c# - How to do DI in asp.net core middleware?

Question

I am trying to inject dependency into my middleware constructor as follows

public class CreateCompanyMiddleware
{
    private readonly RequestDelegate _next;
    private readonly UserManager<ApplicationUser> _userManager;

    public CreateCompanyMiddleware(RequestDelegate next
        , UserManager<ApplicationUser> userManager
        )
    {
        _next = next;
    }

    public async Task Invoke(HttpContext context)
    {
        await _next.Invoke(context);
    }
}

My Startup.cs file looks like

public void ConfigureServices(IServiceCollection services)
{
    services.AddDbContext<ApplicationDbContext>(options =>
        options.UseMySql(Configuration.GetConnectionString("IdentityConnection")));

    services.AddIdentity<ApplicationUser, IdentityRole>()
        .AddEntityFrameworkStores<ApplicationDbContext>()
        .AddDefaultTokenProviders();
    ...

    app.UseMiddleware<CreateCompanyMiddleware>();

    ...

But I am getting this error

An error occurred while starting the application. InvalidOperationException: Cannot resolve scoped service 'Microsoft.AspNetCore.Identity.UserManager`1[Common.Models.ApplicationUser]' from root provider. Microsoft.Extensions.DependencyInjection.ServiceLookup.CallSiteValidator.ValidateResolution(Type serviceType, IServiceScope scope, IServiceScope rootScope)

Answer 1

UserManager<ApplicationUser> is (by default) registered as a scoped dependency, whereas your CreateCompanyMiddleware middleware is constructed at app startup (effectively making it a singleton). This is a fairly standard error saying that you can't take a scoped dependency into a singleton class.

The fix is simple in this case - you can inject the UserManager<ApplicationUser> into your Invoke method:

public async Task Invoke(HttpContext context, UserManager<ApplicationUser> userManager)
{
    await _next.Invoke(context);
}

This is documented in ASP.NET Core Middleware: Per-request middleware dependencies:

Because middleware is constructed at app startup, not per-request, scoped lifetime services used by middleware constructors aren't shared with other dependency-injected types during each request. If you must share a scoped service between your middleware and other types, add these services to the Invoke method's signature. The Invoke method can accept additional parameters that are populated by DI: