Document Library throws NullReferceException everytime I try to grab an item

Question

This code is from a sequential workflow. I am trying to grab the most recent item in a document library. I check to make sure there are more than zero items in the library, then I try to grab the most recent. However, when I try this, I get an "Object reference not set to an instance of an object." error. I have used this exact same code in other places for different document libraries and it works just fine. I can call the library, get an ItemCount from it, but I can't grab any of the items. Could there be some special permission or lock on this document library that I don't know about?

        if (tlList.ItemCount > 0)
        {
            SPListItem item = tlList.Items[tlList.ItemCount - 1];

            name = item["Name"].ToString();
            user = item["Modified By"].ToString();
            url = SharepointState.siteUrl + "/" + SharepointState.TLLibraryName + "/" + DocumentState.TrackingNumber.Replace(':', '_');
            itemNumber = (ulong)DateTime.Now.Ticks;
        }

Edit: I just found some code in the Feature Event Receiver that also calls on tlList and does it successfully. Here is the code:

            tlList = SharepointState.Web.Lists.TryGetList(SharepointState.TLLibraryName);
            SharepointState.Web.AllowUnsafeUpdates = true;
            SPList workflowhistory = SharepointState.Web.Lists["Workflow History"];
            SPList tasks = SharepointState.Web.Lists["Tasks"];

            SPWorkflowAssociation spas = null;
            spas = SPWorkflowAssociation.CreateListAssociation(baseTemplate, "NSI_FCS_Workflow", tasks, workflowhistory);
            spas.AutoStartCreate = true;
            spas.AutoStartChange = true;
            tlList.WorkflowAssociations.Add(spas);
            tlList.Update();
            SharepointState.Web.AllowUnsafeUpdates = false;

Perhaps I need to 'dispose' of the document library before trying to grab it again when the workflow starts? Perhaps I am doing something to the document library in this code that is preventing me from accessing it again in the future? Let me know if you have any suggestions.

Answer 1

Are you able to step through the code? In the immediate window, when within your If statement, enter tlList.ItemCount, and see it's value. Then enter tlList.Items.Count. The ItemCount property is stored as a separate field in the ContentDB, but that should never differ from the actual items list.

Answer 2

Most likely the problem is because you're using tlList.ItemCount -1 as the index to retrieve your SPListItem. I don't think you can ever assume this is the ID number of the last item your library. For example, deleting items will result in a lower item count.

Instead, I would probably use a CAML query with SPQuery and SPQuery.RowLimit = 1:

<OrderBy><FieldRef Ascending="False" Name="Created" /></OrderBy>

Answer 3

If you have folders in your document library, your count will be off. Read the Community Content at the bottom of this MSDN page.

SPList.ItemCount returns the total items in the list/doc lib including folders.

But, SPList.Items.Count only returns the individual items.

Answer 4

Well I made a total 'newb' mistake. I didn't initialize my SPSite or my siteCollection.OpenWeb objects, which was preventing me from properly accessing the document library. Here is the final code that worked:

        using (SPSite siteCollection = new SPSite(SharepointState.siteUrl))
        {
            SPWeb web = siteCollection.OpenWeb();

            SPDocumentLibrary tlLibrary = (SPDocumentLibrary)web.Lists[SharepointState.TLLibraryName];

            //populate some of the class's fields before starting
            if (tlLibrary.ItemCount > 0)
            {
                SPListItem item = tlLibrary.Items[tlLibrary.ItemCount - 1];

                trackingNumber = item["Name"].ToString();
                user = item["Modified By"].ToString();
                url = SharepointState.siteUrl + "/" + SharepointState.TLLibraryName + "/" + DocumentState.TrackingNumber.Replace(':', '_');
                //use the current datetime in ticks to generate a unique number for the MessageSequenceNumber field in the SAR form    
                itemNumber = (ulong)DateTime.Now.Ticks;
            }
        }