Placing the values in a separate sheet is a great solution! Thank you for the recommendation. It's working very well now including support for validation after exporting to Excel. 

My apologies for the second post. The solution I listed below does not work. The actual problem is that Excel has a 256-character limit on data validation lists that are entered as a string (as opposed to validations sourced from elsewhere in a spreadsheet). Source

If the From string exceeds 256 characters, an error message will appear on export.

Hello, I found the cause of my problem and will post it here if it helps others. 

I have a RadSpreadsheet that uses a custom database provider. The spreadsheet has many columns that use a custom editor to provide a drop-down list of selections. The drop-down list is generated from a generic list (from a database). In the below sample code, _fundCodes is a List<FundCode>. 

When I create the column like this, I will see validation error messages when exporting to Excel:

                     

 _columns.Add(new MyColumn()
     {
          Name = "FundCode",
          DisplayName = "FundCode",
          Validation = new Validation()
                    {
                        AllowNulls = true,
                        DataType = "list",
                        ShowButton = true,
                        ComparerType = "list",
  From = "\"" + string.Join(",", _fundCodes.Select(item => item.FundName).ToArray()) + "\"",
                                              Type = "reject"
                    }
                });

 

However, if I move the construction of my comma-delimited list outside this statement, no validation error will appear in Excel and Excel even displays the drop down options (which is awesome):

string strFundCodes = "\"" + string.Join(",", _fundCodes.Select(item => item.FundName).ToArray()) + "\"";
                _columns.Add(new MyColumn()
                {
                    Name = "FundCode",
                    DisplayName = "FundCode",
                    Validation = new Validation()
                    {
                        AllowNulls = true,
                        DataType = "list",
                        ShowButton = true,
                        ComparerType = "list",                     
                        From = strFundCodes,
                        Type = "reject"
                    }
                });

 

Hello, I am encountering the same problem and this solution did not make a difference. Is this answer still correct?