SecureString.MakeReadOnly Method

Definition

Namespace:

System.Security

Assembly:

System.Runtime.InteropServices.dll

Source:

SecureString.cs

Makes the text value of this secure string read-only.

public:
 void MakeReadOnly();

public void MakeReadOnly ();

member this.MakeReadOnly : unit -> unit

Public Sub MakeReadOnly ()

Exceptions

ObjectDisposedException

This secure string has already been disposed.

Examples

The following example demonstrates how the AppendChar and RemoveAt methods can be used to collect the characters in a password. After the password is collected, it is made read-only.

using namespace System;
using namespace System::Security;

void main()
{
   bool go = true;
   ConsoleKeyInfo cki;
   String^ m = L"\nEnter your password (up to 15 letters, numbers, and underscores)\n"
               L"Press BACKSPACE to delete the last character entered. " +
               L"\nPress Enter when done, or ESCAPE to quit:";
   SecureString ^ password = gcnew SecureString;
   int top;
   int left;
   
   // The Console.TreatControlCAsInput property prevents CTRL+C from
   // ending this example.
   Console::TreatControlCAsInput = true;

   Console::Clear();
   Console::WriteLine(m);
   
   top = Console::CursorTop;
   left = Console::CursorLeft;

   do {
      cki = Console::ReadKey(true);
      if (cki.Key == ConsoleKey::Escape)
         break;

      if (cki.Key == ConsoleKey::Backspace){
         if (password->Length > 0) {
            Console::SetCursorPosition(left + password->Length - 1, top);
            Console::Write(' ');
            Console::SetCursorPosition(left + password->Length - 1, top);
            password->RemoveAt(password->Length - 1);
         }
      }
      else {
         if ((password->Length < 15) &&
             (Char::IsLetterOrDigit( cki.KeyChar ) ||
              cki.KeyChar == '_') ) {
            password->AppendChar( cki.KeyChar );
            Console::SetCursorPosition( left + password->Length - 1, top );
            Console::Write("*");
         }
      }
   } while (cki.Key != ConsoleKey::Enter & password->Length < 15);

   // Make the password read-only to prevent modification.
   password->MakeReadOnly();
   // Dispose of the SecureString instance.
   delete password;

}
// The example displays output like the following:
//    Enter your password (up to 15 letters, numbers, and underscores)
//    Press BACKSPACE to delete the last character entered.
//    Press Enter when done, or ESCAPE to quit:
//    ************

using System;
using System.Security;

class Example
{
   public static void Main()
   {
      ConsoleKeyInfo cki;
      String m = "\nEnter your password (up to 15 letters, numbers, and underscores)\n" +
                 "Press BACKSPACE to delete the last character entered. " +
                 "\nPress Enter when done, or ESCAPE to quit:";
      SecureString password = new SecureString();
      int top, left;

      // The Console.TreatControlCAsInput property prevents CTRL+C from
      // ending this example.
      Console.TreatControlCAsInput = true;

      Console.Clear();
      Console.WriteLine(m);

      top  = Console.CursorTop;
      left = Console.CursorLeft;

      // Read user input from the console. Store up to 15 letter, digit, or underscore
      // characters in a SecureString object, or delete a character if the user enters
      // a backspace. Display an asterisk (*) on the console to represent each character
      // that is stored.

      do {
         cki = Console.ReadKey(true);
         if (cki.Key == ConsoleKey.Escape) break;

         if (cki.Key == ConsoleKey.Backspace) {
            if (password.Length > 0) {
               Console.SetCursorPosition(left + password.Length - 1, top);
               Console.Write(' ');
               Console.SetCursorPosition(left + password.Length - 1, top);
               password.RemoveAt(password.Length-1);
            }
         }
         else {
            if ((password.Length < 15) &&
                 (Char.IsLetterOrDigit(cki.KeyChar) || cki.KeyChar == '_')) {
               password.AppendChar(cki.KeyChar);
               Console.SetCursorPosition(left+password.Length-1, top);
               Console.Write('*');
            }
         }
      } while (cki.Key != ConsoleKey.Enter & password.Length < 15);

      // Make the password read-only to prevent modification.
      password.MakeReadOnly();
      // Dispose of the SecureString instance.
      password.Dispose();
   }
}
// This example displays output like the following:
//    Enter your password (up to 15 letters, numbers, and underscores)
//    Press BACKSPACE to delete the last character entered.
//    Press Enter when done, or ESCAPE to quit:
//    ************

Imports System.Security

Class Example
   Public Shared Sub Main()
      Dim cki As ConsoleKeyInfo
      Dim m As String = vbCrLf & "Enter your password (up to 15 letters, numbers, and underscores)" &
                        vbCrLf & "Press BACKSPACE to delete the last character entered. " & vbCrLf &
                        "Press Enter when done, or ESCAPE to quit: "
      Dim password As New SecureString()
      Dim top, left As Integer

      ' The Console.TreatControlCAsInput property prevents CTRL+C from
      ' ending this example.
      Console.TreatControlCAsInput = True

      Console.Clear()
      Console.WriteLine(m)

      top = Console.CursorTop
      left = Console.CursorLeft

      ' Read user input from the console. Store up to 15 letter, digit, or underscore
      ' characters in a SecureString object, or delete a character if the user enters 
      ' a backspace. Display an asterisk (*) on the console to represent each character 
      ' that is stored.
      
      Do
         cki = Console.ReadKey(True)
         If cki.Key = ConsoleKey.Escape Then Exit Do

         If cki.Key = ConsoleKey.Backspace Then
            If password.Length > 0 Then
               Console.SetCursorPosition(left + password.Length - 1, top)
               Console.Write(" "c)
               Console.SetCursorPosition(left + password.Length - 1, top)
               password.RemoveAt(password.Length - 1)
            End If
         Else
            If password.Length < 15 AndAlso([Char].IsLetterOrDigit(cki.KeyChar) _
            OrElse cki.KeyChar = "_"c) Then
               password.AppendChar(cki.KeyChar)
               Console.SetCursorPosition(left + password.Length - 1, top)
               Console.Write("*"c)
            End If
         End If
      Loop While cki.Key <> ConsoleKey.Enter And password.Length < 15

      ' Make the password read-only to prevent modification.
      password.MakeReadOnly()
      ' Dispose of the SecureString instance.
      password.Dispose()
   End Sub
End Class
' The example displays output like the following:
'    Enter your password (up to 15 letters, numbers, and underscores)
'    Press BACKSPACE to delete the last character entered.
'    Press Enter when done, or ESCAPE to quit:
'    ************

Remarks

Initialize the text value of an instance of the SecureString class with the SecureString constructors, and modify the value with the Clear, RemoveAt, SetAt, InsertAt, and AppendChar methods.

After you have made your final modifications, use the MakeReadOnly method to make the value of the instance immutable (read-only). After the value is marked as read-only, any further attempt to modify it throws an InvalidOperationException.

The effect of invoking MakeReadOnly is permanent because the SecureString class provides no means to make the secure string modifiable again. Use the IsReadOnly method to test whether an instance of SecureString is read-only.

See also

• IsReadOnly()