public class DataValidationError : DataValidationMessage 

using( var workbook = Workbook.Create( "AddDataValidation.xlsx" ) ) 
  { 
    // Gets the first Worksheet; a Workbook always contains at least 1 Worksheet. 
    var worksheet = workbook.Worksheets[ 0 ]; 

    // Creates a new DataValidation and assigns it to Cells A1 to A3. 
    var dataValidation = new DataValidation( ValidationType.Decimal, DataComparisonType.LessThanOrEqual ) { Value = 10 }; 
    dataValidation.Error = new DataValidationError( "My Error", "Value must under 10", DataValidationErrorStyle.Warning ); 
    worksheet.Cells[ "A1", "A3" ].DataValidation = dataValidation; 

    // Saves Workbook to disk; 
    workbook.Save(); 
  }