如何从 C# 代码正确使用 Scoped 服务

Question

im using blazor server side and are therefore trying to use AddScoped instead of AddSingleton as the object is used on a per-user basis. I try to split the razor pages and the c# code as much as posible as i find this to be cleaner.

I add a scoped service using

Services.AddScoped<Services.ManageUserService>();

in the ConfigureServices function of the Startup class.

my problem now is to properly access the service from any .cs file (holding the logic of my .razor pages)

I have tried to do an injection like this:

[Inject]
public Services.ManageUserService manageUserService { get; set; }

and then accesed the scoped object using (username for example):

manageUserService.User

and this works. My problem is that if I add a print that is supose to only run once within the scoped service it runs every time the page is reloaded or changed.

for example, lets say i do this:

public class ManageUserService
{
    public string User { get; private set; }

    private bool LoadedStartup = false;

    public ManageUserService() => SetupUser();

    private void SetupUser()
    {
        if (!LoadedStartup)
        {
            User = "me";
            LoadedStartup = true;
            System.Diagnostics.Debug.Print("User:" + User);
        }
    }
}

I then access the class from multiply .cs files using:

[Inject]
public Services.ManageUserService manageUserService { get; set; }

The print "User:me" is supposed to only happen once as the locking bool LoadedStartup is changed, problem is that I get the print every time the Inject is happening (on change page, etc)

What am I doing wrong? aren't the AddScoped() suppose to add a "singelton" instance for every client? am I accessing it wrongly?

I can't find any examples of using AddScoped from separated .cs and .razor pages, only directly from the .razor page, and then it is done using @inject .

Answer 1

I was in the same situation:

1.- Add the scoped services:

Services.AddScoped<Services.ManageUserService>();

2.- Then, in order to have really the scoped instances once per user, in _Hosts.cshtml:

<app>
 <component type="typeof(App)" render-mode="Server" />
</app>

3.- Now the trick I found by myself, instance the scoped services in App.razor

@inject Examples.ViewModels.MainViewModel Main;
@inject Examples.ViewModels.ChildViewModel Child;
@inject Examples.ViewModels.LayoutViewModel Layout;
@inject Examples.ViewModels.TreeViewModel Tree;
@{

Child.Main = Main;
Tree.LayoutViewModel = Layout;
}

4.- And if you have in the constructor something like:

public class MainViewModel
{
  public static MainViewModel Instance;
  public MainViewModel()
  {
  Instance = this;
 }

}

You can access to any class you define as service from anywhere in your code.

 MainViewModel.Instance...

I post about it at my blog: https://expediteapps.net/2020/02/18/scoped-viewmodels-instanced-once-on-start/