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Worksheet protection is meant to lock data for the purpose of presentation only. Worksheet protection should not be confused with encryption, and it should not be used to hide or protect sensitive data. When worksheet protection is activated, users can see the worksheet and all the data within it, but will be prevented from altering the content or formatting the cells.

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//--- To encrypt a workbook, use the Workbook.EncryptPassword property:
ExcelApplication xlw = new ExcelApplication();
Workbook wb = xlw.Create();
wb.EncryptPassword = "myPassword";
xlw.Save(wb, outPath);

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Use the method Worksheet.Protect to active worksheet protection. Pass the method a password for unlocking the worksheet in Excel. Pass "" or nullto protect the worksheet without a password.

ExcelApplication xla = new ExcelApplication();
Workbook wb = xla.Create();
Worksheet ws = wb.Worksheet[0];
ws.Protect("MyPassword");

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With ExcelWriter, you can unlock cells by setting Style.CellLocked to false and applying the style to an individual cell. The following example protects all cells except for A1 and B1:

ExcelApplication xla = new ExcelApplication();
Workbook wb = xla.Create();
Worksheet ws = wb.Worksheet[0];

//--- Turn on worksheet protection.
ws.Protect("MyPassword");

//--- Create an "unlocked" style.
Style unlockedStyle = wb.CreateStyle();
unlockedStyle.CellLocked = false;

//--- Apply the style.
ws.Cells["A1"].Style = unlockedStyle;
ws.Cells["B1"].Style = unlockedStyle;