A few weeks ago we wanted to start a specific workflow on a SharePoint list item, that is located in a list that has multiple custom workflows (none of them has a workflow initiation form) associated with it. We performed the action at the very same time with a colleague of mine (having only 4 seconds difference, as it turned out later), independently from each other. After a while, when we checked the status of the item, we found, that two workflows started on the item. The one, we both wanted to start, was started by my colleague, and I’ve started an other workflow. My colleague said I might have clicked the wrong workflow on the workflow.aspx web page, but I was sure I clicked the right one.
I was able to reproduce the issue in our test environment by loading the Workflow page in two separate browser tabs, and starting the same workflow in each of them. I was pretty confident, that the developers of the page made the mistake to try to start the selected workflow by its index in the array of available workflows, instead of the Id of the workflow association.To be sure, I’ve checked the source of the workflow.aspx file (located in the LAYOUTS folder, having a lot of in-line code) and the class behind it, the Microsoft.SharePoint.ApplicationPages.WorkflowPage class (located in the Microsoft.SharePoint.ApplicationPages assembly).
Note: If you try to start the last workflow on the page (or as a special case of it, you have only a single associated workflow) twice in two separate browser, you get an error on the second try. In this case you have the following entry (Level: Unexpected) in the ULS logs:
System.ArgumentOutOfRangeException: Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: index at System.Collections.ArrayList.get_Item(Int32 index) at Microsoft.SharePoint.ApplicationPages.WorkflowPage.OnLoad(EventArgs e) at System.Web.UI.Control.LoadRecursive() at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)
The OnLoad method of the WorkflowPage class calls the ConstructStartArray method if it is not a page post back. In the ConstructStartArray a new ArrayList is created in the m_alwaStart field, and it is populated by the available workflows, by iterating through the workflow associations on the list, on the content type, and on the web level. In each case, the FCanStartWorkflow method of the base class (WorkflowPageBase) is invoked to ensure the current user has the permission to start the workflow manually (see the SPWorkflowAssociation.PermissionsManual property) and if there is no running instance of this workflow type on the item already (via the FIsRunningWt method of the same class). By the end of the ConstructStartArray method the ArrayList in the m_alwaStart field contains the workflows the user can start on the item. So far so good.
Let’s see how this list is rendered in the in-line code of the workflow.aspx page.
int iwa = 0;
strImageUrl = "/_layouts/images/availableworkflow.gif";
foreach (SPWorkflowAssociation wa in m_alwaStart)
string strIniUrl = GenerateStartWorkflowLink(wa);
if (strIniUrl == null)
As you can see, the parameter used with the StartWorkflow method is really a simple counter, the index of the workflow association in the ArrayList in the m_alwaStart field.
var elIwaStart = document.getElementById("iwaStart");
elIwaStart.value = iwa;
The server side GenerateStartWorkflowLink method of the WorkflowPage class, that you can also see in the inline-code above should display the workflow initiation form for the workflow association, if any exists.
Back to the server side, and let’s see what happens with the value posted back by the StartWorkflow method in the OnLoad method of the WorkflowPage class. If the request is a post back, than it reads the index of the workflow to start, and looks up the workflow by this index from the array of workflow associations in the m_alwaStart field:
int num2 = Convert.ToInt32(base.Request.Form["iwaStart"]);
if (num2 >= 0)
Problem: this array might be not the same, as the one returned on the first page load. If a workflow that precedes the workflow we want to start (or the same workflow) is started in the meantime, the workflow associations are changed (for example, workflows are registered or removed on the web, on the list or on the content type level), or the permissions are changed, it is possible (or even very likely) that the user starts another workflow, not the one he clicked on on the web UI.
Solution: would be to use the Id (of type Guid) of the Microsoft.SharePoint.Workflow.SPWorkflowAssociation instance as the identifier of the item in the array instead of the index / position in the array.
That would mean in the in-line code, instead of using the iwa counter:
and in the OnLoad method, handling the post back as:
Guid waId = Guid.Parse(base.Request.Form["iwaStart"] as string);
base.StartWorkflow(this.m_alwaStart.ToArray().First<SPWorkflowAssociation>(wa => wa.Id == waId));
Note: I could reproduce this buggy behavior in SharePoint 2010 and in SharePoint 2013 using site collections that were not upgraded to the SharePoint 2013 mode. However, as long as I see, “native” SharePoint 2013 sites do suffer from the same kind of problem.