ASP.NET Core Blazor forms validation

This article explains how to use validation in Blazor forms.

Form validation

In basic form validation scenarios, an EditForm instance can use declared EditContext and ValidationMessageStore instances to validate form fields. A handler for the OnValidationRequested event of the EditContext executes custom validation logic. The handler's result updates the ValidationMessageStore instance.

Basic form validation is useful in cases where the form's model is defined within the component hosting the form, either as members directly on the component or in a subclass. Use of a validator component is recommended where an independent model class is used across several components.

In Blazor Web Apps, client-side validation requires an active Blazor SignalR circuit. Client-side validation isn't available to forms in components that have adopted static server-side rendering (static SSR). Forms that adopt static SSR are validated on the server after the form is submitted.

In the following component, the HandleValidationRequested handler method clears any existing validation messages by calling ValidationMessageStore.Clear before validating the form.

Starship8.razor:

@page "/starship-8"
@implements IDisposable
@inject ILogger<Starship8> Logger

<h2>Holodeck Configuration</h2>

<EditForm EditContext="editContext" OnValidSubmit="Submit" FormName="Starship8">
    <div>
        <label>
            <InputCheckbox @bind-Value="Model!.Subsystem1" />
            Safety Subsystem
        </label>
    </div>
    <div>
        <label>
            <InputCheckbox @bind-Value="Model!.Subsystem2" />
            Emergency Shutdown Subsystem
        </label>
    </div>
    <div>
        <ValidationMessage For="() => Model!.Options" />
    </div>
    <div>
        <button type="submit">Update</button>
    </div>
</EditForm>

@code {
    private EditContext? editContext;

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

    private ValidationMessageStore? messageStore;

    protected override void OnInitialized()
    {
        Model ??= new();
        editContext = new(Model);
        editContext.OnValidationRequested += HandleValidationRequested;
        messageStore = new(editContext);
    }

    private void HandleValidationRequested(object? sender,
        ValidationRequestedEventArgs args)
    {
        messageStore?.Clear();

        // Custom validation logic
        if (!Model!.Options)
        {
            messageStore?.Add(() => Model.Options, "Select at least one.");
        }
    }

    private void Submit() => Logger.LogInformation("Submit: Processing form");

    public class Holodeck
    {
        public bool Subsystem1 { get; set; }
        public bool Subsystem2 { get; set; }
        public bool Options => Subsystem1 || Subsystem2;
    }

    public void Dispose()
    {
        if (editContext is not null)
        {
            editContext.OnValidationRequested -= HandleValidationRequested;
        }
    }
}

Data Annotations Validator component and custom validation

The DataAnnotationsValidator component attaches data annotations validation to a cascaded EditContext. Enabling data annotations validation requires the DataAnnotationsValidator component. To use a different validation system than data annotations, use a custom implementation instead of the DataAnnotationsValidator component. The framework implementations for DataAnnotationsValidator are available for inspection in the reference source:

• DataAnnotationsValidator
• AddDataAnnotationsValidation.

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).

Blazor performs two types of validation:

• Field validation is performed when the user tabs out of a field. During field validation, the DataAnnotationsValidator component associates all reported validation results with the field.
• Model validation is performed when the user submits the form. During model validation, the DataAnnotationsValidator component attempts to determine the field based on the member name that the validation result reports. Validation results that aren't associated with an individual member are associated with the model rather than a field.

Validator components

Validator components support form validation by managing a ValidationMessageStore for a form's EditContext.

The Blazor framework provides the DataAnnotationsValidator component to attach validation support to forms based on validation attributes (data annotations). You can create custom validator components to process validation messages for different forms on the same page or the same form at different steps of form processing (for example, client validation followed by server validation). The validator component example shown in this section, CustomValidation, is used in the following sections of this article:

• Business logic validation with a validator component
• Server validation with a validator component

Of the data annotation built-in validators, only the [Remote] validation attribute isn't supported in Blazor.

Note

Custom data annotation validation attributes can be used instead of custom validator components in many cases. Custom attributes applied to the form's model activate with the use of the DataAnnotationsValidator component. When used with server validation, any custom attributes applied to the model must be executable on the server. For more information, see the Custom validation attributes section.

Create a validator component from ComponentBase:

• The form's EditContext is a cascading parameter of the component.
• When the validator component is initialized, a new ValidationMessageStore is created to maintain a current list of form errors.
• The message store receives errors when developer code in the form's component calls the DisplayErrors method. The errors are passed to the DisplayErrors method in a Dictionary<string, List<string>>. In the dictionary, the key is the name of the form field that has one or more errors. The value is the error list.
• Messages are cleared when any of the following have occurred:
  • Validation is requested on the EditContext when the OnValidationRequested event is raised. All of the errors are cleared.
  • A field changes in the form when the OnFieldChanged event is raised. Only the errors for the field are cleared.
  • The ClearErrors method is called by developer code. All of the errors are cleared.

Update the namespace in the following class to match your app's namespace.

CustomValidation.cs:

using Microsoft.AspNetCore.Components;
using Microsoft.AspNetCore.Components.Forms;

namespace BlazorSample;

public class CustomValidation : ComponentBase
{
    private ValidationMessageStore? messageStore;

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

    protected override void OnInitialized()
    {
        if (CurrentEditContext is null)
        {
            throw new InvalidOperationException(
                $"{nameof(CustomValidation)} requires a cascading " +
                $"parameter of type {nameof(EditContext)}. " +
                $"For example, you can use {nameof(CustomValidation)} " +
                $"inside an {nameof(EditForm)}.");
        }

        messageStore = new(CurrentEditContext);

        CurrentEditContext.OnValidationRequested += (s, e) => 
            messageStore?.Clear();
        CurrentEditContext.OnFieldChanged += (s, e) => 
            messageStore?.Clear(e.FieldIdentifier);
    }

    public void DisplayErrors(Dictionary<string, List<string>> errors)
    {
        if (CurrentEditContext is not null)
        {
            foreach (var err in errors)
            {
                messageStore?.Add(CurrentEditContext.Field(err.Key), err.Value);
            }

            CurrentEditContext.NotifyValidationStateChanged();
        }
    }

    public void ClearErrors()
    {
        messageStore?.Clear();
        CurrentEditContext?.NotifyValidationStateChanged();
    }
}

Important

Specifying a namespace is required when deriving from ComponentBase. Failing to specify a namespace results in a build error:

Tag helpers cannot target tag name '<global namespace>.{CLASS NAME}' because it contains a ' ' character.

The {CLASS NAME} placeholder is the name of the component class. The custom validator example in this section specifies the example namespace BlazorSample.

Note

Anonymous lambda expressions are registered event handlers for OnValidationRequested and OnFieldChanged in the preceding example. It isn't necessary to implement IDisposable and unsubscribe the event delegates in this scenario. For more information, see ASP.NET Core Razor component lifecycle.

Business logic validation with a validator component

For general business logic validation, use a validator component that receives form errors in a dictionary.

Basic validation is useful in cases where the form's model is defined within the component hosting the form, either as members directly on the component or in a subclass. Use of a validator component is recommended where an independent model class is used across several components.

In the following example:

• A shortened version of the Starfleet Starship Database form (Starship3 component) of the Example form section of the Input components article is used that only accepts the starship's classification and description. Data annotation validation isn't triggered on form submission because the DataAnnotationsValidator component isn't included in the form.
• The CustomValidation component from the Validator components section of this article is used.
• The validation requires a value for the ship's description (Description) if the user selects the "Defense" ship classification (Classification).

When validation messages are set in the component, they're added to the validator's ValidationMessageStore and shown in the EditForm's validation summary.

Starship9.razor:

@page "/starship-9"
@inject ILogger<Starship9> Logger

<h1>Starfleet Starship Database</h1>

<h2>New Ship Entry Form</h2>

<EditForm Model="Model" OnValidSubmit="Submit" FormName="Starship9">
    <CustomValidation @ref="customValidation" />
    <ValidationSummary />
    <div>
        <label>
            Primary Classification:
            <InputSelect @bind-Value="Model!.Classification">
                <option value="">
                    Select classification ...
                </option>
                <option checked="@(Model!.Classification == "Exploration")" 
                    value="Exploration">
                    Exploration
                </option>
                <option checked="@(Model!.Classification == "Diplomacy")" 
                    value="Diplomacy">
                    Diplomacy
                </option>
                <option checked="@(Model!.Classification == "Defense")" 
                    value="Defense">
                    Defense
                </option>
            </InputSelect>
        </label>
    </div>
    <div>
        <label>
            Description (optional):
            <InputTextArea @bind-Value="Model!.Description" />
        </label>
    </div>
    <div>
        <button type="submit">Submit</button>
    </div>
</EditForm>

@code {
    private CustomValidation? customValidation;

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

    protected override void OnInitialized() =>
        Model ??= new() { ProductionDate = DateTime.UtcNow };

    private void Submit()
    {
        customValidation?.ClearErrors();

        var errors = new Dictionary<string, List<string>>();

        if (Model!.Classification == "Defense" &&
                string.IsNullOrEmpty(Model.Description))
        {
            errors.Add(nameof(Model.Description),
                new() { "For a 'Defense' ship classification, " +
                "'Description' is required." });
        }

        if (errors.Any())
        {
            customValidation?.DisplayErrors(errors);
        }
        else
        {
            Logger.LogInformation("Submit called: Processing the form");
        }
    }
}

Note

As an alternative to using validation components, data annotation validation attributes can be used. Custom attributes applied to the form's model activate with the use of the DataAnnotationsValidator component. When used with server validation, the attributes must be executable on the server. For more information, see the Custom validation attributes section.

Server validation with a validator component

This section is focused on Blazor Web App scenarios, but the approach for any type of app that uses server validation with web API adopts the same general approach.

Server validation is supported in addition to client validation:

• Process client validation in the form with the DataAnnotationsValidator component.
• When the form passes client validation (OnValidSubmit is called), send the EditContext.Model to a backend server API for form processing.
• Process model validation on the server.
• The server API includes both the built-in framework data annotations validation and custom validation logic supplied by the developer. If validation passes on the server, process the form and send back a success status code (200 - OK). If validation fails, return a failure status code (400 - Bad Request) and the field validation errors.
• Either disable the form on success or display the errors.

Basic validation is useful in cases where the form's model is defined within the component hosting the form, either as members directly on the component or in a subclass. Use of a validator component is recommended where an independent model class is used across several components.

The following example is based on:

• A Blazor Web App with Interactive WebAssembly components created from the Blazor Web App project template.
• The Starship model (Starship.cs) of the Example form section of the Input components article.
• The CustomValidation component shown in the Validator components section.

Place the Starship model (Starship.cs) into a shared class library project so that both the client and server projects can use the model. Add or update the namespace to match the namespace of the shared app (for example, namespace BlazorSample.Shared). Since the model requires data annotations, confirm that the shared class library uses the shared framework or add the System.ComponentModel.Annotations package to the shared project.

Note

For guidance on adding packages to .NET apps, see the articles under Install and manage packages at Package consumption workflow (NuGet documentation). Confirm correct package versions at NuGet.org.

In the main project of the Blazor Web App, add a controller to process starship validation requests and return failed validation messages. Update the namespaces in the last using statement for the shared class library project and the namespace for the controller class. In addition to client and server data annotations validation, the controller validates that a value is provided for the ship's description (Description) if the user selects the Defense ship classification (Classification).

The validation for the Defense ship classification only occurs on the server in the controller because the upcoming form doesn't perform the same validation client-side when the form is submitted to the server. Server validation without client validation is common in apps that require private business logic validation of user input on the server. For example, private information from data stored for a user might be required to validate user input. Private data obviously can't be sent to the client for client validation.

Note

The StarshipValidation controller in this section uses Microsoft Identity 2.0. The Web API only accepts tokens for users that have the "API.Access" scope for this API. Additional customization is required if the API's scope name is different from API.Access.

For more information on security, see:

• ASP.NET Core Blazor authentication and authorization (and the other articles in the Blazor Security and Identity node)
• Microsoft identity platform documentation

Controllers/StarshipValidation.cs:

using Microsoft.AspNetCore.Authorization;
using Microsoft.AspNetCore.Mvc;
using BlazorSample.Shared;

namespace BlazorSample.Server.Controllers;

[Authorize]
[ApiController]
[Route("[controller]")]
public class StarshipValidationController(
    ILogger<StarshipValidationController> logger) 
    : ControllerBase
{
    static readonly string[] scopeRequiredByApi = new[] { "API.Access" };

    [HttpPost]
    public async Task<IActionResult> Post(Starship model)
    {
        HttpContext.VerifyUserHasAnyAcceptedScope(scopeRequiredByApi);

        try
        {
            if (model.Classification == "Defense" && 
                string.IsNullOrEmpty(model.Description))
            {
                ModelState.AddModelError(nameof(model.Description),
                    "For a 'Defense' ship " +
                    "classification, 'Description' is required.");
            }
            else
            {
                logger.LogInformation("Processing the form asynchronously");

                // async ...

                return Ok(ModelState);
            }
        }
        catch (Exception ex)
        {
            logger.LogError("Validation Error: {Message}", ex.Message);
        }

        return BadRequest(ModelState);
    }
}

Confirm or update the namespace of the preceding controller (BlazorSample.Server.Controllers) to match the app's controllers' namespace.

When a model binding validation error occurs on the server, an ApiController (ApiControllerAttribute) normally returns a default bad request response with a ValidationProblemDetails. The response contains more data than just the validation errors, as shown in the following example when all of the fields of the Starfleet Starship Database form aren't submitted and the form fails validation:

{
  "title": "One or more validation errors occurred.",
  "status": 400,
  "errors": {
    "Id": ["The Id field is required."],
    "Classification": ["The Classification field is required."],
    "IsValidatedDesign": ["This form disallows unapproved ships."],
    "MaximumAccommodation": ["Accommodation invalid (1-100000)."]
  }
}

Note

To demonstrate the preceding JSON response, you must either disable the form's client validation to permit empty field form submission or use a tool to send a request directly to the server API, such as Firefox Browser Developer.

If the server API returns the preceding default JSON response, it's possible for the client to parse the response in developer code to obtain the children of the errors node for forms validation error processing. It's inconvenient to write developer code to parse the file. Parsing the JSON manually requires producing a Dictionary<string, List<string>> of errors after calling ReadFromJsonAsync. Ideally, the server API should only return the validation errors, as the following example shows:

{
  "Id": ["The Id field is required."],
  "Classification": ["The Classification field is required."],
  "IsValidatedDesign": ["This form disallows unapproved ships."],
  "MaximumAccommodation": ["Accommodation invalid (1-100000)."]
}

To modify the server API's response to make it only return the validation errors, change the delegate that's invoked on actions that are annotated with ApiControllerAttribute in the Program file. For the API endpoint (/StarshipValidation), return a BadRequestObjectResult with the ModelStateDictionary. For any other API endpoints, preserve the default behavior by returning the object result with a new ValidationProblemDetails.

Add the Microsoft.AspNetCore.Mvc namespace to the top of the Program file in the main project of the Blazor Web App:

using Microsoft.AspNetCore.Mvc;

In the Program file, add or update the following AddControllersWithViews extension method and add the following call to ConfigureApiBehaviorOptions:

builder.Services.AddControllersWithViews()
    .ConfigureApiBehaviorOptions(options =>
    {
        options.InvalidModelStateResponseFactory = context =>
        {
            if (context.HttpContext.Request.Path == "/StarshipValidation")
            {
                return new BadRequestObjectResult(context.ModelState);
            }
            else
            {
                return new BadRequestObjectResult(
                    new ValidationProblemDetails(context.ModelState));
            }
        };
    });

If you're adding controllers to the main project of the Blazor Web App for the first time, map controller endpoints when you place the preceding code that registers services for controllers. The following example uses default controller routes:

app.MapDefaultControllerRoute();

Note

The preceding example explicitly registers controller services by calling AddControllersWithViews to automatically mitigate Cross-Site Request Forgery (XSRF/CSRF) attacks. If you merely use AddControllers, antiforgery isn't enabled automatically.

For more information on controller routing and validation failure error responses, see the following resources:

• Routing to controller actions in ASP.NET Core
• Handle errors in ASP.NET Core controller-based web APIs

In the .Client project, add the CustomValidation component shown in the Validator components section. Update the namespace to match the app (for example, namespace BlazorSample.Client).

In the .Client project, the Starfleet Starship Database form is updated to show server validation errors with help of the CustomValidation component. When the server API returns validation messages, they're added to the CustomValidation component's ValidationMessageStore. The errors are available in the form's EditContext for display by the form's validation summary.

In the following component, update the namespace of the shared project (@using BlazorSample.Shared) to the shared project's namespace. Note that the form requires authorization, so the user must be signed into the app to navigate to the form.

Starship10.razor:

Note

Forms based on EditForm automatically enable antiforgery support. The controller should use AddControllersWithViews to register controller services and automatically enable antiforgery support for the web API.

@page "/starship-10"
@rendermode InteractiveWebAssembly
@using System.Net
@using System.Net.Http.Json
@using Microsoft.AspNetCore.Authorization
@using Microsoft.AspNetCore.Components.WebAssembly.Authentication
@using BlazorSample.Shared
@attribute [Authorize]
@inject HttpClient Http
@inject ILogger<Starship10> Logger

<h1>Starfleet Starship Database</h1>

<h2>New Ship Entry Form</h2>

<EditForm FormName="Starship10" Model="Model" OnValidSubmit="Submit">
    <DataAnnotationsValidator />
    <CustomValidation @ref="customValidation" />
    <ValidationSummary />
    <div>
        <label>
            Identifier: 
            <InputText @bind-Value="Model!.Id" disabled="@disabled" />
        </label>
    </div>
    <div>
        <label>
            Description (optional):
            <InputTextArea @bind-Value="Model!.Description" 
                disabled="@disabled" />
        </label>
    </div>
    <div>
        <label>
            Primary Classification:
            <InputSelect @bind-Value="Model!.Classification" disabled="@disabled">
                <option value="">Select classification ...</option>
                <option value="Exploration">Exploration</option>
                <option value="Diplomacy">Diplomacy</option>
                <option value="Defense">Defense</option>
            </InputSelect>
        </label>
    </div>
    <div>
        <label>
            Maximum Accommodation:
            <InputNumber @bind-Value="Model!.MaximumAccommodation" 
                disabled="@disabled" />
        </label>
    </div>
    <div>
        <label>
            Engineering Approval:
            <InputCheckbox @bind-Value="Model!.IsValidatedDesign" 
                disabled="@disabled" />
        </label>
    </div>
    <div>
        <label>
            Production Date:
            <InputDate @bind-Value="Model!.ProductionDate" disabled="@disabled" />
        </label>
    </div>
    <div>
        <button type="submit" disabled="@disabled">Submit</button>
    </div>
    <div style="@messageStyles">
        @message
    </div>
</EditForm>

@code {
    private CustomValidation? customValidation;
    private bool disabled;
    private string? message;
    private string messageStyles = "visibility:hidden";

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

    protected override void OnInitialized() => 
        Model ??= new() { ProductionDate = DateTime.UtcNow };

    private async Task Submit(EditContext editContext)
    {
        customValidation?.ClearErrors();

        try
        {
            var response = await Http.PostAsJsonAsync<Starship>(
                "StarshipValidation", (Starship)editContext.Model);

            var errors = await response.Content
                .ReadFromJsonAsync<Dictionary<string, List<string>>>() ?? 
                new Dictionary<string, List<string>>();

            if (response.StatusCode == HttpStatusCode.BadRequest && 
                errors.Any())
            {
                customValidation?.DisplayErrors(errors);
            }
            else if (!response.IsSuccessStatusCode)
            {
                throw new HttpRequestException(
                    $"Validation failed. Status Code: {response.StatusCode}");
            }
            else
            {
                disabled = true;
                messageStyles = "color:green";
                message = "The form has been processed.";
            }
        }
        catch (AccessTokenNotAvailableException ex)
        {
            ex.Redirect();
        }
        catch (Exception ex)
        {
            Logger.LogError("Form processing error: {Message}", ex.Message);
            disabled = true;
            messageStyles = "color:red";
            message = "There was an error processing the form.";
        }
    }
}

The .Client project of a Blazor Web App must also register an HttpClient for HTTP POST requests to a backend web API controller. Confirm or add the following to the .Client project's Program file:

builder.Services.AddScoped(sp => 
    new HttpClient { BaseAddress = new Uri(builder.HostEnvironment.BaseAddress) });

The preceding example sets the base address with builder.HostEnvironment.BaseAddress (IWebAssemblyHostEnvironment.BaseAddress), which gets the base address for the app and is typically derived from the <base> tag's href value in the host page.

Note

As an alternative to the use of a validation component, data annotation validation attributes can be used. Custom attributes applied to the form's model activate with the use of the DataAnnotationsValidator component. When used with server validation, the attributes must be executable on the server. For more information, see the Custom validation attributes section.

InputText based on the input event

Use the InputText component to create a custom component that uses the oninput event (input) instead of the onchange event (change). Use of the input event triggers field validation on each keystroke.

The following CustomInputText component inherits the framework's InputText component and sets event binding to the oninput event (input).

CustomInputText.razor:

@inherits InputText

<input @attributes="AdditionalAttributes" 
       class="@CssClass" 
       @bind="CurrentValueAsString" 
       @bind:event="oninput" />

The CustomInputText component can be used anywhere InputText is used. The following component uses the shared CustomInputText component.

Starship11.razor:

@page "/starship-11"
@using System.ComponentModel.DataAnnotations
@inject ILogger<Starship11> Logger

<EditForm Model="Model" OnValidSubmit="Submit" FormName="Starship11">
    <DataAnnotationsValidator />
    <ValidationSummary />
    <div>
        <label>
            Identifier: 
            <CustomInputText @bind-Value="Model!.Id" />
        </label>
    </div>
    <div>
        <button type="submit">Submit</button>
    </div>
</EditForm>

<div>
    CurrentValue: @Model?.Id
</div>

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

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

    private void Submit() => Logger.LogInformation("Submit: Processing form");

    public class Starship
    {
        [Required]
        [StringLength(10, ErrorMessage = "Id is too long.")]
        public string? Id { get; set; }
    }
}

Validation Summary and Validation Message components

The ValidationSummary component summarizes all validation messages, which is similar to the Validation Summary Tag Helper:

<ValidationSummary />

Output validation messages for a specific model with the Model parameter:

<ValidationSummary Model="Model" />

The ValidationMessage<TValue> component displays validation messages for a specific field, which is similar to the Validation Message Tag Helper. Specify the field for validation with the For attribute and a lambda expression naming the model property:

<ValidationMessage For="@(() => Model!.MaximumAccommodation)" />

The ValidationMessage<TValue> and ValidationSummary components support arbitrary attributes. Any attribute that doesn't match a component parameter is added to the generated <div> or <ul> element.

Control the style of validation messages in the app's stylesheet (wwwroot/css/app.css or wwwroot/css/site.css). The default validation-message class sets the text color of validation messages to red:

.validation-message {
    color: red;
}

Determine if a form field is valid

Use EditContext.IsValid to determine if a field is valid without obtaining validation messages.

❌ Supported, but not recommended:

var isValid = !editContext.GetValidationMessages(fieldIdentifier).Any();

✔️ Recommended:

var isValid = editContext.IsValid(fieldIdentifier);

Custom validation attributes

To ensure that a validation result is correctly associated with a field when using a custom validation attribute, pass the validation context's MemberName when creating the ValidationResult.

CustomValidator.cs:

using System;
using System.ComponentModel.DataAnnotations;

public class CustomValidator : ValidationAttribute
{
    protected override ValidationResult IsValid(object value, 
        ValidationContext validationContext)
    {
        ...

        return new ValidationResult("Validation message to user.",
            new[] { validationContext.MemberName });
    }
}

Inject services into custom validation attributes through the ValidationContext. The following example demonstrates a salad chef form that validates user input with dependency injection (DI).

The SaladChef class indicates the approved starship ingredient list for a Ten Forward salad.

SaladChef.cs:

namespace BlazorSample;

public class SaladChef
{
    public string[] SaladToppers = { "Horva", "Kanda Root", "Krintar", "Plomeek",
        "Syto Bean" };
}

Register SaladChef in the app's DI container in the Program file:

builder.Services.AddTransient<SaladChef>();

The IsValid method of the following SaladChefValidatorAttribute class obtains the SaladChef service from DI to check the user's input.

SaladChefValidatorAttribute.cs:

using System.ComponentModel.DataAnnotations;

namespace BlazorSample;

public class SaladChefValidatorAttribute : ValidationAttribute
{
    protected override ValidationResult? IsValid(object? value,
        ValidationContext validationContext)
    {
        var saladChef = validationContext.GetRequiredService<SaladChef>();

        if (saladChef.SaladToppers.Contains(value?.ToString()))
        {
            return ValidationResult.Success;
        }

        return new ValidationResult("Is that a Vulcan salad topper?! " +
            "The following toppers are available for a Ten Forward salad: " +
            string.Join(", ", saladChef.SaladToppers));
    }
}

The following component validates user input by applying the SaladChefValidatorAttribute ([SaladChefValidator]) to the salad ingredient string (SaladIngredient).

Starship12.razor:

@page "/starship-12"
@inject SaladChef SaladChef

<EditForm Model="this" autocomplete="off" FormName="Starship12">
    <DataAnnotationsValidator />
    <div>
        <label>
            Salad topper (@saladToppers):
            <input @bind="SaladIngredient" />
        </label>
    </div>
    <div>
        <button type="submit">Submit</button>
    </div>
    <ul>
        @foreach (var message in context.GetValidationMessages())
        {
            <li class="validation-message">@message</li>
        }
    </ul>
</EditForm>

@code {
    private string? saladToppers;

    [SaladChefValidator]
    public string? SaladIngredient { get; set; }

    protected override void OnInitialized() =>
        saladToppers ??= string.Join(", ", SaladChef.SaladToppers);
}

Custom validation CSS class attributes

Custom validation CSS class attributes are useful when integrating with CSS frameworks, such as Bootstrap.

To specify custom validation CSS class attributes, start by providing CSS styles for custom validation. In the following example, valid (validField) and invalid (invalidField) styles are specified.

Add the following CSS classes to the app's stylesheet:

.validField {
    border-color: lawngreen;
}

.invalidField {
    background-color: tomato;
}

Create a class derived from FieldCssClassProvider that checks for field validation messages and applies the appropriate valid or invalid style.

CustomFieldClassProvider.cs:

using Microsoft.AspNetCore.Components.Forms;

public class CustomFieldClassProvider : FieldCssClassProvider
{
    public override string GetFieldCssClass(EditContext editContext, 
        in FieldIdentifier fieldIdentifier)
    {
        var isValid = editContext.IsValid(fieldIdentifier);

        return isValid ? "validField" : "invalidField";
    }
}

Set the CustomFieldClassProvider class as the Field CSS Class Provider on the form's EditContext instance with SetFieldCssClassProvider.

Starship13.razor:

@page "/starship-13"
@using System.ComponentModel.DataAnnotations
@inject ILogger<Starship13> Logger

<EditForm EditContext="editContext" OnValidSubmit="Submit" FormName="Starship13">
    <DataAnnotationsValidator />
    <ValidationSummary />
    <div>
        <label>
            Identifier: 
            <InputText @bind-Value="Model!.Id" />
        </label>
    </div>
    <div>
        <button type="submit">Submit</button>
    </div>
</EditForm>

@code {
    private EditContext? editContext;

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

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

    private void Submit() => Logger.LogInformation("Submit: Processing form");

    public class Starship
    {
        [Required]
        [StringLength(10, ErrorMessage = "Id is too long.")]
        public string? Id { get; set; }
    }
}

The preceding example checks the validity of all form fields and applies a style to each field. If the form should only apply custom styles to a subset of the fields, make CustomFieldClassProvider apply styles conditionally. The following CustomFieldClassProvider2 example only applies a style to the Name field. For any fields with names not matching Name, string.Empty is returned, and no style is applied. Using reflection, the field is matched to the model member's property or field name, not an id assigned to the HTML entity.

CustomFieldClassProvider2.cs:

using Microsoft.AspNetCore.Components.Forms;

public class CustomFieldClassProvider2 : FieldCssClassProvider
{
    public override string GetFieldCssClass(EditContext editContext,
        in FieldIdentifier fieldIdentifier)
    {
        if (fieldIdentifier.FieldName == "Name")
        {
            var isValid = editContext.IsValid(fieldIdentifier);

            return isValid ? "validField" : "invalidField";
        }

        return string.Empty;
    }
}

Note

Matching the field name in the preceding example is case sensitive, so a model property member designated "Name" must match a conditional check on "Name":

• ✔️Correctly matches: fieldId.FieldName == "Name"
• ❌Fails to match: fieldId.FieldName == "name"
• ❌Fails to match: fieldId.FieldName == "NAME"
• ❌Fails to match: fieldId.FieldName == "nAmE"

Add an additional property to Model, for example:

[StringLength(10, ErrorMessage = "Description is too long.")]
public string? Description { get; set; } 

Add the Description to the CustomValidationForm component's form:

<InputText @bind-Value="Model!.Description" />

Update the EditContext instance in the component's OnInitialized method to use the new Field CSS Class Provider:

editContext?.SetFieldCssClassProvider(new CustomFieldClassProvider2());

Because a CSS validation class isn't applied to the Description field, it isn't styled. However, field validation runs normally. If more than 10 characters are provided, the validation summary indicates the error:

Description is too long.

In the following example:

• The custom CSS style is applied to the Name field.

• Any other fields apply logic similar to Blazor's default logic and using Blazor's default field CSS validation styles, modified with valid or invalid. Note that for the default styles, you don't need to add them to the app's stylesheet if the app is based on a Blazor project template. For apps not based on a Blazor project template, the default styles can be added to the app's stylesheet:

  .valid.modified:not([type=checkbox]) {
      outline: 1px solid #26b050;
  }
  
  .invalid {
      outline: 1px solid red;
  }
  

CustomFieldClassProvider3.cs:

using Microsoft.AspNetCore.Components.Forms;

public class CustomFieldClassProvider3 : FieldCssClassProvider
{
    public override string GetFieldCssClass(EditContext editContext,
        in FieldIdentifier fieldIdentifier)
    {
        var isValid = editContext.IsValid(fieldIdentifier);

        if (fieldIdentifier.FieldName == "Name")
        {
            return isValid ? "validField" : "invalidField";
        }
        else
        {
            if (editContext.IsModified(fieldIdentifier))
            {
                return isValid ? "modified valid" : "modified invalid";
            }
            else
            {
                return isValid ? "valid" : "invalid";
            }
        }
    }
}

Update the EditContext instance in the component's OnInitialized method to use the preceding Field CSS Class Provider:

editContext.SetFieldCssClassProvider(new CustomFieldClassProvider3());

Using CustomFieldClassProvider3:

• The Name field uses the app's custom validation CSS styles.
• The Description field uses logic similar to Blazor's logic and Blazor's default field CSS validation styles.

Class-level validation with IValidatableObject

Class-level validation with IValidatableObject (API documentation) is supported for Blazor form models. IValidatableObject validation only executes when the form is submitted and only if all other validation succeeds.

Blazor data annotations validation package

The Microsoft.AspNetCore.Components.DataAnnotations.Validation is a package that fills validation experience gaps using the DataAnnotationsValidator component. The package is currently experimental.

Warning

The Microsoft.AspNetCore.Components.DataAnnotations.Validation package has a latest version of release candidate at NuGet.org. Continue to use the experimental release candidate package at this time. Experimental features are provided for the purpose of exploring feature viability and may not ship in a stable version. Watch the Announcements GitHub repository, the dotnet/aspnetcore GitHub repository, or this topic section for further updates.

Nested models, collection types, and complex types

Blazor provides support for validating form input using data annotations with the built-in DataAnnotationsValidator. However, the DataAnnotationsValidator only validates top-level properties of the model bound to the form that aren't collection- or complex-type properties.

To validate the bound model's entire object graph, including collection- and complex-type properties, use the ObjectGraphDataAnnotationsValidator provided by the experimental Microsoft.AspNetCore.Components.DataAnnotations.Validation package:

<EditForm ...>
    <ObjectGraphDataAnnotationsValidator />
    ...
</EditForm>

Annotate model properties with [ValidateComplexType]. In the following model classes, the ShipDescription class contains additional data annotations to validate when the model is bound to the form:

Starship.cs:

using System;
using System.ComponentModel.DataAnnotations;

public class Starship
{
    ...

    [ValidateComplexType]
    public ShipDescription ShipDescription { get; set; } = new();

    ...
}

ShipDescription.cs:

using System;
using System.ComponentModel.DataAnnotations;

public class ShipDescription
{
    [Required]
    [StringLength(40, ErrorMessage = "Description too long (40 char).")]
    public string? ShortDescription { get; set; }

    [Required]
    [StringLength(240, ErrorMessage = "Description too long (240 char).")]
    public string? LongDescription { get; set; }
}

Enable the submit button based on form validation

To enable and disable the submit button based on form validation, the following example:

• Uses a shortened version of the earlier Starfleet Starship Database form (Starship3 component) of the Example form section of the Input components article that only accepts a value for the ship's Id. The other Starship properties receive valid default values when an instance of the Starship type is created.
• Uses the form's EditContext to assign the model when the component is initialized.
• Validates the form in the context's OnFieldChanged callback to enable and disable the submit button.
• Implements IDisposable and unsubscribes the event handler in the Dispose method. For more information, see ASP.NET Core Razor component lifecycle.

Note

When assigning to the EditForm.EditContext, don't also assign an EditForm.Model to the EditForm.

Starship14.razor:

@page "/starship-14"
@implements IDisposable
@inject ILogger<Starship14> Logger

<EditForm EditContext="editContext" OnValidSubmit="Submit" FormName="Starship14">
    <DataAnnotationsValidator />
    <ValidationSummary />
    <div>
        <label>
            Identifier:
            <InputText @bind-Value="Model!.Id" />
        </label>
    </div>
    <div>
        <button type="submit" disabled="@formInvalid">Submit</button>
    </div>
</EditForm>

@code {
    private bool formInvalid = false;
    private EditContext? editContext;

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

    protected override void OnInitialized()
    {
        Model ??=
            new()
                {
                    Id = "NCC-1701",
                    Classification = "Exploration",
                    MaximumAccommodation = 150,
                    IsValidatedDesign = true,
                    ProductionDate = new DateTime(2245, 4, 11)
                };
        editContext = new(Model);
        editContext.OnFieldChanged += HandleFieldChanged;
    }

    private void HandleFieldChanged(object? sender, FieldChangedEventArgs e)
    {
        if (editContext is not null)
        {
            formInvalid = !editContext.Validate();
            StateHasChanged();
        }
    }

    private void Submit() => Logger.LogInformation("Submit: Processing form");

    public void Dispose()
    {
        if (editContext is not null)
        {
            editContext.OnFieldChanged -= HandleFieldChanged;
        }
    }
}

If a form isn't preloaded with valid values and you wish to disable the Submit button on form load, set formInvalid to true.

A side effect of the preceding approach is that a validation summary (ValidationSummary component) is populated with invalid fields after the user interacts with any one field. Address this scenario in either of the following ways:

• Don't use a ValidationSummary component on the form.
• Make the ValidationSummary component visible when the submit button is selected (for example, in a Submit method).

<EditForm ... EditContext="editContext" OnValidSubmit="Submit" ...>
    <DataAnnotationsValidator />
    <ValidationSummary style="@displaySummary" />

    ...

    <button type="submit" disabled="@formInvalid">Submit</button>
</EditForm>

@code {
    private string displaySummary = "display:none";

    ...

    private void Submit()
    {
        displaySummary = "display:block";
    }
}