public class Protection : Xceed.Workbooks.NET.StyleBase<IReadonlyProtection>, IReadonlyProtection  

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

    // Sets the Worksheet protection : only formatting Cells and inserting Rows/Columns will be allowed. 
    var protection = new WorksheetProtection() { AllowFormatCells = true, AllowInsertRows = true, AllowInsertColumns = true }; 

    // Protects the 1st Worksheet with a password. 
    worksheet.Protect( protection, "xceed" ); 

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