purpose of ? in asp.net

foreach (ListEntry mySpread in myFeed.Entries)
          {
              DataRow myDR = myTable.NewRow();
              foreach (ListEntry.Custom listrow in mySpread.Elements)
              {
              DC = myTable.Columns[listrow.LocalName] ?? myTable.Columns.Add(listrow.LocalName);

                  myDR[DC] = listrow.Value;


              }
              myTable.Rows.Add(myDR);
          }