ExecuteScalar: Connection 属性尚未初始化

我用三层写了个校程序，可是运行即报错，我调了半天都找不到点上，各位大人们帮帮忙，先行谢过……
部分代码：
(DAL层)class DBHelp的内容：
private static SqlConnection connection;
public static SqlConnection Connection
{
get
{
string connectionString = ConfigurationManager.ConnectionStrings["人员管理.Properties.Settings.OperationManagerConnectionString"].ConnectionString;
if (connection == null)
{
connection = new SqlConnection(connectionString );
connection.Open();
}
else if (connection.State == System.Data.ConnectionState.Closed)
{
connection.Open();
}
else if (connection.State == System.Data.ConnectionState.Broken)
{
connection.Close();
connection.Open();
}
return connection;
}
}
public static string GetScalar(string safeSql)
{
SqlCommand cmd = new SqlCommand(safeSql,connection);
string result = (cmd.ExecuteScalar()).ToString(); //报错行：ExecuteScalar: Connection 属性尚未初始化
return result;
}

class AdminService的内容：
#region 得到数据库的连接
public static readonly string strCon
= ConfigurationManager.ConnectionStrings["人员管理.Properties.Settings.OperationManagerConnectionString"].ToString();
#endregion
public static SqlConnection GetConnection()
{
SqlConnection connection = new SqlConnection(AdminService.strCon);
return connection;
}
public static string GetAdminUserPwdByName(string salesman)
{
using (SqlConnection connection = AdminService.GetConnection())
{
connection.Open();
string strSql = "select OPwd from Office where OSalesman ='" + salesman + "' ";
return DBHelp.GetScalar(strSql);
}
}

(BLL层)class AdminManager 的内容：
public static string GetAdminByName(string salesman)
{
return DAL.AdminService.GetAdminUserPwdByName(salesman);
}