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MSDN Library

Marshal.ZeroFreeCoTaskMemAnsi Method

Frees an unmanaged string pointer that was allocated using the SecureStringToCoTaskMemAnsi method.

Namespace:  System.Runtime.InteropServices
Assemblies:   System.Runtime.InteropServices (in System.Runtime.InteropServices.dll)
  mscorlib (in mscorlib.dll)

'Declaration
Public Shared Sub ZeroFreeCoTaskMemAnsi ( _
	s As IntPtr _
)

Parameters

s
Type: System.IntPtr

The address of the unmanaged string to free.

The ZeroFreeCoTaskMemAnsi method first zeros out and then frees unmanaged memory that was allocated using the SecureStringToCoTaskMemAnsi method.

The following example uses the SecureStringToCoTaskMemAnsi method to marshal and decrypt the contents of a SecureString object to a block of unmanaged memory. It then uses the ZeroFreeCoTaskMemAnsi method to zero out and dispose the unmanaged block.

Imports System
Imports System.Diagnostics
Imports System.Runtime.InteropServices
Imports System.Security
Imports System.Security.Principal
Imports System.Text




Module MarshalExample



    Sub Main(ByVal args() As String)

        Dim unmanagedRef As IntPtr

        Try 
            ' Ask the user for a password.
            Console.Write("Please enter your password:")

            Dim passWord As SecureString = GetPassword()

            Console.WriteLine("Copying and decrypting the string to unmanaged memory...")

            ' Copy the Secure string to unmanaged memory (and decrypt it).
            unmanagedRef = Marshal.SecureStringToCoTaskMemAnsi(passWord)


        Catch e As Exception
            Console.WriteLine(e.Message)
        Finally 

            If unmanagedRef <> IntPtr.Size Then

                Console.WriteLine("Zeroing out unmanaged memory...")

                Marshal.ZeroFreeCoTaskMemAnsi(unmanagedRef)

            End If 

        End Try

        Console.WriteLine("Done.")

        Console.ReadLine()

    End Sub 



    Function GetPassword() As SecureString
        Dim password As New SecureString()

        ' get the first character of the password 
        Dim nextKey As ConsoleKeyInfo = Console.ReadKey(True)

        While nextKey.Key <> ConsoleKey.Enter
            If nextKey.Key = ConsoleKey.BackSpace Then 
                If password.Length > 0 Then
                    password.RemoveAt(password.Length - 1)

                    ' erase the last * as well
                    Console.Write(nextKey.KeyChar)
                    Console.Write(" ")
                    Console.Write(nextKey.KeyChar)
                End If 
            Else
                password.AppendChar(nextKey.KeyChar)
                Console.Write("*")
            End If

            nextKey = Console.ReadKey(True)
        End While

        Console.WriteLine()

        ' lock the password down
        password.MakeReadOnly()
        Return password

    End Function 
End Module

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

.NET for Windows Phone apps

Supported in: Windows Phone 8.1

Portable Class Library

Supported in: Portable Class Library

  • SecurityCriticalAttribute 

    requires full trust for the immediate caller. This member cannot be used by partially trusted or transparent code.

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