How do I validate the DI container in ASP.NET Core?

A built-in DI container validation was added in ASP.NET Core 3 and it is enabled only in the Development environment by default. If something is missing, the container throws a fatal exception on startup.

Keep in mind that controllers aren’t created in the DI container by default, so a typical web app won’t get much from this check until the controllers are registered in the DI:

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers()
        .AddControllersAsServices();
}

To disable/customize the validation, add a IHostBuilder.UseDefaultServiceProvider call:

public class Program
{
    public static IHostBuilder CreateHostBuilder(string[] args) =>
        Host.CreateDefaultBuilder(args)
            //...
            .UseDefaultServiceProvider((context, options) =>
            {
                options.ValidateOnBuild = false;
            });

This validation feature has several limitations, read more here: https://andrewlock.net/new-in-asp-net-core-3-service-provider-validation/