How to set ViewBag properties for all Views without using a base class for Controllers?

The best way is using the ActionFilterAttribute. I’ll show you how to use it in .Net Core and .Net Framework.

.Net Core 2.1 & 3.1

public class ViewBagActionFilter : ActionFilterAttribute
{

    public ViewBagActionFilter(IOptions<Settings> settings){
        //DI will inject what you need here
    }

    public override void OnResultExecuting(ResultExecutingContext context)
    {
        // for razor pages
        if (context.Controller is PageModel)
        {
            var controller = context.Controller as PageModel;
            controller.ViewData.Add("Avatar", $"~/avatar/empty.png");
            // or
            controller.ViewBag.Avatar = $"~/avatar/empty.png";

            //also you have access to the httpcontext & route in controller.HttpContext & controller.RouteData
        }

        // for Razor Views
        if (context.Controller is Controller)
        {
            var controller = context.Controller as Controller;
            controller.ViewData.Add("Avatar", $"~/avatar/empty.png");
            // or
            controller.ViewBag.Avatar = $"~/avatar/empty.png";

            //also you have access to the httpcontext & route in controller.HttpContext & controller.RouteData
        }

        base.OnResultExecuting(context);
    }
}

Then you need to register this in your startup.cs.

.Net Core 3.1

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllersWithViews(options => { 
        options.Filters.Add<Components.ViewBagActionFilter>();
    });
}

.Net Core 2.1

public void ConfigureServices(IServiceCollection services)
{
    services.AddMvc(options =>
        {
            options.Filters.Add<Configs.ViewBagActionFilter>();
        });
}

Then you can use it in all views and pages

@ViewData["Avatar"]
@ViewBag.Avatar

---

.Net Framework (ASP.NET MVC .Net Framework)

public class UserProfilePictureActionFilter : ActionFilterAttribute
{

    public override void OnResultExecuting(ResultExecutingContext filterContext)
    {
        filterContext.Controller.ViewBag.IsAuthenticated = MembershipService.IsAuthenticated;
        filterContext.Controller.ViewBag.IsAdmin = MembershipService.IsAdmin;

        var userProfile = MembershipService.GetCurrentUserProfile();
        if (userProfile != null)
        {
            filterContext.Controller.ViewBag.Avatar = userProfile.Picture;
        }
    }

}

register your custom class in the global. asax (Application_Start)

protected void Application_Start()
    {
        AreaRegistration.RegisterAllAreas();

        GlobalFilters.Filters.Add(new UserProfilePictureActionFilter(), 0);

    }

Then you can use it in all views

@ViewBag.IsAdmin
@ViewBag.IsAuthenticated
@ViewBag.Avatar

---

Also there is another way

Creating an extension method on HtmlHelper

[Extension()]
public string MyTest(System.Web.Mvc.HtmlHelper htmlHelper)
{
    return "This is a test";
}

Then you can use it in all views

@Html.MyTest()