. Inputs based on InputBase<TValue> generate form value names that match the names Blazor uses for model binding. Unlike component parameter properties ([Parameter]), properties annotated with [SupplyParameterFromForm] aren't required to be marked public.

You can specify the following form binding parameters to the [SupplyParameterFromForm] attribute:

• Name: Gets or sets the name for the parameter. The name is used to determine the prefix to use to match the form data and decide whether or not the value needs to be bound.
• FormName: Gets or sets the name for the handler. The name is used to match the parameter to the form by form name to decide whether or not the value needs to be bound.

The following example independently binds two forms to their models by form name.

Starship6.razor:

@page "/starship-6"
@inject ILogger<Starship6> Logger

<EditForm Model="Model1" OnSubmit="Submit1" FormName="Holodeck1">
    <div>
        <label>
            Holodeck 1 Identifier: 
            <InputText @bind-Value="Model1!.Id" />
        </label>
    </div>
    <div>
        <button type="submit">Submit</button>
    </div>
</EditForm>

<EditForm Model="Model2" OnSubmit="Submit2" FormName="Holodeck2">
    <div>
        <label>
            Holodeck 2 Identifier: 
            <InputText @bind-Value="Model2!.Id" />
        </label>
    </div>
    <div>
        <button type="submit">Submit</button>
    </div>
</EditForm>

@code {
    [SupplyParameterFromForm(FormName = "Holodeck1")]
    private Holodeck? Model1 { get; set; }

    [SupplyParameterFromForm(FormName = "Holodeck2")]
    private Holodeck? Model2 { get; set; }

    protected override void OnInitialized()
    {
        Model1 ??= new();
        Model2 ??= new();
    }

    private void Submit1() => Logger.LogInformation("Submit1: Id={Id}", Model1?.Id);

    private void Submit2() => Logger.LogInformation("Submit2: Id={Id}", Model2?.Id);

    public class Holodeck
    {
        public string? Id { get; set; }
    }
}

Nest and bind forms

The following guidance demonstrates how to nest and bind child forms.

The following ship details class (ShipDetails) holds a description and length for a subform.

ShipDetails.cs:

namespace BlazorSample;

public class ShipDetails
{
    public string? Description { get; set; }
    public int? Length { get; set; }
}

The following Ship class names an identifier (Id) and includes the ship details.

Ship.cs:

namespace BlazorSample
{
    public class Ship
    {
        public string? Id { get; set; }
        public ShipDetails Details { get; set; } = new();
    }
}

The following subform is used for editing values of the ShipDetails type. This is implemented by inheriting Editor<T> at the top of the component. Editor<T> ensures that the child component generates the correct form field names based on the model (T), where T in the following example is ShipDetails.

StarshipSubform.razor:

@inherits Editor<ShipDetails>

<div>
    <label>
        Description: 
        <InputText @bind-Value="Value!.Description" />
    </label>
</div>
<div>
    <label>
        Length: 
        <InputNumber @bind-Value="Value!.Length" />
    </label>
</div>

The main form is bound to the Ship class. The StarshipSubform component is used to edit ship details, bound as Model!.Details.

Starship7.razor:

@page "/starship-7"
@inject ILogger<Starship7> Logger

<EditForm Model="Model" OnSubmit="Submit" FormName="Starship7">
    <div>
        <label>
            Identifier: 
            <InputText @bind-Value="Model!.Id" />
        </label>
    </div>
    <StarshipSubform @bind-Value="Model!.Details" />
    <div>
        <button type="submit">Submit</button>
    </div>
</EditForm>

@code {
    [SupplyParameterFromForm]
    private Ship? Model { get; set; }

    protected override void OnInitialized() => Model ??= new();

    private void Submit() => 
        Logger.LogInformation("Id = {Id} Desc = {Description} Length = {Length}",
            Model?.Id, Model?.Details?.Description, Model?.Details?.Length);
}

Initialize form data with static SSR

When a component adopts static SSR, the OnInitialized{Async} lifecycle method and the OnParametersSet{Async} lifecycle method fire when the component is initially rendered and on every form POST to the server. To initialize form model values, confirm if the model already has data before assigning new model values in OnParametersSet{Async}, as the following example demonstrates.

StarshipInit.razor:

@page "/starship-init"
@inject ILogger<StarshipInit> Logger

<EditForm Model="Model" OnValidSubmit="Submit" FormName="StarshipInit">
    <div>
        <label>
            Identifier:
            <InputText @bind-Value="Model!.Id" />
        </label>
    </div>
    <div>
        <button type="submit">Submit</button>
    </div>
</EditForm>

@code {
    [SupplyParameterFromForm]
    private Starship? Model { get; set; }

    protected override void OnInitialized() => Model ??= new();

    protected override void OnParametersSet()
    {
        if (Model!.Id == default)
        {
            LoadData();
        }
    }

    private void LoadData()
    {
        Model!.Id = "Set by LoadData";
    }

    private void Submit()
    {
        Logger.LogInformation("Id = {Id}", Model?.Id);
    }

    public class Starship
    {
        public string? Id { get; set; }
    }
}

Advanced form mapping error scenarios

The framework instantiates and populates the FormMappingContext for a form, which is the context associated with a given form's mapping operation. Each mapping scope (defined by a FormMappingScope component) instantiates FormMappingContext. Each time a [SupplyParameterFromForm] asks the context for a value, the framework populates the FormMappingContext with the attempted value and any mapping errors.

Developers aren't expected to interact with FormMappingContext directly, as it's mainly a source of data for InputBase<TValue>, EditContext, and other internal implementations to show mapping errors as validation errors. In advanced custom scenarios, developers can access FormMappingContext directly as a [CascadingParameter] to write custom code that consumes the attempted values and mapping errors.

Custom input components

For custom input processing scenarios, the following subsections demonstrate custom input components:

• Input component based on InputBase<T>: The component inherits from InputBase<TValue>, which provides a base implementation for binding, callbacks, and validation. Components that inherit from InputBase<TValue> must be used in a Blazor form (EditForm).

• Input component with full developer control: The component takes full control of input processing. The component's code must manage binding, callbacks, and validation. The component can be used inside or outside of a Blazor form.

We recommend that you derive your custom input components from InputBase<TValue> unless specific requirements prevent you from doing so. The InputBase<TValue> class is actively maintained by the ASP.NET Core team, ensuring it stays up-to-date with the latest Blazor features and framework changes.

Input component based on InputBase<T>

The following example component:

• Inherits from InputBase<TValue>. Components that inherit from InputBase<TValue> must be used in a Blazor form (EditForm).
• Takes boolean input from a checkbox.
• Sets the background color of its container <div> based on the checkbox's state, which occurs when the AfterChange method executes after binding (@bind:after).
• Is required to override the base class's TryParseValueFromString method but doesn't process string input data because a checkbox doesn't provide string data. Example implementations of TryParseValueFromString for other types of input components that process string input are available in the ASP.NET Core reference source.

Note

Documentation links to .NET reference source usually load the repository's default branch, which represents the current development for the next release of .NET. To select a tag for a specific release, use the Switch branches or tags dropdown list. For more information, see How to select a version tag of ASP.NET Core source code (dotnet/AspNetCore.Docs #26205).

EngineeringApprovalInputDerived.razor:

@using System.Diagnostics.CodeAnalysis
@inherits InputBase<bool>

<div class="@divCssClass">
    <label>
        Engineering Approval:
        <input @bind="CurrentValue" @bind:after="AfterChange" class="@CssClass" 
            type="checkbox" />
    </label>
</div>

@code {
    private string? divCssClass;

    private void AfterChange()
    {
        divCssClass = CurrentValue ? "bg-success text-white" : null;
    }

    protected override bool TryParseValueFromString(
        string? value, out bool result, 
        [NotNullWhen(false)] out string? validationErrorMessage)
            => throw new NotSupportedException(
                "This component does not parse string inputs. " +
                $"Bind to the '{nameof(CurrentValue)}' property, " +
                $"not '{nameof(CurrentValueAsString)}'.");
}

To use the preceding component in the starship example form (Starship3.razor/Starship.cs), replace the <div> block for the engineering approval field with an EngineeringApprovalInputDerived component instance bound to the model's IsValidatedDesign property:

- <div>
-     <label>
-         Engineering Approval: 
-         <InputCheckbox @bind-Value="Model!.IsValidatedDesign" />
-     </label>
- </div>
+ <EngineeringApprovalInputDerived @bind-Value="Model!.IsValidatedDesign" />

If the component that inherits from InputBase<TValue> is ever statically rendered, assign the InputBase<TValue>.NameAttributeValue property to the name attribute of <input> elements:

<input @bind="CurrentValue" @bind:after="AfterChange" class="@CssClass"
    type="checkbox" name="@NameAttributeValue" />

The preceding assignment isn't necessary if the component is guaranteed to always render interactively.

Input component with full developer control

The following example component:

• Doesn't inherit from InputBase<TValue>. The component takes full control of input processing, including binding, callbacks, and validation. The component can be used inside or outside of a Blazor form (EditForm).
• Takes boolean input from a checkbox.
• Changes the background color if the checkbox is checked.

Code in the component includes:

• The Value property is used with two-way binding to get or set the value of the input. ValueChanged is the callback that updates the bound value.

• When used in a Blazor form:

  • The EditContext is a cascading value.
  • fieldCssClass styles the field based on the result of EditContext validation.
  • ValueExpression is an expression (Expression<Func<T>>) assigned by the framework that identifies the bound value.
  • FieldIdentifier uniquely identifies a single field that can be edited, usually corresponding to a model property. The field identifier is created with the expression that identifies the bound value (ValueExpression).

• In the OnChange event handler:

  • The value of the checkbox input is obtained from InputFileChangeEventArgs.
  • The background color and text color of the container <div> element are set.
  • EventCallback.InvokeAsync invokes the delegate associated with the binding and dispatches an event notification to consumers that the value has changed.
  • If the component is used in an EditForm (the EditContext property isn't null), EditContext.NotifyFieldChanged is called to trigger validation.

EngineeringApprovalInputStandalone.razor:

@using System.Globalization
@using System.Linq.Expressions

<div class="@divCssClass">
    <label>
        Engineering Approval:
        <input class="@fieldCssClass" @onchange="OnChange" type="checkbox" 
            value="@Value" />
    </label>
</div>

@code {
    private string? divCssClass;
    private FieldIdentifier fieldIdentifier;
    private string? fieldCssClass => EditContext?.FieldCssClass(fieldIdentifier);

    [CascadingParameter]
    private EditContext? EditContext { get; set; }

    [Parameter]
    public bool? Value { get; set; }

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

    [Parameter]
    public Expression<Func<bool>>? ValueExpression { get; set; }

    protected override void OnInitialized()
    {
        fieldIdentifier = FieldIdentifier.Create(ValueExpression!);
    }

    private async Task OnChange(ChangeEventArgs args)
    {
        BindConverter.TryConvertToBool(args.Value, CultureInfo.CurrentCulture, 
            out var value);

        divCssClass = value ? "bg-success text-white" : null;

        await ValueChanged.InvokeAsync(value);
        EditContext?.NotifyFieldChanged(fieldIdentifier);
    }
}

To use the preceding component in the starship example form (Starship3.razor/Starship.cs), replace the <div> block for the engineering approval field with a EngineeringApprovalInputStandalone component instance bound to the model's IsValidatedDesign property:

- <div>
-     <label>
-         Engineering Approval: 
-         <InputCheckbox @bind-Value="Model!.IsValidatedDesign" />
-     </label>
- </div>
+ <EngineeringApprovalInputStandalone @bind-Value="Model!.IsValidatedDesign" />

The EngineeringApprovalInputStandalone component is also functional outside of an EditForm:

<EngineeringApprovalInputStandalone @bind-Value="ValidDesign" />

<div>
    <b>ValidDesign:</b> @ValidDesign
</div>

@code {
    private bool ValidDesign { get; set; }
}

Radio buttons

The example in this section is based on the Starfleet Starship Database form (Starship3 component) of the Example form section of this article.

Add the following enum types to the app. Create a new file to hold them or add them to the Starship.cs file.

public class ComponentEnums
{
    public enum Manufacturer { SpaceX, NASA, ULA, VirginGalactic, Unknown }
    public enum Color { ImperialRed, SpacecruiserGreen, StarshipBlue, VoyagerOrange }
    public enum Engine { Ion, Plasma, Fusion, Warp }
}

Make the ComponentEnums class accessible to the:

• Starship model in Starship.cs (for example, using static ComponentEnums;).
• Starfleet Starship Database form (Starship3.razor) (for example, @using static ComponentEnums).

Use InputRadio<TValue> components with the InputRadioGroup<TValue> component to create a radio button group. In the following example, properties are added to the Starship model described in the Example form section of the Input components article:

[Required]
[Range(typeof(Manufacturer), nameof(Manufacturer.SpaceX), 
    nameof(Manufacturer.VirginGalactic), ErrorMessage = "Pick a manufacturer.")]
public Manufacturer Manufacturer { get; set; } = Manufacturer.Unknown;

[Required, EnumDataType(typeof(Color))]
public Color? Color { get; set; } = null;

[Required, EnumDataType(typeof(Engine))]
public Engine? Engine { get; set; } = null;

Update the Starfleet Starship Database form (Starship3 component) of the Example form section of the Input components article. Add the components to produce:

• A radio button group for the ship manufacturer.
• A nested radio button group for engine and ship color.

Note

Nested radio button groups aren't often used in forms because they can result in a disorganized layout of form controls that may confuse users. However, there are cases when they make sense in UI design, such as in the following example that pairs recommendations for two user inputs, ship engine and ship color. One engine and one color are required by the form's validation. The form's layout uses nested InputRadioGroup<TValue>s to pair engine and color recommendations. However, the user can combine any engine with any color to submit the form.

Note

Be sure to make the ComponentEnums class available to the component for the following example:

@using static ComponentEnums

<fieldset>
    <legend>Manufacturer</legend>
    <InputRadioGroup @bind-Value="Model!.Manufacturer">
        @foreach (var manufacturer in Enum.GetValues<Manufacturer>())
        {
            <div>
                <label>
                    <InputRadio Value="manufacturer" />
                    @manufacturer
                </label>
            </div>
        }
    </InputRadioGroup>
</fieldset>

<fieldset>
    <legend>Engine and Color</legend>
    <p>
        Engine and color pairs are recommended, but any
        combination of engine and color is allowed.
    </p>
    <InputRadioGroup Name="engine" @bind-Value="Model!.Engine">
        <InputRadioGroup Name="color" @bind-Value="Model!.Color">
            <div style="margin-bottom:5px">
                <div>
                    <label>
                        <InputRadio Name="engine" Value="Engine.Ion" />
                        Ion
                    </label>
                </div>
                <div>
                    <label>
                        <InputRadio Name="color" Value="Color.ImperialRed" />
                        Imperial Red
                    </label>
                </div>
            </div>
            <div style="margin-bottom:5px">
                <div>
                    <label>
                        <InputRadio Name="engine" Value="Engine.Plasma" />
                        Plasma
                    </label>
                </div>
                <div>
                    <label>
                        <InputRadio Name="color" Value="Color.SpacecruiserGreen" />
                        Spacecruiser Green
                    </label>
                </div>
            </div>
            <div style="margin-bottom:5px">
                <div>
                    <label>
                        <InputRadio Name="engine" Value="Engine.Fusion" />
                        Fusion
                    </label>
                </div>
                <div>
                    <label>
                        <InputRadio Name="color" Value="Color.StarshipBlue" />
                        Starship Blue
                    </label>
                </div>
            </div>
            <div style="margin-bottom:5px">
                <div>
                    <label>
                        <InputRadio Name="engine" Value="Engine.Warp" />
                        Warp
                    </label>
                </div>
                <div>
                    <label>
                        <InputRadio Name="color" Value="Color.VoyagerOrange" />
                        Voyager Orange
                    </label>
                </div>
            </div>
        </InputRadioGroup>
    </InputRadioGroup>
</fieldset>

Note

If Name is omitted, InputRadio<TValue> components are grouped by their most recent ancestor.

If you implemented the preceding Razor markup in the Starship3 component of the Example form section of the Input components article, update the logging for the Submit method:

Logger.LogInformation("Id = {Id} Description = {Description} " +
    "Classification = {Classification} MaximumAccommodation = " +
    "{MaximumAccommodation} IsValidatedDesign = " +
    "{IsValidatedDesign} ProductionDate = {ProductionDate} " +
    "Manufacturer = {Manufacturer}, Engine = {Engine}, " +
    "Color = {Color}",
    Model?.Id, Model?.Description, Model?.Classification,
    Model?.MaximumAccommodation, Model?.IsValidatedDesign,
    Model?.ProductionDate, Model?.Manufacturer, Model?.Engine, 
    Model?.Color);