How to properly use Dependency Injection in Backend in Blazor Server?

Question

I have a Blazor-service that needs a specific "LocalStorage"-service injected. It is done this way:

 public class UserApi : IUserApi
    {
        private readonly LocalStorage _localStorage;

        public UserApi(LocalStorage localStorage)
        {
            _localStorage = localStorage;
        }
...
}

"LocalStorage" is injected within the Startup.cs (ConfigureServices):

services.AddStorage();

(I think it is safe to assume that this works as it should)

I also added my own service with the following line:

services.AddSingleton<IUserApi, UserApi>();

On the razor-page I inject my Service:

@inject IUserApi UserApi

and access a function there:

  private void HandleValidSubmit()
    {
        if (UserApi.Login( _loginUser.Mail, _loginUser.Password, out string error, out Guid? guid))
        {
            // DoMagic
        }
    }

When I try to start my application I receive the following error:

System.AggregateException

Inner Exception 1:

InvalidOperationException: Error while validating the service descriptor 'ServiceType: BlazorFood.IUserApi Lifetime: Singleton ImplementationType: BlazorFood.UserApi': Cannot consume scoped service 'Cloudcrate.AspNetCore.Blazor.Browser.Storage.LocalStorage' from singleton 'BlazorFood.IUserApi'.

Inner Exception 2: InvalidOperationException: Cannot consume scoped service 'Cloudcrate.AspNetCore.Blazor.Browser.Storage.LocalStorage' from singleton 'BlazorFood.IUserApi'

When I remove the AddSingleton-line from ConfigureServices I can start my application but retrieve an error when trying to access the razor-page containing the IUserApi-Injection:

InvalidOperationException: Cannot provide a value for property 'UserApi' on type 'BlazorFood.Pages.admin.Login'. There is no registered service of type 'BlazorFood.IUserApi'

What has to be done to inject my services properly?

Nkosi Nkosi · Accepted Answer · 2020-03-24T15:36:19

This part of the error message

Cannot consume scoped service 'Cloudcrate.AspNetCore.Blazor.Browser.Storage.LocalStorage' from singleton 'BlazorFood.IUserApi'

as also indicated from the docs

It's dangerous to resolve a scoped service from a singleton. It may cause the service to have incorrect state when processing subsequent requests.

Reference Dependency injection in ASP.NET Core

Don't make your service singleton when registering it and that should fix the issue

services.AddScoped<IUserApi, UserApi>();

How to properly use Dependency Injection in Backend in Blazor Server?

1 Answers