Startup_HttpClient_DI.cs

using HttpTracer;
using System.Net.Http;
using Microsoft.AspNetCore.Components.Browser;
using Microsoft.AspNetCore.Components.Builder;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.AspNetCore.Components;
using System;
using Microsoft.AspNetCore.Blazor.Services;
using Microsoft.AspNetCore.Blazor.Http;
using System.Threading.Tasks;

namespace WebApplication1
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.AddTransient<IMyService, MyService>();
            services.AddTransient<MyHandler>();
            services.AddSingleton<HttpClient>(s =>
            {
                // Creating the URI helper needs to wait until the JS Runtime is initialized, so defer it.
                var uriHelper = s.GetRequiredService<IUriHelper>();
                var myHandler = s.GetRequiredService<MyHandler>();
                myHandler.InnerHandler = new WebAssemblyHttpMessageHandler();
                return new HttpClient(myHandler)
                {
                    BaseAddress = new Uri(WebAssemblyUriHelper.Instance.GetBaseUri())
                };
            });
        }

        public void Configure(IComponentsApplicationBuilder app)
        {
            app.AddComponent<App>("app");
        }
    }
    
    // I know, I'm a monster.
    public class MyHandler : DelegatingHandler
    {
        private readonly IMyService _myService;
        public MyHandler(IMyService myService)
        {
            _myService = myService;
        }

        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, System.Threading.CancellationToken cancellationToken)
        {
            _myService.MyMethod();
            return base.SendAsync(request, cancellationToken);
        }
    }   

    public interface IMyService
    {
        void MyMethod();
    }

    public class MyService : IMyService
    {
        public void MyMethod() => Console.WriteLine("Hello from MyService.MyMethod");
    }
}