ADO.NET工厂模式下数据删除错误

       private void DeleteBtn_Click(object sender, EventArgs e)

        {

            //int result = DALClass.DeleteManager(this.managerNameTxt.Text);

            //if (result > -1)

            //{

            //    parent.TaxstatusBar.Panels[0].Text = "删除成功";

            //}

            //else

            //{

            //    parent.TaxstatusBar.Panels[0].Text = "删除失败";

            //}



            int ret =-1;

            string dbProviderName = ConfigurationSettings.AppSettings["DbProviderName"];

            //private static string dbp = "Provider=System.Data.OleDb";

            string dbConnectionString = ConfigurationManager.AppSettings["TaxInvoiceConn"];



            DbProviderFactory dbfactory = DbProviderFactories.GetFactory(dbProviderName);

            DbConnection dbconn = dbfactory.CreateConnection();

            dbconn.ConnectionString = dbConnectionString;



            DbCommand dbCommand = dbconn.CreateCommand();

            dbCommand.CommandText = "Delete From TaxManagers Where MangerNumbuer=@TaxManagerNum";

            dbCommand.CommandType = CommandType.Text;



            DbParameter dbParameter = dbCommand.CreateParameter();

            dbParameter.DbType = DbType.String;

            dbParameter.ParameterName = "@TaxManagerNum";

            dbParameter.Value = this.managerNumTxt.Text;

            dbParameter.Direction = ParameterDirection.Input;

            dbCommand.Parameters.Add(dbParameter);



            dbCommand.Connection.Open();

            ret = dbCommand.ExecuteNonQuery();

            dbCommand.Connection.Close();



            if (ret > -1)

            {

                MessageBox.Show("删除成功");

            }

        }

<?xml version="1.0" encoding="utf-8" ?>

<configuration>

    <configSections>

    </configSections>

    <connectionStrings>

        <add name="TaxConnectionString"

            connectionString="Provider=Microsoft.ACE.OLEDB.12.0;Data Source=|DataDirectory|\Tax.accdb;Persist Security Info=True"

            providerName="System.Data.OleDb" />

      <add name="TaxInvoiceConn" connectionString="Provider=Microsoft.Jet.OLEDB.4.0; Data Source=D:\TaxInvionce\TaxInvionce\Tax.mdb"

           providerName="System.Data.OleDb" />

    </connectionStrings>

    

    <appSettings>

        <add key="UseDepartment" value=""/>

        <add key="DepartmentNum" value=""/>

        <add key="DbProviderName" value="System.Data.OleDb"/>

        <add key="TaxInvoiceConn" value="Provider=Microsoft.ACE.OLEDB.12.0;Data Source=|DataDirectory|\Tax.accdb;Persist Security Info=True"/>

    </appSettings>

</configuration>



执行到ret = dbCommand.ExecuteNonQuery();时出现如下错误信息：

至少有一个参数未指定