如何有效防止同一账户去重复登录系统 

public virtual void Application_Start(object sender, EventArgs e) 
{ 
// reset the mailer indicator 
Application["MailerStatus"] = "All Mailings Complete"; 

// initialize a datatable for users online 
DataTable objUserTable = new DataTable(); 
objUserTable.Columns.Add("SessionID",System.Type.GetType("System.Guid")); 
objUserTable.Columns.Add("PeopleID",System.Type.GetType("System.Int32")); 
objUserTable.Columns.Add("ShowDetail",System.Type.GetType("System.Boolean")); 
DataColumn[] pk = new DataColumn[1]; 
pk[0] = objUserTable.Columns[0]; 
objUserTable.PrimaryKey = pk; 
Application["UserTable"] = objUserTable; 
} 

/**//// 
/// The Session_Start event adds user session information to 
/// Application["UserTable"]. 
/// 
public virtual void Session_Start(object sender, EventArgs e) 
{ 
Application.Lock(); 
//Application.Lock (); 
DataTable objUserTable = (DataTable)Application["UserTable"]; 
DataRow objRow = objUserTable.NewRow(); 
Guid objGuid = Guid.NewGuid(); 
objRow[0] = objGuid; 
Session["PfSessionID"] = objRow[0]; 
objRow[1] = 0; 
objRow[2] = false; 
objUserTable.Rows.Add(objRow); 
Application["UserTable"] = objUserTable; 
Application.UnLock(); 
} 


/**//// 
/// The Session_End event deletes user session information from 
/// Application["UserTable"]. 
/// 
public virtual void Session_End(object sender, EventArgs e) 
{ 
Application.Lock(); 
DataTable objUserTable = (DataTable)Application["UserTable"]; 
objUserTable.Rows.Find((Guid)Session["PfSessionID"]).Delete(); 
Application["UserTable"] = objUserTable; 
Application.UnLock(); 
}