如何在Visual Studio 2010中基于connectionString修复SystemArgumentNull

每当我单击表单上的提交按钮时，我都会收到以下错误消息：

错误： System.ArgumentNullException： 值不能为空。参数名称：连接字符串位于 Microsoft.ApplicationBlocks.Data.SqlHelper.ExecuteDataset（String connectionString， CommandType commandType， String commandText， SqlParameter[] commandParameters） at ClassesnWorkshops._Default.GetHousesBySearchCriteria（）

. . .

我是 c# 和 Visual Studio 的初学者。谁能告诉我为什么我的连接字符串可能为空？任何想法如何解决此错误？我使用项目"属性"的"设置"按钮创建了一个名为"connectionString"的连接字符串，连接到我的类数据库。但这可能是不必要的，因为它没有改变错误。

//Code for my submit button click event:
protected void cmdSearch_Click(object sender, EventArgs e)
        {
            DataTable objDT = null;
            try
            {
                //Query the database for houses
                objDT = GetHousesBySearchCriteria();
                //Any houses found?
                if (objDT.Rows.Count == 0)
                {
                    //None found - hide repeater, show message
                    rptHouses.Visible = false;
                    lblMessage.Text = "No results found";
                }
                else
                {
                    //Houses found - show repeater, hide message
                    rptHouses.DataSource = objDT;
                    rptHouses.DataBind();
                    rptHouses.Visible = true;
                    lblMessage.Text = "";
                }
            }
            catch (Exception ex)
            {
                //Add your own error handling here
                lblMessage.Text = "Error: " + ex.ToString();
            }
            finally
            {
                //Release memory
                objDT = null;
            }
        }
        #endregion

        #region GetHousesBySearchCriteria
        public DataTable GetHousesBySearchCriteria()
        {
            //Use the Microsoft Data Application Blocks to query database
            DataSet objDS = new DataSet();
            SqlParameter[] arParms = new SqlParameter[2];
            arParms[0] = new SqlParameter("@Zipcode", SqlDbType.Char);
            arParms[0].Value = txtZipCode.Text;
            arParms[1] = new SqlParameter("@Miles", SqlDbType.Decimal);
            arParms[1].Value = Int16.Parse(cboWithin.SelectedItem.Value);
            lblTestParms0.Text = txtZipCode.Text;
            lblTestParms1.Text = cboWithin.SelectedValue.ToString();
            //Return a DataTable
            return SqlHelper.ExecuteDataset(ConfigurationManager.AppSettings["ConnectionString"],
                CommandType.StoredProcedure, "spHouses_GetNearZipcode", arParms).Tables[0];

        }
        #endregion

存储过程代码：

CREATE PROCEDURE [dbo].[spHouses_GetNearZipcode]
@Zipcode char(5),
@Miles decimal(11,6)
AS
--Load close zipcodes into temp table
SELECT ZIP.ZipCode, ZIP.City, 
    dbo.DistanceFunc(ZIP.Latitude, ZIP.Longitude, RAD.Latitude, RAD.Longitude) As Distance
    INTO #TempZips
FROM ZipCodes ZIP, RadiusFunc(@ZipCode, @Miles) RAD
WHERE (ZIP.Latitude BETWEEN RAD.MinLatitude AND RAD.MaxLatitude) AND
    (ZIP.Longitude BETWEEN RAD.MinLongitude AND RAD.MaxLongitude) AND
    (dbo.DistanceFunc(ZIP.Latitude,ZIP.Longitude,RAD.Latitude,RAD.Longitude) <= @Miles)
--Search Houses table and JOIN to temp zipcodes table
SELECT H.*, Zips.Distance AS Miles
FROM Schools H INNER JOIN 
    #TempZips Zips ON Zips.ZipCode = H.zip
ORDER BY Zips.Distance
RETURN

return SqlHelper.ExecuteDataset(ConfigurationManager.AppSettings["ConnectionString"].ConnectionString, CommandType.StoredProcedure, "spHouses_GetNearZipcode", arParms).Tables[0];

connectionString or ConnectionString
^                   ^