How can i declare global variable to use all mvc view?

You have a couple of choices, but don't think of it as a variable, think of it as state. When you start thinking of it as state, you can start asking questions around storage, manipulation and/or access.

You could always use session state. You can access this from any controller or view. By default, this is stored in memory and lost when a user's session dies and/or your application pool is reset on the server. It is tied to a single user.

Session["YourVariable"] = "Customer Name";

or

Session["Customers"] = new List<customer>();</customer>

or similar.

Better would be to store this information in a database. There are many resources for this here on the site and across the web.

Cheers.