Protect an Excel worksheet with a password in C#

Restrict editing on an Excel worksheet while leaving its contents readable.

dotnet add package \
  RecursiveAI.OneDocument.Spreadsheets

Requires .NET 10 (net10.0); .NET 9 and earlier aren’t supported.

using RecursiveAI.OneDocument.Spreadsheets;
 
using var book = Workbook.Open("input.xlsx");
book.Sheets["Data"].Protection.Protect(
    password: "sample");
book.SaveAs("protected.xlsx");

Writes protected.xlsx; the snippet prints nothing.

Unzip the example, then dotnet run.

Full program

input.xlsx has a Data sheet with these cells:

A1: Customer   B1: Total
A2: Ada        B2: 42
A3: Lin        B3: 18

using RecursiveAI.OneDocument.Spreadsheets;
 
using var book = Workbook.Open("input.xlsx");
book.Sheets["Data"].Protection.Protect(
    password: "sample");
book.SaveAs("protected.xlsx");
Console.WriteLine(book.Sheets["Data"].Protection.Verify("sample"));

dotnet run prints:

Match

Protection is not encryption: the content stays readable and restrictions bind only applications that honor them. Chart and macro sheets throw NotSupportedException on Protect.

Variations

Allow sorting and selection of unlocked cells

using RecursiveAI.OneDocument.Spreadsheets;
 
using var book = Workbook.Open("input.xlsx");
book.Sheets["Data"].Protection.Protect(
    SheetActions.Sort |
    SheetActions.SelectUnlockedCells, "sample");
book.SaveAs("protected.xlsx");

Writes protected.xlsx with sorting and unlocked-cell selection allowed. It prints nothing.

Remove protection with the password

After running the full program:

using RecursiveAI.OneDocument.Spreadsheets;
 
using var book = Workbook.Open("protected.xlsx");
Console.WriteLine(
    book.Sheets["Data"].Protection.TryRemove("sample"));
book.SaveAs("unprotected.xlsx");

Match

Writes unprotected.xlsx after the password matches.

Related recipes

• Create an Excel file
• Read an Excel file
• Fill a Word template

The short answer

Open the workbook, call Worksheet.Protection.Protect with a password, and save it as XLSX. The password controls lifting an editing restriction, not opening the file. This distinction matters more than the choice of password: sheet protection is not a way to hide salaries, customer data or other confidential information.

The example uses the default allowed actions, selecting locked and unlocked cells. The argument describes what a user may do, not what you want to prohibit. The protection documentation explains how those flags map to the stored XML.

How do I make my Excel sheet password protected?

book.Sheets["Data"] selects the worksheet named Data, not every worksheet in the workbook. Repeat the operation for other worksheets if each needs its own restrictions. The sample contains a Data worksheet with two customer totals; its contents remain readable after protection.

Protect replaces the sheet’s existing protection settings. Decide which actions should remain available before overwriting settings in a supplied workbook. SheetActions includes actions such as sorting and formatting cells. The variation sets the entire allowed-action list; it does not add two flags to the previous list.

Saving persists the metadata to protected.xlsx; setting protection without saving only changes the open model. The full program prints Match because Verify compares the supplied password with the saved restriction’s verifier in the model. It does not launch Excel or demonstrate an interactive password prompt. Not yet tested in desktop Excel.

What’s the difference between protect sheet and protect workbook?

Worksheet protection controls actions on a worksheet. Workbook protection controls structure and windows through Workbook.Protection.Protect(lockStructure, lockWindows, password). Neither encrypts the file’s contents. A workbook-level restriction is therefore not a stronger version of worksheet confidentiality; it is a restriction on a different set of operations.

A write-reservation password is another separate setting, exposed by Workbook.FileSharing. If the requirement is that an unauthorized reader cannot inspect values at all, this recipe does not meet it. Use file encryption rather than editing restrictions.

How do I remove password protection from an Excel file?

For these worksheet editing restrictions, TryRemove(password) removes protection only when verification matches. Remove() lifts the restriction without checking a password, leaving authorization to your application. Do not expose that operation to users merely because it is available in the library.

Verify returns Match, Mismatch or UnsupportedAlgorithm. An unsupported verifier is not a wrong password; it means the library cannot compute that stored algorithm. A restriction with no stored password matches any input. Reading the workbook never requires the sheet-protection password because the values are not encrypted.

Keep XLSX output when you need the modern protection metadata described here. The documented XLS conversion loses the modern hash and allowed-action list, and the binary writers do not carry every workbook-level protection setting.