SampleAnonymousSessionMiddleware

AnonymousSessionMiddleware.cs

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Authentication;
using System.Security.Claims;

public class AnonymousSessionMiddleware
{
    private readonly RequestDelegate _next;

    public AnonymousSessionMiddleware(RequestDelegate next)
    {
        _next = next;
    }

    public async System.Threading.Tasks.Task InvokeAsync(HttpContext context)
    {
        if (!context.User.Identity.IsAuthenticated)
        {
            if (string.IsNullOrEmpty(context.User.FindFirstValue(ClaimTypes.Anonymous)))
            {
                var claim = new Claim(ClaimTypes.Anonymous, System.Guid.NewGuid().ToString());
                context.User.AddIdentity(new ClaimsIdentity(new[] { claim }));

                string scheme = Microsoft.AspNetCore.Authentication.Cookies.CookieAuthenticationDefaults.AuthenticationScheme;
                await context.SignInAsync(scheme, context.User, new AuthenticationProperties { IsPersistent = false });
            }
        }

        await _next(context);
    }
}

Startup.cs

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

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

    public IConfiguration Configuration { get; }

    // This method gets called by the runtime. Use this method to add services to the container.
    public void ConfigureServices(IServiceCollection services)
    {
        string scheme = Microsoft.AspNetCore.Authentication.Cookies.CookieAuthenticationDefaults.AuthenticationScheme;
        services.AddAuthentication(scheme)
            .AddCookie(scheme);

        services.AddMvc()
            .AddNewtonsoftJson();
    }

    // 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
        {
            // 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.UseRouting(routes =>
        {
            routes.MapControllers();
        });

        app.UseAuthorization();
        app.UseAuthentication();
        
        app.UseMiddleware<AnonymousSessionMiddleware>();
    }
}