ASP.NET / 求助：System.TypeInitializationException: “ComputerManage.OleDb

************** 异常文本 **************
System.TypeInitializationException: “ComputerManage.OleDbDAL.DbHelperOleDb”的类型初始值设定项引发异常。 ---> System.NullReferenceException: 未将对象引用设置到对象的实例。
   在 ComputerManage.OleDbDAL.DbHelperOleDb..cctor() 位置 E:\ComputerManage\ComputerManage\OleDbDAL\DbHelperOleDb.cs:行号 15
   --- 内部异常堆栈跟踪的结尾 ---
   在 ComputerManage.OleDbDAL.DbHelperOleDb.GetSingle(String SQLString, OleDbParameter[] cmdParms)
   在 ComputerManage.OleDbDAL.UserDAL.Exists(String uName) 位置 E:\ComputerManage\ComputerManage\OleDbDAL\UserDAL.cs:行号 52
   在 ComputerManage.BLL.UserBLL.Exists(String uName) 位置 E:\ComputerManage\ComputerManage\BLL\UserBLL.cs:行号 41
   在 FormUI.frmLogin.btnOK_Click(Object sender, EventArgs e) 位置 E:\ComputerManage\ComputerManage\FormUI\frmLogin.cs:行号 41
   在 System.Windows.Forms.Control.OnClick(EventArgs e)
   在 System.Windows.Forms.Button.OnClick(EventArgs e)
   在 System.Windows.Forms.Button.PerformClick()
   在 System.Windows.Forms.Form.ProcessDialogKey(Keys keyData)
   在 System.Windows.Forms.TextBoxBase.ProcessDialogKey(Keys keyData)
   在 System.Windows.Forms.Control.PreProcessMessage(Message& msg)
   在 System.Windows.Forms.Control.PreProcessControlMessageInternal(Control target, Message& msg)
   在 System.Windows.Forms.Application.ThreadContext.PreTranslateMessage(MSG& msg)


操作数据方法：
public static int Exists(string uName)
        {
            StringBuilder strSql = new StringBuilder();
            strSql.Append("select uID from tbUser");
            strSql.Append(" where uName= @uName");
            OleDbParameter[] parameters = {
     new OleDbParameter("@uName", OleDbType.VarChar,50)
    };
            parameters[0].Value = uName;
            object obj = DbHelperOleDb.GetSingle(strSql.ToString(), parameters);   //经调试，发现错误出现在此
            if (obj != null)
            {
                return int.Parse(obj.ToString());
            }
            else
            {
                return 0;
            }
        }

 DbHelperOleDb.GetSingle方法：

 public static object GetSingle(string SQLString, params OleDbParameter[] cmdParms)
        {
            using (OleDbConnection connection = new OleDbConnection(connectionString))
            {
                using (OleDbCommand cmd = new OleDbCommand())
                {
                    try
                    {
                        PrepareCommand(cmd, connection, null, SQLString, cmdParms);
                        object obj = cmd.ExecuteScalar();
                        cmd.Parameters.Clear();
                        if ((Object.Equals(obj, null)) || (Object.Equals(obj, System.DBNull.Value)))
                        {
                            return null;
                        }
                        else
                        {
                            return obj;
                        }
                    }
                    catch (System.Data.OleDb.OleDbException e)
                    {
                        throw new Exception(e.Message);
                    }
                }
            }
        }