item.SystemUpdate() - It is used when we do not want fields such as Modified,ModifiedBy gets updated or if we do not want a new version should be created.
In the below code I used to get SharePoint List Item from different method and then update it.
  1. using (SPSite site = new SPSite(SPContext.Current.Site.ID))
  3. {
  5. using (SPWeb web = site.OpenWeb())
  7. {
  9. SPListItem item = GetItem("TestListSite1", 1);
  11. web.AllowUnsafeUpdates = true;
  13. item["Title"] = "Update List Item";
  15. item.SystemUpdate();
  17. web.AllowUnsafeUpdates = false;
  19. //remaining code
  21. }
  23. }
Executing the above code gave an error below.
After searching I found that we should make AllowUnsafeUpdates = true of the item that we are updating and since I was getting item from a different method I used the below code.
The only change I made is this item.Web.AllowUnsafeUpdates = true;
  1. using (SPSite site = new SPSite(SPContext.Current.Site.ID))
  3. {
  5. using (SPWeb web = site.OpenWeb())
  7. {
  9. SPListItem item = GetItem("TestListSite1", 1);
  11. item.Web.AllowUnsafeUpdates = true;
  13. item["Title"] = "Update List Item";
  15. item.SystemUpdate();
  17. item.Web.AllowUnsafeUpdates = false;
  19. }
  21. }
By using above code I was successfully able to update the list item using item.SystemUpdate();