What's the difference between ApplicationServices and RequestServices in Microsoft.AspNet.Http.HttpContext?

Question

In Startup class, I can add services in ConfigureServices method. When I want to get a certain service, there are 2 properties in HttpContext instance, ApplicationServices and RequestServices. I wonder the difference between them and how to use them properly.

Edit #1: For example, in Startup class:

public void ConfigureServices(IServiceCollection services)
{
    services.AddSingleton<ICache, InProcessCache>();  // Line 1
    services.AddSingleton<ISystemClientResolver>(SystemClientResolver.CreateInstance); // Line 2
    services.AddScoped<SystemClient>(); // Line 3
    services.AddRvcBusiness(); // Line 4
    services.AddMvc(); // Line 5
}

If I don't call app.UseRequestServices() in Configure(IApplicationBuilder app) method, I can get SystemClient instance by Context.ApplicationServices.GetService(typeof(SystemClient)), but it's not per-request basis, it seems singleton and Context.RequestServices is null. Once I call app.UseRequestServices() like this:

public void Configure(IApplicationBuilder app)
{
    app.UseRequestServices();
    // ...
}

Context.RequestServices is not null and I can get SystemClient instance by Context.RequestServices.GetService(typeof(SystemClient)) and the instance is per-request basis.

Answer 1

ApplicationServices are for the lifetime of your app, while RequestServices are scoped to a particular request (HttpContext). Furthermore, RequestServices can be null. A special RequestContainer middleware is what initializes RequestServices:

app.UseRequestServices() will use the result of ConfigureServices as RequestServices

app.UseServices() and its overloads let you specify/modify the request services

Also if you use routing/or any security auth middleware, they will automatically ensure RequestServices are populated as well, since they rely on request scoped services themselves.