System.InvalidCastException指定的强制转换无效

Question

描述：执行当前Web请求期间发生了未处理的异常。请查看堆栈跟踪以获取有关错误及其源自代码的位置的更多信息。

异常详细信息：System.InvalidCastException：指定的强制转换无效。

我无法弄清楚为什么即使我的detailview正确完成，我仍然会收到错误。 变量已事先声明，大多数类都是正确的。 我正在使用visual C＃为学校模块做一个电子商务网站。

    //extract the QuantityOnHand from the database - based on the product ID
    strSQLSelect = "SELECT pQuantity FROM Product WHERE pProdNo = @ProductID";
    cmd = new OleDbCommand(strSQLSelect, mDB);
    cmd.Parameters.Add("@ProductId", OleDbType.VarChar).Value = strProductId;
    object oQty = cmd.ExecuteScalar();
    intQuantityOnHand = (int)oQty; //error here

建议和帮助将不胜感激！

以下页面的整个编码：

static readonly string scriptStockOut = "<script language=\"javascript\">\n" +
"alert (\"Sorry Stock Out!  Please choose a smaller quantity or another product \");\n" +
"</script>";

static readonly string scriptErrorLogin = "<script language=\"javascript\">\n" +
   "alert (\"Please login or create account first to facilitate buying\");\n</script>";

protected void Page_Load(object sender, EventArgs e)
{

}
protected void btnBuy_Click(object sender, EventArgs e)
{
    //declare variables
    string strProductId, strSQLSelect, strSQL;
    int intQuantityOnHand, intBuyQuantity, newQty, intOrderNo;
    decimal decUnitPrice;


    // test to remind customer to login first
    if ((string)Session["sFlag"] != "T")
    {
        Type csType = this.GetType();
        ClientScript.RegisterStartupScript(csType, "Error", scriptErrorLogin);
    }

    // connect to database and then check stock quantity
    OleDbConnection mDB = new OleDbConnection();
    mDB.ConnectionString = "Provider = Microsoft.ACE.OLEDB.12.0;Data source="
         + Server.MapPath("~/Database.accdb");
    mDB.Open(); OleDbCommand cmd;

    //retrieve the product ID from the first row of the DetailsView
    DetailsViewRow row0 = DetailsView1.Rows[0];
    strProductId = row0.Cells[1].Text;

    //extract the QuantityOnHand from the database - based on the product ID
    strSQLSelect = "SELECT pQuantity FROM Product WHERE pProdNo = @ProductID";
    cmd = new OleDbCommand(strSQLSelect, mDB);
    cmd.Parameters.Add("@ProductId", OleDbType.VarChar).Value = strProductId;
    object oQty = cmd.ExecuteScalar();
    intQuantityOnHand = (int)oQty;

    //extract the Promotion Price from the database - based on the product ID
    strSQLSelect = "SELECT pPromoCost FROM Product WHERE pProdNo = @ProductID";
    cmd = new OleDbCommand(strSQLSelect, mDB);
    cmd.Parameters.Add("@ProductId", OleDbType.VarChar).Value = strProductId;
    object oUnitPrice = cmd.ExecuteScalar();
    decUnitPrice = (decimal)oUnitPrice;

    //extract the quantity purchased from the dropdown list
    intBuyQuantity = int.Parse(ddlQty.Items[ddlQty.SelectedIndex].ToString());

    //calculate the new quantity left
    newQty = intQuantityOnHand - intBuyQuantity;

    //checking for out of stock situation
    if (intQuantityOnHand < intBuyQuantity)
    {
        Type csType = this.GetType();
        ClientScript.RegisterStartupScript(csType, "StockOut", scriptStockOut);
    }

    // save productID, unitPrice and quantity as Session variables
    Session["sProductId"] = strProductId;
    Session["sUnitPrice"] = decUnitPrice.ToString();
    Session["sQuantity"] = newQty.ToString();

    // insert product ordered to the orderItems table
    intOrderNo = (int)Session["sOrderNo"];
    strSQL = "INSERT INTO InvoiceDetails(idOrderNo, idProdID, idQty, idUnitPrice)"
         + "VALUES (@OrderNo, @ProductId, @Qty, @UnitPrice)";
    cmd = new OleDbCommand(strSQL, mDB);
    cmd.Parameters.Add("@OrderNO", OleDbType.Integer).Value = intOrderNo;
    cmd.Parameters.Add("@ProductId", OleDbType.VarChar).Value = strProductId;
    cmd.Parameters.Add("@Qty", OleDbType.Integer).Value = intBuyQuantity;
    cmd.Parameters.Add("@UnitPrice", OleDbType.Currency).Value = decUnitPrice;
    cmd.ExecuteNonQuery();

    //update the quantity on hand in the products table
    strSQL = "UPDATE Product SET pQuantity = @NewQty WHERE pProdNo = @ProductId";
    cmd = new OleDbCommand(strSQL, mDB);
    cmd.Parameters.Add("@ProductId", OleDbType.Integer).Value = newQty;
    cmd.Parameters.Add("@NewQty", OleDbType.VarChar).Value = strProductId;
    cmd.ExecuteNonQuery();

    mDB.Close();

    // redirect the user to the Shopping Cart page
    Response.Redirect("ShoppingCart.aspx");

}

}