New To C# And Trying To Use A Global Variable?

Is it possible to use global variables in C#? I'm coming from mainly a PHP background so variables are either accessible everywhere or just a global definition away.

My main issue is I have a User class that I built myself to wrap around the current users table on my company's database. I am defining it in the MasterPage but can't seem to access it from the actual pages (I don't know if there's a better word to describe them but they are the pages that inherit the styles and format from the MasterPage) Any general tips or implementation practices for me?

EDIT: here's some code snippets of what I'm trying to do:

Site.master.cs

public partial class SiteMaster : System.Web.UI.MasterPage
{
public User user = new User();
}
logout.aspx
<%@ Page Title="" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true" CodeFile="logout.aspx.cs" Inherits="logout" %>
<%@ MasterType virtualPath="~/Site.master"%>
logout.aspx.cs
public partial class _Default : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
User user = Master.user;
}
}