title: Excel data validation missing after re-opening file
body: <p>I am adding some data validation on the fly using the following: </p>
  
  <pre><code>var cell = sheet.Range["A2", "A1048576"];
  cell.Validation.Add(
      XlDVType.xlValidateList,
      XlDVAlertStyle.xlValidAlertStop,
      XlFormatConditionOperator.xlBetween,
      "ABC,DEF,123");
  </code></pre>
  
  <p>This works grand and you will see the ABC,DEF and 123 in the datavalidation dropdown for the column. The issue is after re-opening the sheet later the data validation is gone.  An error is shown <img src="https://i.stack.imgur.com/CZEE7.png" alt="enter image description here"> 
  Click yes then you are presented with: <img src="https://i.stack.imgur.com/OS6L7.png" alt="enter image description here">
  Click close and your sheet is displayed with all the data already entered but the data validation is gone entirely. </p>
  
  <p>How do I add the validation so it persists and survives save and re-open? </p>