I have an event receiver that is looping through a group of lists and associating a workflow to it if they have content approval enabled. The feature fails to activate with this error:
Error occurred in deployment step ‘Activate Features’: Collection was modified; enumeration operation may not execute.
Here is the code snippet. If I comment out the line in bold, it activates without error:
public override void FeatureActivated(SPFeatureReceiverProperties properties)
{
   using (SPSite site = properties.Feature.Parent as SPSite)
   {
     using (SPWeb web = site.OpenWeb(“/marketingdc”))
     {
        SPWorkflowTemplate wfTemplate = web.WorkflowTemplates.GetTemplateByName(“CPMDApproval”, web.Locale);
        SPList tasks = web.Lists[“Tasks”];
        SPList wfHistory = web.Lists[“Workflow History”];
        foreach (SPList list in web.Lists.Cast<SPList>().Where(l => l.EnableModeration == true))
        {
          SPWorkflowAssociation wfAssociation = list.WorkflowAssociations.GetAssociationByName(“Marketing Docs Approval”, web.Locale);
          if (wfAssociation == null)
          {
             wfAssociation = SPWorkflowAssociation.CreateListAssociation(wfTemplate, “Marketing Docs Approval”, tasks, wfHistory);
             wfAssociation.AllowManual = true;
             wfAssociation.AutoStartChange = false;
             wfAssociation.AutoStartCreate = false;
             list.WorkflowAssociations.Add(wfAssociation);
          }
          else
          {
             wfAssociation.AllowManual = true;
             wfAssociation.AutoStartChange = false;
             wfAssociation.AutoStartCreate = false;
             list.WorkflowAssociations.Update(wfAssociation);
          }
        }
     }
   }
}
