In my West Wind Web Store I have an issue where about once a week or so I get an error that relates to a specific field. I blogged about this issue before a long while back and I have still problems with this one.
The error occurs against any direct access of an Item Url in the store. For example:
Give it a try and see if it fails for you . In most cases it won’t. The errors get logged so the more errors I see the more I can maybe see a pattern. If it fails, see if it fails more than once and with a different browser. Try switching your browser’s language (if you’re not using US-English). I’d be interested to see what comes up.
Here’s what goes on where the error occurs. In my business object layer I have a simple entity mapping scheme that basically maps a DataRow to front end object that accesses the DataRow fields in order to provide strong typing. The use of this approach is essentially to get strong typing without all the mess and overhead that typed DataSets incur.
The problem occurs in this block of code which is used in the store to display an item. Basically the following code uses a business object to retrieve an inventory item by sku and then sets a few properties with various fixups to display the data. The C# code in the ASP.NET Codebehind page looks like this:
private void DisplayItem()
// *** Allow item disply from URL query string
string Qty = Request.QueryString["qty"];
if (Qty != null)
this.txtQty.Text = Qty;
this.txtQty.Text = "1";
// *** If Action=Add was used force to Button_Click
if (this.FirstAccess && Request.QueryString["Action"] == "Add")
this.FirstAccess = false;
// Put user code to initialize the page here
this.IvtItem = WebStoreFactory.GetbusItem();
if (this.Sku == null)
// *** Can't display an item with out a sku
// *** Retrieve the inventory item for checking stock and binding
MessageDisplay.DisplayMessage("Invalid Item","The item you selected is no longer available. " +
"If you typed the Url in manually make sure the SKU is typed correctly.");
this.rowItem = this.IvtItem.GetTypedDataRow(false);
if (this.rowItem.Redirect != "")
Response.Redirect( this.rowItem.Redirect );
// *** If we have a subitem show the parent item instead
if (rowItem.Parentsku != null && rowItem.Parentsku != "" )
// *** We redirect here so our URL is correct -
// *** otherwise we have issues picking up querystring
Response.Redirect( "item.aspx?Sku=" + this.rowItem.Parentsku );
// *** We'll bind in code here because we need to fix up a few things
this.lblTitle.Text = rowItem.Descript;
// *** Insert a list of subitems if user has specified it
string Description = rowItem.Ldescript.Replace("<%= InsertSubItemList() %>",IvtItem.SubItemHtmlList(this.Sku));
// *** Force HTML linebreaks into the field text
this.lblDescription.Text = this.DisplayMemo( Description );
// *** Display price as Currency
this.lblPrice.Text = rowItem.Price.ToString("C");
// *** Display Item image only if image is not blank
--> if (!wwUtils.Empty( rowItem.Itemimage ) )
this.imgProduct.ImageUrl = "itemimages/" + rowItem.Itemimage;
this.imgProduct.Visible = false;
if (rowItem.Url != null && rowItem.Url != "")
this.lblMoreInfo.Text = " target='_MoreInfo'>More Info";
// *** Don't allow adding if stock is out
if ( !IvtItem.CheckStock(1))
this.btnSubmit.Text = "Out of Stock";
The failure occurs towards the end of the method with the code in bold. ItemImage is a field in the wws_Items table which is loaded up inside the GetItemBySku business object method. GetItemBySku is pretty simple – it basically runs a SELECT * FROM wws_items to retrieve all fields of the database table. The code looks like this:
public bool GetItemBySku(string Sku)
int RecCount = this.Execute("select * from " + this.Tablename + " where sku=@Sku",this.Tablename,
if (RecCount < 0)
if (RecCount < 1)
this.SetError("Item not found.");
this.DataRow = this.DataSet.Tables[this.Tablename].Rows;
The method handles errors, so if an exception occurs in the Execute call the error is bubbled up and RecCount returns -1. So I’m certain that the ADO.NET code beneath is actually working in retrieving the data. The ASP.NET code checks the result from the method and shows an error if it fails. If this method succeeds an item was loaded.
When this code returns it ends up with a single DataRow that contains all the fields. If you look at the first block of code again, you can see there’s a GetTypedDataRow() method which returns the pseudo-entity object which behind the scenes maps the DataRow fields to properties.
A number of other fields/properties are accessed before the error occurs so it appears that code is working as it’s supposed to. But an error occurs when accessing the ItemImage field – about once a week.
As I mentioned all errors are logged by a custom WebErrorHandler which produces this output for the error:
Column 'Itemimage' does not belong to table wws_items.
on 16.05.2005 03:22:45
--- Stack Trace ---
at System.Data.DataRow.get_Item(String columnName)
at Westwind.WebStore.Item.Page_Load(Object sender, EventArgs e)
at System.Web.UI.Control.OnLoad(EventArgs e)
--- Request Information ---
Full Url: http://www.west-wind.com/wwstore/item.aspx?sku=WWHELP40
Browser: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.0; MyIE2; .NET CLR 1.0.3705; .NET CLR 1.1.4322)
This is REALLY, REALLY odd, because 99.9% of the time this code works perfectly fine. In fact, I’ve never been able to duplicate this error at all. Itemimage is a varchar field in the database and does not contain any nulls. It holds a string to an image file that is used to display the item’s image. It’s something like wwhelp.gif for example. The actual implementation of the property looks like this:
public String Itemimage
if (this.DataRow == null)
return (String) this.DataRow[this.ItemimageColumn];
return (String) this.DataRow["Itemimage"];
if (DataRow != null)
this.DataRow["Itemimage"] = value;
this._Itemimage = value;
The actual line of code that fails is the DataRow access for the ItemImage field.
What’s really odd here is this: At this point of failure the code for this request has already accessed a number of the other fields/entity properties of the same object/DataRow without errors! So the DataRow is there. The other fields are there and the property accessors for them are working just fine.
What this error suggests is that the SELECT * FROM wws_items is not returning the ItemImage field even though it is part of the SQL Server Table it’s retrieved from. And remember it does work 99.9% of the time.
I noticed reviewing my logs that the errors always occur with foreign locales. I just today added logging the Locale information to possibly illuminate me on whether there some locale consistency to the failure but all the errors have dates in European formats. 16.05.2005. Just to be sure I tried switching the browser into German, but it didn’t seem to make a difference – I see no failure. Besides why would the thread locale setting cause the field not be retrieved when we’re dealing with a string field?
I’ve tried all sorts of stuff since the last post. I changed the field name from Image to ItemImage thinking that maybe in some odd way Image interfered with the reserved word of the Image field type. Still have errors. I moved access of the ItemImage further down in the processing code so it’s now next to the last access. Still fails and still fails only on the ItemImage field. All the preceeding field accesses work just fine.
I also tried bypassing the Entity field altogether, going directly to the DataRow value. Same result – still have intermittent errors.
This is very, very odd and I am absolutely stumped? Anybody have any ideas what else to look at?
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