If I understand the question correctly, one simple way is to use a static class to store application wide information. For example
public static class MyAppData {
public bool LoggedIn { get; set; }
public string UserName { get; set; }
}
You can access the data simply using for example
if (!MyAppData.LoggedIn) {
...
}
...
MessageBox.Show("$"Welcome {MyAppData.UserName}");