C# Signature:

[DllImport("advapi32", CharSet=CharSet.Auto, SetLastError=true)]
static extern bool ConvertSidToStringSid(
    [MarshalAs(UnmanagedType.LPArray)] byte [] pSID,
    out IntPtr ptrSid);

Can't get the above to work, but this works for me

[DllImport("advapi32.dll", CharSet=CharSet.Auto, SetLastError=true)]
static extern bool ConvertSidToStringSid(  IntPtr ptrSid,
    [ MarshalAs(UnmanagedType.LPTStr)] ref string pSID );

VB Signature:

Declare Auto Function ConvertSidToStringSid Lib "advapi32.dll" (ByVal pSID() As Byte, _
   ByRef ptrSid As IntPtr) As Boolean

Notes:

As explained here, memory for the returned unmanaged string must be freed by calling LocalFree. Therefore, you must define the string as an IntPtr in managed code and call LocalFree yourself. Otherwise, the marshaler would call CoTaskMemFree which is not correct in this case.

Tips & Tricks:

Please add some!

Sample Code:

// C# sample
public static string GetSidString(byte[] sid)
{
   IntPtr ptrSid;
   string sidString;
   if (!ConvertSidToStringSid(sid,out ptrSid))
     throw new System.ComponentModel.Win32Exception();
   try
   {
     sidString = Marshal.PtrToStringAuto(ptrSid);
   }
   finally
   {
     LocalFree(ptrSid);
   }
   return sidString;
}

//Another C# Sample that converts a sid from a DirectoryEntry object

private string GetTextualSID(DirectoryEntry objGroup){
  string sSID = string.Empty;
  byte[] SID = objGroup.Properties["objectSID"].Value as byte[];
  IntPtr sidPtr = Marshal.AllocHGlobal( SID.Length);
  sSID = "";
  System.Runtime.InteropServices.Marshal.Copy(SID, 0, sidPtr, SID.Length);
  ConvertSidToStringSid((IntPtr)sidPtr, ref sSID);
  System.Runtime.InteropServices.Marshal.FreeHGlobal( sidPtr );
  return sSID;}

'VB Sample
Public Shared Function ByteArrayToStringSid(ByRef bArray As Byte()) As String
   Dim ptrSID As IntPtr = Nothing
   Try
     Dim sSID As String = String.Empty
     If ConvertSidToStringSid(bArray, ptrSID) = True Then
       'The PtrToStringXXX call here needs to match the CharSet used on your
       'ConvertSidToStringSid DllImport.  The default is CharSet.Ansi.
       sSID = System.Runtime.InteropServices.Marshal.PtrToStringAnsi(ptrSID)
     End If
     Return sSID
   Finally
     LocalFree(ptrSID)
   End Try
End Function

'Alternative VB Sample
Public Shared Function ByteArrayToStringSid(ByRef bArray As Byte()) As String
   Dim ptrSID As IntPtr = Nothing
   Try
     Dim sSID As String = String.Empty
     If ConvertSidToStringSid(bArray, ptrSID) = True Then
       sSID = System.Runtime.InteropServices.Marshal.PtrToStringAuto(ptrSID)
     End If
     Return sSID
   Finally
     System.Runtime.InteropServices.Marshal.FreeHGlobal(ptrSID)
   End Try
End Function

Alternative Managed API:

Do you know one? Please contribute it!

Alternative Managed Code:

private string ConvertByteToStringSid(Byte[] sidBytes)

    {
        short sSubAuthorityCount = 0;
        StringBuilder strSid = new StringBuilder();
        strSid.Append("S-");
        try
        {
        // Add SID revision.
        strSid.Append(sidBytes[0].ToString());

        sSubAuthorityCount = Convert.ToInt16(sidBytes[1]);

        // Next six bytes are SID authority value.
        if (sidBytes[2] != 0 || sidBytes[3] != 0)
        {
            string strAuth = String.Format("0x{0:2x}{1:2x}{2:2x}{3:2x}{4:2x}{5:2x}",
                           (Int16) sidBytes[2],
                           (Int16) sidBytes[3],
                           (Int16) sidBytes[4],
                           (Int16) sidBytes[5],
                           (Int16) sidBytes[6],
                           (Int16) sidBytes[7]);
            strSid.Append("-");
            strSid.Append(strAuth);
        }
        else
        {
            Int64 iVal = sidBytes[7] +
                 (sidBytes[6] << 8) +
                 (sidBytes[5] << 16) +
                 (sidBytes[4] << 24);
            strSid.Append("-");
            strSid.Append(iVal.ToString());
        }

        // Get sub authority count...
        int idxAuth = 0;
        for (int i = 0; i < sSubAuthorityCount; i++)
        {
            idxAuth = 8 + i*4;
            UInt32 iSubAuth = BitConverter.ToUInt32(sidBytes, idxAuth);
            strSid.Append("-");
            strSid.Append(iSubAuth.ToString());
        }
        }
        catch (Exception ex)
        {
        Trace.TraceWarning(ex.Message);
        throw;
        }
        return strSid.ToString();
    }