//Program.cs
public class Program {
    public static User user = null;
    public static void main() {
        //stuff
    }
}

//User.cs
public class User {
    //whatever you have in here
    public String aMethodInUser() {
        //stuff
    }
    public static int totalUsers() {
        //return total users (static method)
    }
}

//ANY other file can now access user
public class MyOtherClass {
    public int doSomethingCool() {
        //you can then access your user object like this;
        System.out.println(Program.user.name);
        doSomethingElse(Program.user);
        Program.user.aMethodInUser();
        //and if User has any static methods, you don't need a reference
        //to any particular instance, and can do things like;
        System.out.println(User.totalUsers());
    }
}