Options data annotations validation not working in Blazor apps

[bgillet]

Here is a sample options class.

using System.ComponentModel.DataAnnotations;

namespace SampleApp.Options
{
    public class SampleOptions
    {
        public const string ConfigSectionName = "Sample";

        [Required(AllowEmptyStrings = false)]
        [RegularExpression(pattern: @"^[0-9]{14,14}$")]
        public string Dummy { get; set; }
    }
}

Below is code used in Startup class.

using SampleApp.Options;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

namespace SampleApp
{
    public class Startup
    {
        public IConfiguration Configuration { get; }

        public Startup(IConfiguration configuration)
        {
            Configuration = configuration;
        }

        // This method gets called by the runtime. Use this method to add services to the container.
        // For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddOptions<SampleOptions>()
                    .Bind(Configuration.GetSection(SampleOptions.ConfigSectionName))
                    .ValidateDataAnnotations();

            services.AddRazorPages();
            services.AddServerSideBlazor();
            services.AddSingleton<WeatherForecastService>();
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            else
            {
                app.UseExceptionHandler("/Error");
                // The default HSTS value is 30 days. You may want to change this for production scenarios, see https://aka.ms/aspnetcore-hsts.
                app.UseHsts();
            }

            app.UseHttpsRedirection();
            app.UseStaticFiles();

            app.UseRouting();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapBlazorHub();
                endpoints.MapFallbackToPage("/_Host");
            });
        }
    }
}

When executing the code, I would expect an OptionsValidationException being thrown on application startup if I have bad option values set in my appsetting.json file.

But when I start my Blazor server application, nothing happens !!!

[davidfowl]

Options validation doesn't happen at startup but that feature has been added in .NET 6. See dotnet/runtime#36391 for more information.

[bgillet]

Ok. Thanks. But when does it happen then ? On DI ?

This limitation should be clearly explained in documentation. It seems a very important to know.

[davidfowl]

Ok. Thanks. But when does it happen then ? On DI ?

It happens on host startup as part of an IHostedService implementation.

This limitation should be clearly explained in documentation. It seems a very important to know.

Agreed, can you file an issue on this topic https://docs.microsoft.com/en-us/aspnet/core/fundamentals/configuration/options?view=aspnetcore-5.0#options-validation.