Where-to-use-the-using-keyword.md

Note: The "using" keyword is not needed if you turn off event tracking on the workbook. Please see Turning off events for more details.

You should use the "using" keyword whenever you get a hold of a range-like object inside a loop. That includes workbook, worksheet, row, rows, column, and columns.

The following recommendations assume you're dealing with big files.

Not so good:

var wb = new XLWorkbook();
var ws = wb.Worksheets.Add("Sheet1");
for (int ro = 1; ro <= 1000000; ro++)
{
  ws.Row(ro).FirstCell().Value = ro;
}

Best way:

using (var wb = new XLWorkbook())
{
  // No need to put the worksheet inside a "using" block because
  // the workbook will dispose of the sheets. The worksheet is not
  // created inside a loop and the workbook's dispose is being
  // called immediately after using the worksheet.
  var ws = wb.Worksheets.Add("Sheet1");
  for (int ro = 1; ro <= 1000000; ro++)
  {
    // Dispose of the row once we're done with it
    using(var row = ws.Row(ro))
    row.FirstCell().Value = ro;
  }
}

Not so good:

var wb = new XLWorkbook();
for (int wsNum = 1; wsNum <= 10; wsNum++)
{
  var ws = wb.Worksheets.Add("Sheet" + wsNum);

  // Add stuff to the worksheet...
}

Best way:

using (var wb = new XLWorkbook())
{
  for (int wsNum = 1; wsNum <= 10; wsNum++)
  {
    // Dispose the worksheet right away
    // (Don't wait for the workbook to do so)
    using (var ws = wb.Worksheets.Add("Sheet" + wsNum))
    {
      // Add stuff to the worksheet...
    }
  }
}