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title author description monikerRange ms.author ms.custom ms.date uid
ASP.NET Core Blazor forms validation
guardrex
Learn how to use validation in Blazor forms.
>= aspnetcore-3.1
riande
mvc
01/12/2024
blazor/forms/validation

ASP.NET Core Blazor forms validation

[!INCLUDE]

This article explains how to use validation in Blazor forms.

[!INCLUDE]

Form validation

In basic form validation scenarios, an xref:Microsoft.AspNetCore.Components.Forms.EditForm instance can use declared xref:Microsoft.AspNetCore.Components.Forms.EditContext and xref:Microsoft.AspNetCore.Components.Forms.ValidationMessageStore instances to validate form fields. A handler for the xref:Microsoft.AspNetCore.Components.Forms.EditContext.OnValidationRequested event of the xref:Microsoft.AspNetCore.Components.Forms.EditContext executes custom validation logic. The handler's result updates the xref:Microsoft.AspNetCore.Components.Forms.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 the following component, the HandleValidationRequested handler method clears any existing validation messages by calling xref:Microsoft.AspNetCore.Components.Forms.ValidationMessageStore.Clear%2A?displayProperty=nameWithType before validating the form.

Starship8.razor:

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

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/Pages/Starship8.razor":::

:::moniker-end

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

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

<h2>Holodeck Configuration</h2>

<EditForm EditContext="editContext" OnValidSubmit="@Submit">
    <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;

    public 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 called: Processing the 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;
        }
    }
}

:::moniker-end

Data Annotations Validator component and custom validation

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

[!INCLUDE]

Blazor performs two types of validation:

  • Field validation is performed when the user tabs out of a field. During field validation, the xref:Microsoft.AspNetCore.Components.Forms.DataAnnotationsValidator component associates all reported validation results with the field.
  • Model validation is performed when the user submits the form. During model validation, the xref:Microsoft.AspNetCore.Components.Forms.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 xref:Microsoft.AspNetCore.Components.Forms.ValidationMessageStore for a form's xref:Microsoft.AspNetCore.Components.Forms.EditContext.

The Blazor framework provides the xref:Microsoft.AspNetCore.Components.Forms.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:

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 xref:Microsoft.AspNetCore.Components.Forms.DataAnnotationsValidator component. When used with server validation, any custom attributes applied to the model must be executable on the server. For more information, see xref:mvc/models/validation#alternatives-to-built-in-attributes.

Create a validator component from xref:Microsoft.AspNetCore.Components.ComponentBase:

  • The form's xref:Microsoft.AspNetCore.Components.Forms.EditContext is a cascading parameter of the component.
  • When the validator component is initialized, a new xref:Microsoft.AspNetCore.Components.Forms.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 xref:Microsoft.AspNetCore.Components.Forms.EditContext when the xref:Microsoft.AspNetCore.Components.Forms.EditContext.OnValidationRequested event is raised. All of the errors are cleared.
    • A field changes in the form when the xref:Microsoft.AspNetCore.Components.Forms.EditContext.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:

:::code language="csharp" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/CustomValidation.cs":::

Important

Specifying a namespace is required when deriving from xref:Microsoft.AspNetCore.Components.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 xref:Microsoft.AspNetCore.Components.Forms.EditContext.OnValidationRequested and xref:Microsoft.AspNetCore.Components.Forms.EditContext.OnFieldChanged in the preceding example. It isn't necessary to implement xref:System.IDisposable and unsubscribe the event delegates in this scenario. For more information, see xref:blazor/components/lifecycle#component-disposal-with-idisposable-and-iasyncdisposable.

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 is not triggered on form submission because the xref:Microsoft.AspNetCore.Components.Forms.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 xref:Microsoft.AspNetCore.Components.Forms.ValidationMessageStore and shown in the xref:Microsoft.AspNetCore.Components.Forms.EditForm's validation summary.

Starship9.razor:

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

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/Pages/Starship9.razor":::

:::moniker-end

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

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

<h1>Starfleet Starship Database</h1>

<h2>New Ship Entry Form</h2>

<EditForm Model="@Model" OnValidSubmit="@Submit">
    <CustomValidation @ref="customValidation" />
    <ValidationSummary />
    <div>
        <label>
            Primary Classification:
            <InputSelect @bind-Value="Model!.Classification">
                <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>
            Description (optional):
            <InputTextArea @bind-Value="Model!.Description" />
        </label>
    </div>
    <div>
        <button type="submit">Submit</button>
    </div>
</EditForm>

@code {
    private CustomValidation? customValidation;

    public 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");
        }
    }
}

:::moniker-end

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 xref:Microsoft.AspNetCore.Components.Forms.DataAnnotationsValidator component. When used with server validation, the attributes must be executable on the server. For more information, see xref:mvc/models/validation#alternatives-to-built-in-attributes.

Server validation with a validator component

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

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.

:::moniker-end

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

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

:::moniker-end

Server validation is supported in addition to client validation:

  • Process client validation in the form with the xref:Microsoft.AspNetCore.Components.Forms.DataAnnotationsValidator component.
  • When the form passes client validation (xref:Microsoft.AspNetCore.Components.Forms.EditForm.OnValidSubmit is called), send the xref:Microsoft.AspNetCore.Components.Forms.EditContext.Model?displayProperty=nameWithType 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:

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

  • 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.

[!INCLUDE]

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).

:::moniker-end

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

  • A hosted Blazor WebAssembly solution created from the Blazor WebAssembly project template. The approach is supported for any of the secure hosted Blazor solutions described in the hosted Blazor WebAssembly security documentation.
  • 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 the solution's Shared project so that both the client and server apps 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, add the System.ComponentModel.Annotations package to the Shared project.

[!INCLUDE]

In the :::no-loc text="Server"::: project, add a controller to process starship validation requests and return failed validation messages. Update the namespaces in the last using statement for the Shared 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).

:::moniker-end

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:

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 : ControllerBase
{
    private readonly ILogger<StarshipValidationController> logger;

    public StarshipValidationController(
        ILogger<StarshipValidationController> logger)
    {
        this.logger = logger;
    }

    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 (xref:Microsoft.AspNetCore.Mvc.ApiControllerAttribute) normally returns a default bad request response with a xref:Microsoft.AspNetCore.Mvc.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 or Postman.

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 xref:System.Net.Http.Json.HttpContentJsonExtensions.ReadFromJsonAsync%2A. 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 xref:Microsoft.AspNetCore.Mvc.ApiControllerAttribute in the Program file. For the API endpoint (/StarshipValidation), return a xref:Microsoft.AspNetCore.Mvc.BadRequestObjectResult with the xref:Microsoft.AspNetCore.Mvc.ModelBinding.ModelStateDictionary. For any other API endpoints, preserve the default behavior by returning the object result with a new xref:Microsoft.AspNetCore.Mvc.ValidationProblemDetails.

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

Add the xref:Microsoft.AspNetCore.Mvc?displayProperty=fullName 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 xref:Microsoft.Extensions.DependencyInjection.MvcServiceCollectionExtensions.AddControllersWithViews%2A extension method and add the following call to xref:Microsoft.Extensions.DependencyInjection.MvcCoreMvcBuilderExtensions.ConfigureApiBehaviorOptions%2A:

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 xref:Microsoft.Extensions.DependencyInjection.MvcServiceCollectionExtensions.AddControllersWithViews%2A to automatically mitigate Cross-Site Request Forgery (XSRF/CSRF) attacks. If you merely use xref:Microsoft.Extensions.DependencyInjection.MvcServiceCollectionExtensions.AddControllers%2A, anti-forgery is not enabled automatically.

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

  • xref:mvc/controllers/routing
  • xref:web-api/handle-errors#validation-failure-error-response

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 xref:Microsoft.AspNetCore.Components.Forms.ValidationMessageStore. The errors are available in the form's xref:Microsoft.AspNetCore.Components.Forms.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.

:::moniker-end

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

Add the xref:Microsoft.AspNetCore.Mvc?displayProperty=fullName namespace to the top of the Program file in the :::no-loc text="Server"::: app:

using Microsoft.AspNetCore.Mvc;

In the Program file, locate the xref:Microsoft.Extensions.DependencyInjection.MvcServiceCollectionExtensions.AddControllersWithViews%2A extension method and add the following call to xref:Microsoft.Extensions.DependencyInjection.MvcCoreMvcBuilderExtensions.ConfigureApiBehaviorOptions%2A:

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));
            }
        };
    });

Note

The preceding example explicitly registers controller services by calling xref:Microsoft.Extensions.DependencyInjection.MvcServiceCollectionExtensions.AddControllersWithViews%2A to automatically mitigate Cross-Site Request Forgery (XSRF/CSRF) attacks. If you merely use xref:Microsoft.Extensions.DependencyInjection.MvcServiceCollectionExtensions.AddControllers%2A, anti-forgery is not enabled automatically.

In the :::no-loc text="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 :::no-loc text="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 xref:Microsoft.AspNetCore.Components.Forms.ValidationMessageStore. The errors are available in the form's xref:Microsoft.AspNetCore.Components.Forms.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.

:::moniker-end

Starship10.razor:

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

Note

By default, forms based on xref:Microsoft.AspNetCore.Components.Forms.EditForm automatically enable anti-forgery support. The controller should use xref:Microsoft.Extensions.DependencyInjection.MvcServiceCollectionExtensions.AddControllersWithViews%2A to register controller services and automatically enable anti-forgery 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 Model="@Model" OnValidSubmit="@Submit" FormName="Starship10">
    <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]
    public 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 xref:System.Net.Http.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) });

:::moniker-end

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

@page "/starship-10"
@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 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";
    
    public 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.";
        }
    }
}

:::moniker-end

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 xref:Microsoft.AspNetCore.Components.Forms.DataAnnotationsValidator component. When used with server validation, the attributes must be executable on the server. For more information, see xref:mvc/models/validation#alternatives-to-built-in-attributes.

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

Note

The server validation approach in this section is suitable for any of the hosted Blazor WebAssembly solution examples in this documentation set:

  • Microsoft Entra ID (ME-ID)
  • Azure Active Directory (AAD) B2C
  • Identity Server

:::moniker-end

InputText based on the input event

Use the xref:Microsoft.AspNetCore.Components.Forms.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:

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

The CustomInputText component can be used anywhere xref:Microsoft.AspNetCore.Components.Forms.InputText is used. The following component uses the shared CustomInputText component.

Starship11.razor:

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

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/Pages/Starship11.razor":::

:::moniker-end

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

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

<EditForm Model="@Model" OnValidSubmit="@Submit">
    <DataAnnotationsValidator />
    <ValidationSummary />
    <CustomInputText @bind-Value="Model!.Id" />
    <button type="submit">Submit</button>
</EditForm>

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

@code {
    public Starship? Model { get; set; }

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

    private void Submit()
    {
        Logger.LogInformation("Submit called: Processing the form");
    }

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

:::moniker-end

Validation Summary and Validation Message components

The xref:Microsoft.AspNetCore.Components.Forms.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 xref:Microsoft.AspNetCore.Components.Forms.ValidationMessage%601 component displays validation messages for a specific field, which is similar to the Validation Message Tag Helper. Specify the field for validation with the xref:Microsoft.AspNetCore.Components.Forms.ValidationMessage%601.For%2A attribute and a lambda expression naming the model property:

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

The xref:Microsoft.AspNetCore.Components.Forms.ValidationMessage%601 and xref:Microsoft.AspNetCore.Components.Forms.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;
}

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

Determine if a form field is valid

Use EditContext.IsValid(fieldIdentifier) 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);

:::moniker-end

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 xref:System.ComponentModel.DataAnnotations.ValidationContext.MemberName when creating the xref:System.ComponentModel.DataAnnotations.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 xref:System.ComponentModel.DataAnnotations.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:

:::code language="csharp" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/SaladChef.cs":::

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:

:::code language="csharp" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/SaladChefValidatorAttribute.cs":::

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

Starship12.razor:

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

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/Pages/Starship12.razor":::

:::moniker-end

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

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

<EditForm Model="@this" autocomplete="off">
    <DataAnnotationsValidator />
    <p>
        <label>
            Salad topper (@saladToppers):
            <input @bind="SaladIngredient" />
        </label>
    </p>
    <button type="submit">Submit</button>
    <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);
}

:::moniker-end

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

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 xref:Microsoft.AspNetCore.Components.Forms.FieldCssClassProvider that checks for field validation messages and applies the appropriate valid or invalid style.

CustomFieldClassProvider.cs:

:::moniker-end

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

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";
    }
}

:::moniker-end

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

using Microsoft.AspNetCore.Components.Forms;

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

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

:::moniker-end

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

Set the CustomFieldClassProvider class as the Field CSS Class Provider on the form's xref:Microsoft.AspNetCore.Components.Forms.EditContext instance with xref:Microsoft.AspNetCore.Components.Forms.EditContextFieldClassExtensions.SetFieldCssClassProvider%2A.

Starship13.razor:

:::moniker-end

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

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/Pages/Starship13.razor":::

:::moniker-end

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

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

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

@code {
    private EditContext? editContext;

    public Starship? Model { get; set; }

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

    private void Submit()
    {
        Logger.LogInformation("Submit called: Processing the form");
    }

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

:::moniker-end

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

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:

:::moniker-end

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

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;
    }
}

:::moniker-end

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

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.GetValidationMessages(fieldIdentifier).Any();

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

        return string.Empty;
    }
}

:::moniker-end

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

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 xref:Microsoft.AspNetCore.Components.Forms.EditForm.EditContext%2A 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:

:::moniker-end

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

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";
            }
        }
    }
}

:::moniker-end

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

using Microsoft.AspNetCore.Components.Forms;

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

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

:::moniker-end

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

Update the xref:Microsoft.AspNetCore.Components.Forms.EditForm.EditContext%2A 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.

:::moniker-end

Blazor data annotations validation package

The Microsoft.AspNetCore.Components.DataAnnotations.Validation is a package that fills validation experience gaps using the xref:Microsoft.AspNetCore.Components.Forms.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.

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

[CompareProperty] attribute

The xref:System.ComponentModel.DataAnnotations.CompareAttribute doesn't work well with the xref:Microsoft.AspNetCore.Components.Forms.DataAnnotationsValidator component because it doesn't associate the validation result with a specific member. This can result in inconsistent behavior between field-level validation and when the entire model is validated on a submit. The Microsoft.AspNetCore.Components.DataAnnotations.Validation experimental package introduces an additional validation attribute, ComparePropertyAttribute, that works around these limitations. In a Blazor app, [CompareProperty] is a direct replacement for the [Compare] attribute.

:::moniker-end

Nested models, collection types, and complex types

Blazor provides support for validating form input using data annotations with the built-in xref:Microsoft.AspNetCore.Components.Forms.DataAnnotationsValidator. However, the xref:Microsoft.AspNetCore.Components.Forms.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 xref:Microsoft.AspNetCore.Components.Forms.EditContext to assign the model when the component is initialized.
  • Validates the form in the context's xref:Microsoft.AspNetCore.Components.Forms.EditContext.OnFieldChanged callback to enable and disable the submit button.
  • Implements xref:System.IDisposable and unsubscribes the event handler in the Dispose method. For more information, see xref:blazor/components/lifecycle#component-disposal-with-idisposable-and-iasyncdisposable.

Note

When assigning to the xref:Microsoft.AspNetCore.Components.Forms.EditForm.EditContext?displayProperty=nameWithType, don't also assign an xref:Microsoft.AspNetCore.Components.Forms.EditForm.Model?displayProperty=nameWithType to the xref:Microsoft.AspNetCore.Components.Forms.EditForm.

Starship14.razor:

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

:::code language="razor" source="~/../blazor-samples/8.0/BlazorSample_BlazorWebApp/Components/Pages/Starship14.razor":::

:::moniker-end

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

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

<EditForm EditContext="@editContext" OnValidSubmit="@Submit">
    <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;

    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 called: Processing the form");
    }

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

:::moniker-end

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 (xref:Microsoft.AspNetCore.Components.Forms.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 xref:Microsoft.AspNetCore.Components.Forms.ValidationSummary component on the form.
  • Make the xref:Microsoft.AspNetCore.Components.Forms.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";
    }
}