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ASP.NET Core MVC - Partial view only renders on one page

I have a partial view for ViewBag.count, which is defined in the ShoppingCartController. The problem is that the ViewBag will only show when you are on the ShoppingCart View. I want the ViewBag to be seen on all views. How do I fix this? I am currently rendering the partial like this:
#Html.Partial("_ShoppingCart", new List<bytme.Models.ShoppingCartModel>())
The partial view called _ShoppingCart:
<span class="badge">#ViewBag.count</span>

You should create a seperate action method which returns HTML markup needed to render the cart section of your page and include that in all your view using Html.Action method.
You may also decorate this action method with ChildActionOnly attribute so that users's cannot directly access this action method by requesting the url /ShoppingCart/Cart.
[ChildActionOnly]
public ActionResult Cart()
{
ViewBag.ItemCount = 2; // replace hard coded value with your actual value
return PartialView();
}
and in your partial view (~/Views/Shared/Cart.cshtml), you may write the HTML code which is needed for the cart segment of the page.
<span class="mycart">
Total items in cart #ViewBag.ItemCount
</span>
Here we are using ViewBag to pass the item count numeric value from the action method to it's partial view. But you may use a view model and use the strongly typed view approach to pass data from your action method to the partial view (this is my preferred approach).
Now in other views/layout file where you want to render the cart HTML, you can call the Html.Action method
<div>
#Html.Action("Cart","ShoppingCart")
</div>
<h1>Welcome to my site</h1>
When razor execute your view, it will see this Html.Action method and that will be executed and the output of that (the HTML markup generated fro the action method), will be included in the final output generated for the current view.
I am using the PartialView method, so that it will not try to execute the Layout code. (People make this mistake and gets an infinite calls to the Cart action method.
For Asp.Net Core projects
If you want to do the same thing in asp.net core projects, you may use View components to achieve the same results.
Create a view component to render the cart.
public class CartViewComponent : ViewComponent
{
public IViewComponentResult Invoke(string name)
{
var totalItemCount = 3;
return View(totalItemCount);
}
}
Create a razor view for this view component with the name Default.cshtml inside ~/Views/Shared/Components/Cart directory and you can have your razor code/HTML markup inside that to render the desired HTML. In this example, I am using a strongly typed approach where my view is stongly typed to int type and I am passing an int value from the the Invoke method when calling the View method.
#model int
<span>
Total items : #Model
</span>
Now you can invoke this view component in other views/ layout file by calling the Component.InvokeAsync method.
<div>
#await Component.InvokeAsync("Cart")
</div>
<h1>Welcome to my site</h1>

Can I Display User Specific Data throughout the site without adding to each view model

I have a site (MVC5) with a partial that is a header. This header displays the users name, and a logo of the organisation that they represent.
Each page also has a ViewModel of page specific data.
Is there any way I can have this Partial rendered on each page from a common model / object behind the scenes, or do I need to add my 'userheader' viewmodel to the viewmodel on each page?

You can get your requirement done through ChildActionOnly, lets say -
[ChildActionOnly]
public ActionResult LoggedIn()
{
// create your User View Model and pass it to Login Partial View
return PartialView("_LoginPartial", user);
}
Now create a Partial View with a stringly typed model what you are returning from the controller action above.
And in your Layout you can get the partial view like shown below -
#Html.Action("LoggedIn", "ControllerName")
In this way there is no need for you to include the same models across different views.

Getting the Controller and Action used to call

In a Partial View _MyView.cshtml I need to get the Controller name and the Action used to create the parent calling view.
So if /Equity/Sell was the Controller/Action used to call the view which rendered the partial view _MyView.cshtml , then I need the value /Equity/Sell.
I don't see an object that can do that though.

In the view use:
ViewContext.RouteData.GetRequiredString("controller");
ViewContext.RouteData.GetRequiredString("action");
For posterity in a child action it is like this:
ControllerContext.ParentActionViewContext.RouteData.Values["action"];
ControllerContext.ParentActionViewContext.RouteData.Values["controller"];

How can 2 controllers communicate with 1 view in asp.net mvc

I would like one view to have access in 2 controllers. What I mean
I have Controller A and Controller B
and on the view i have 2 divs: Div A with content info from Controller A and Div b with content info from Controller B. Is it possible? Can someone give me a hint on how or show me a simple example?

You'll need to look at partial views: http://www.dotnetfunda.com/articles/article1831-how-to-create-partial-views-mvc-tutorial-number-10.aspx
Also you can place your view file into Shared folder and reference it by a name: return View("commonView");

you can achieve this by using partial views.
Create a different partial views to show content from 2ND controllers render it on first view that has already data from 1st controller.
you can render partial view using following razor syntax:
#Html.Partial("name of partial view").

Html.Partial in Razor View Engine

I have a simple PartialView setup in my MVC3 Project using the Razor View Engine.
The Partial will render but if I set a breakpoint in the controller on the Action for the Partial, it never gets hit. If I change the URL to go directly to the PartialView, i.e. http://localhost:13965/Home/GridControl, then the breakpoint is hit. What am I missing?
My view:
#model MyModel
#Html.Partial("GridControl", Model)
My Controller:
public ActionResult GridControl()
{
return PartialView();
}

Html.Partial doesn't call a controller action. It is a simple include of a partial view at the place you called it.
If you want to call the controller action you need to use the Html.Action or Html.RenderAction helper like this:
#Html.Action("GridControl")
or:
#{Html.RenderAction("GridControl");}
And obviously in this case you are not passing any model as your controller action doesn't expect any model as argument and it is its responsibility to fetch a model and pass it to the partial view that will be rendered and included at the place where you called this helper.