PM> Install-Package DNTCommon.Web.Core

using System.Security.Claims;
using Microsoft.AspNetCore.Authorization; // dotnet add package Microsoft.AspNetCore.Authorization

namespace BlazorServer.Common
{
    public static class PolicyTypes
    {
        public const string RequireAdmin = nameof(RequireAdmin);
        public const string RequireCustomer = nameof(RequireCustomer);
        public const string RequireEmployee = nameof(RequireEmployee);
        public const string RequireEmployeeOrCustomer = nameof(RequireEmployeeOrCustomer);

        public static AuthorizationOptions AddAppPolicies(this AuthorizationOptions options)
        {
            options.AddPolicy(RequireAdmin, policy => policy.RequireRole(ConstantRoles.Admin));
            options.AddPolicy(RequireCustomer, policy =>
                    policy.RequireAssertion(context =>
                        context.User.HasClaim(claim => claim.Type == ClaimTypes.Role
                            && (claim.Value == ConstantRoles.Admin || claim.Value == ConstantRoles.Customer))
                    ));
            options.AddPolicy(RequireEmployee, policy =>
                    policy.RequireAssertion(context =>
                        context.User.HasClaim(claim => claim.Type == ClaimTypes.Role
                            && (claim.Value == ConstantRoles.Admin || claim.Value == ConstantRoles.Employee))
                    ));

            options.AddPolicy(RequireEmployeeOrCustomer, policy =>
                                policy.RequireAssertion(context =>
                                    context.User.HasClaim(claim => claim.Type == ClaimTypes.Role
                                        && (claim.Value == ConstantRoles.Admin ||
                                            claim.Value == ConstantRoles.Employee ||
                                            claim.Value == ConstantRoles.Customer))
                                ));
            return options;
        }
    }
}

namespace BlazorWasm.WebApi
{
    public class Startup
    {
        // ...

        public void ConfigureServices(IServiceCollection services)
        {
            // ...

            services.AddAuthorization(options => options.AddAppPolicies());

            // ...

namespace BlazorWasm.Client
{
    public class Program
    {
        public static async Task Main(string[] args)
        {
            // ...

            builder.Services.AddAuthorizationCore(options => options.AddAppPolicies());

            // ...
        }
    }
}

@page "/hotel-room-details/{Id:int}"

// ...

@*
@attribute [Roles(ConstantRoles.Customer, ConstantRoles.Employee)]
*@

@attribute [Authorize(Policy = PolicyTypes.RequireEmployeeOrCustomer)]

<AuthorizeView  Policy="@PolicyTypes.RequireEmployeeOrCustomer">
    <p>You can only see this if you're an admin or an employee or a customer.</p>
</AuthorizeView>

using System.ComponentModel.DataAnnotations;

namespace BlazorAsyncValidation.Shared;

public class UserDto
{
    [Required] public string Name { set; get; } = default!;
}

using BlazorAsyncValidation.Shared;
using Microsoft.AspNetCore.Mvc;

namespace BlazorAsyncValidation.Server.Controllers;

[ApiController, Route("api/[controller]/[action]")]
public class RegisterController : ControllerBase
{
    [HttpPost]
    public IActionResult IsUserNameUnique([FromBody] UserDto userModel)
    {
        if (string.Equals(userModel?.Name, "Vahid", StringComparison.OrdinalIgnoreCase))
        {
            return Ok(false);
        }

        return Ok(true);
    }
}

@page "/"

<PageTitle>Index</PageTitle>

<h2>Register</h2>

<EditForm EditContext="@EditContext" OnValidSubmit="DoSubmitAsync">
    <DataAnnotationsValidator/>
    <div class="row mb-2">
        <label class="col-form-label col-lg-2">Name:</label>
        <div class="col-lg-10">
            <InputText @bind-Value="Model.Name" class="form-control"/>
            <ValidationMessage For="@(() => Model.Name)"/>
        </div>
    </div>

    <button class="btn btn-secondary" type="submit">Submit</button>
</EditForm>

public partial class Index
{
    private const string UserValidationUrl = "/api/Register/IsUserNameUnique";

    private ValidationMessageStore? _messageStore;
    [Inject] private HttpClient HttpClient { set; get; } = default!;
    private EditContext? EditContext { set; get; }

    private UserDto Model { get; } = new();

    protected override void OnInitialized()
    {
        EditContext = new EditContext(Model);
        _messageStore = new ValidationMessageStore(EditContext);
        EditContext.OnFieldChanged += (sender, eventArgs) =>
        {
            var fieldIdentifier = eventArgs.FieldIdentifier;
            _messageStore?.Clear(fieldIdentifier);
            _ = InvokeAsync(async () =>
            {
                var errors = await OnValidateFieldAsync(fieldIdentifier.FieldName);
                if (errors?.Any() != true)
                {
                    return;
                }

                foreach (var error in errors)
                {
                    _messageStore?.Add(fieldIdentifier, error);
                }

                EditContext.NotifyValidationStateChanged();
            });
            StateHasChanged();
        };
        EditContext.OnValidationStateChanged += (sender, eventArgs) => StateHasChanged();
        EditContext.OnValidationRequested += (sender, eventArgs) => _messageStore?.Clear();
    }

    private async Task<IList<string>?> OnValidateFieldAsync(string fieldName)
    {
        switch (fieldName)
        {
            case nameof(UserDto.Name):
                var response = await HttpClient.PostAsJsonAsync(
                    UserValidationUrl,
                    new UserDto { Name = Model.Name });
                var responseContent = await response.Content.ReadAsStringAsync();
                if (string.Equals(responseContent, "false", StringComparison.OrdinalIgnoreCase))
                {
                    return new List<string>
                    {
                        $"`{Model.Name}` is in use. Please choose another name."
                    };
                }

                // TIP: It's better to use the `DntDebounceInputText` component for this case to reduce the network round-trips.

                break;
        }

        return null;
    }