AddMediatR.cs

public void ConfigureServices(IServiceCollection services)
{
    services.Configure<AppSettings>(Configuration.GetSection("AppSettings"));

    // Add EF services to the services container
    if (_platform.UseInMemoryStore)
    {
        services.AddDbContext<MusicStoreContext>(options =>
            options.UseInMemoryDatabase());
    }
    else
    {
        services.AddDbContext<MusicStoreContext>(options =>
            options.UseSqlServer(Configuration[StoreConfig.ConnectionStringKey.Replace("__", ":")]));
    }

    // Add Identity services to the services container
    services.AddIdentity<ApplicationUser, IdentityRole>(options =>
            {
                options.Cookies.ApplicationCookie.AccessDeniedPath = "/Home/AccessDenied";
            })
            .AddEntityFrameworkStores<MusicStoreContext>()
            .AddDefaultTokenProviders();

    services.AddMediatR(typeof(Startup).GetTypeInfo().Assembly);