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()

Since ViewBag properties are, by definition, tied to the view presentation and any light view logic that may be necessary, I'd create a base WebViewPage and set the properties on page initialization. It's very similar to the concept of a base controller for repeated logic and common functionality, but for your views:

    public abstract class ApplicationViewPage<T> : WebViewPage<T>
    {
        protected override void InitializePage()
        {
            SetViewBagDefaultProperties();
            base.InitializePage();
        }

        private void SetViewBagDefaultProperties()
        {
            ViewBag.GlobalProperty = "MyValue";
        }
    }

And then in \Views\Web.config, set the pageBaseType property:

<system.web.webPages.razor>
    <host factoryType="System.Web.Mvc.MvcWebRazorHostFactory, System.Web.Mvc, Version=3.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" />
    <pages pageBaseType="MyNamespace.ApplicationViewPage">
      <namespaces>
        <add namespace="System.Web.Mvc" />
        <add namespace="System.Web.Mvc.Ajax" />
        <add namespace="System.Web.Mvc.Html" />
        <add namespace="System.Web.Routing" />
      </namespaces>
    </pages>
  </system.web.webPages.razor>

Un-tried by me, but you might look at registering your views and then setting the view data during the activation process.

Because views are registered on-the-fly, the registration syntax doesn't help you with connecting to the Activated event, so you'd need to set it up in a Module:

class SetViewBagItemsModule : Module
{
    protected override void AttachToComponentRegistration(
        IComponentRegistration registration,
        IComponentRegistry registry)
    {
        if (typeof(WebViewPage).IsAssignableFrom(registration.Activator.LimitType))
        {
            registration.Activated += (s, e) => {
                ((WebViewPage)e.Instance).ViewBag.Global = "global";
            };
        }
    }
}

This might be one of those "only tool's a hammer"-type suggestions from me; there may be simpler MVC-enabled ways to get at it.

Edit: Alternate, less code approach - just attach to the Controller

public class SetViewBagItemsModule: Module
{
    protected override void AttachToComponentRegistration(IComponentRegistry cr,
                                                      IComponentRegistration reg)
    {
        Type limitType = reg.Activator.LimitType;
        if (typeof(Controller).IsAssignableFrom(limitType))
        {
            registration.Activated += (s, e) =>
            {
                dynamic viewBag = ((Controller)e.Instance).ViewBag;
                viewBag.Config = e.Context.Resolve<Config>();
                viewBag.Identity = e.Context.Resolve<IIdentity>();
            };
        }
    }
}

Edit 2: Another approach that works directly from the controller registration code:

builder.RegisterControllers(asm)
    .OnActivated(e => {
        dynamic viewBag = ((Controller)e.Instance).ViewBag;
        viewBag.Config = e.Context.Resolve<Config>();
        viewBag.Identity = e.Context.Resolve<IIdentity>();
    });