string UserName = System.Web.HttpContext.Current.User.Identity.Name

public interface IAuditable{
    DateTime CreateDate{get;set;}
    string UserName{get;set;}
}

public class MyEntity:IAuditable{
    public int ID{get;set;}
    public string Name{get;set;}
    public string Information{get;set;}
}

public static class Auditor{
    public static IAuditable Stamp(IAuditable model){
        model.CreateDate=DateTime.UtcNow;
        model.CreatedBy=System.Web.HttpContext.Current.User.Identity.Name;
        return model;
    }
}

public sealed class MyService:IDisposable{
    MyDb db=new MyDb();
    public async Task<int> Create(MyEntity model){
        Auditor.Stamp(model);
        db.MyEntities.Add(model);
        return await db.SaveAsync();
    }
    public void Dispose(){
        db.Dispose();
    }
}

[HttpPost]
[ValidateAntiForgeryToken]
public async Task<IActionResult> Create(MyEntity model)
{
    await service.Create(model);
    return RedirectToAction("Index")
}

public class UserResolverService
{
    private readonly IHttpContextAccessor _context;
    public UserResolverService(IHttpContextAccessor context)
    {
        _context = context;
    }

    public string GetUser()
    {
       return await _context.HttpContext.User?.Identity?.Name;
    }
}

public interface IAuditor {
    IAuditable Stamp(IAuditable model);
}

public class Auditor : IAuditor {
    private readonly IHttpContextAccessor accessor;

    public Auditor(IHttpContextAccessor accessor) {
        this.accessor = accessor;
    }

    public IAuditable Stamp(IAuditable model){
        model.CreateDate = DateTime.UtcNow;
        model.CreatedBy = accessor.HttpContext.User?.Identity?.Name;
        return model;
    }
}

public sealed class MyService:IDisposable {
    MyDb db = new MyDb();
    private readonly IAuditor auditor;

    public MyService(IAuditor auditor) {
        this.auditor = auditor;
    }

    public async Task<int> Create(MyEntity model) {
        auditor.Stamp(model);
        db.MyEntities.Add(model);
        return await db.SaveAsync();
    }

    public void Dispose() {
        db.Dispose();
    }
}

public static IServiceCollection AddMyLibrary(this IServiceCollection services) {

    services.AddSingleton<IHttpContextAccessor, HttpContextAccessor>();
    services.AddSingleton<IAuditor, Auditor>();
    services.AddScoped<IMyService, MyService>();

    //...add other services as needed

    return services.
}

public void ConfigureServices(IServiceCollection services) {

    //...

    services.AddMyLibrary();

    //...
}