Problem

When you specify List data validation in Excel with a sequence of comma-separated values, Excel prevents you from entering more than 255 characters, including commas. Analogously, when you create List data validation in ExcelWriter with an array of values, the CreateDataValidation method throws the exception: Items in the array exceed 256 characters, if the combined length of all of the elements in the array is longer than 255 characters. The limit is actually 255 rather than 256 as stated in the exception message. In practice, the actual limit would be lower if there are multiple elements in the array, as ExcelWriter has to insert tokens to demarcate individual values.

Solution

In order to prevent the exception, you can specify the list as a formula pointing to a range containing the desired values rather than as an array. This approach parallels Excel’s second way to create List data validation. If it’s desirable to hide the validation values, you can place the range on a non-visible area of the worksheet, or on a separate hidden worksheet.

 // Create validation object based on desired cell range 
DataValidation dv = workbook.CreateDataValidation(DataValidation.ValidationType.List, "=Sheet1!A1:A4"); 

// Alternatively, use an existing named range 
DataValidation dv2 = workbook.CreateDataValidation(DataValidation.ValidationType.List, "=ValidationRange"); 

// Set cell B1 to use validation 
worksheet.Cells["B1"].DataValidation = dv; 

For more information about creating data validations, please refer to our documentation