C# Signature:

[DllImport("advapi32.dll", SetLastError=true)]
static extern bool ConvertStringSidToSid(
            string StringSid,
            out IntPtr ptrSid
            );

VB Signature:

    Private Declare Auto Function ConvertSidToStringSid Lib "advapi32.dll" ( _
    ByVal Sid As IntPtr, _
    ByRef StringSid As IntPtr _
    ) As Boolean

User-Defined Types:

None.

Notes:

Dispose of the sid pointer using Marshal.FreeHGlobal() to avoid a memory leak

Tips & Tricks:

Please add some!

C#.Net Sample Code:

public void CopyFolderPermissions(string FromPath, string ToPath)

{

    ManagementBaseObject Descriptor = null;
    ManagementObjectCollection UserSearch = new ManagementObjectSearcher("Select * From Win32_LogicalFileSecuritySetting Where Path='" + FromPath + "'").Get();
    try
    {
        foreach(ManagementObject UserObject in UserSearch)
        {
            ManagementBaseObject inParams1 = UserObject.GetMethodParameters("GetSecurityDescriptor");
            ManagementBaseObject outParams1 = UserObject.InvokeMethod("GetSecurityDescriptor", inParams1, null);
            Descriptor = ((ManagementBaseObject)(outParams1.Properties["Descriptor"].Value));
        }
    }
    catch (Exception se)
    {
        Trace.WriteLine(se.Message);
    }

    ManagementObject Share = new ManagementObject("Win32_Directory='" + ToPath + "'");
    ManagementBaseObject inParams = Share.GetMethodParameters("ChangeSecurityPermissions");

    inParams["Option"] = 4;
    inParams["SecurityDescriptor"] = Descriptor;

    ManagementBaseObject outParams = Share.InvokeMethod("ChangeSecurityPermissions", inParams, null);

}

internal class Win32_Ace

{

    [DllImport("advapi32.dll", SetLastError=true)]
    static extern bool ConvertStringSidToSid(
        string lbBuffer,
        out IntPtr ptrSid);

    [DllImport("advapi32.dll", EntryPoint = "GetLengthSid", CharSet = CharSet.Auto)]
    static extern int GetLengthSid(IntPtr pSID);

    #region AccessMask
    public class AccessMask
    {
        public static uint FullAccess = 0x1F01FF;
        public static uint FILE_LIST_DIRECTORY = 0x1;
        public static uint FILE_ADD_FILE = 0x2;
        public static uint FILE_ADD_SUBDIRECTORY = 0x4;
        public static uint FILE_READ_EA = 0x8;
        public static uint FILE_WRITE_EA = 0x10;
        public static uint FILE_TRAVERSE = 0x20;
        public static uint FILE_DELETE_CHILD = 0x40;
        public static uint FILE_READ_ATTRIBUTES = 0x80;
        public static uint FILE_WRITE_ATTRIBUTES = 0x100;
        public static uint DELETE  = 0x10000;
        public static uint READ_CONTROL = 0x20000;
        public static uint WRITE_DAC = 0x40000;
        public static uint WRITE_OWNER = 0x80000;
        public static uint SYNCHRONIZE = 0x100000;
    }
    #endregion
    #region AceFlags
    public class AceFlags
    {
        public static uint OBJECT_INHERIT_ACE = 0x1;
        public static uint CONTAINER_INHERIT_ACE = 0x2;
        public static uint NO_PROPAGATE_INHERIT_ACE = 0x4;
        public static uint INHERIT_ONLY_ACE = 0x8;
        public static uint INHERITED_ACE = 0x10;
        public static uint SUCCESSFUL_ACCESS_ACE_FLAG = 0x40;
        public static uint FAILED_ACCESS_ACE_FLAG = 0x80;
    }
    #endregion
    #region AceType
    public class AceType
    {
        public static uint ACCESS_ALLOWED_ACE = 0;
        public static uint ACCESS_DENIED_ACE = 1;
        public static uint AUDIT_ACE = 2;
    }
    #endregion
            #region SecurtyDescriptor
    public class ControlFlags
    {
        public static uint SE_OWNER_DEFAULTED = 0x1;
        public static uint SE_GROUP_DEFAULTED = 0x2;
        public static uint SE_DACL_PRESENT = 0x4;
        public static uint SE_DACL_DEFAULTED = 0x8;
        public static uint SE_SACL_PRESENT = 0x10;
        public static uint SE_SACL_DEFAULTED = 0x20;
        public static uint SE_DACL_AUTO_INHERIT_REQ = 0x100;
        public static uint SE_SACL_AUTO_INHERIT_REQ = 0x200;
        public static uint SE_DACL_AUTO_INHERITED = 0x400;
        public static uint SE_SACL_AUTO_INHERITED = 0x800;
        public static uint SE_DACL_PROTECTED = 0x1000;
        public static uint SE_SACL_PROTECTED = 0x2000;
        public static uint SE_SELF_RELATIVE = 0x800;
    }
    #endregion

    public static ManagementObject GetInstance(string Account, uint
        AccessMask, uint AceType, uint AceFlags)
    {
        ManagementObject Ace = new
            System.Management.ManagementClass("Win32_Ace").CreateInstance();
        Ace["Trustee"] = Win32_Trustee(Account);
        Ace["AccessMask"] = AccessMask;
        Ace["AceType"] = AceType;
        Ace["AceFlags"] = AceFlags;
        return Ace;
    }

    private static ManagementObject Win32_Trustee(string Account)
    {
        byte[] SID = null;
        ManagementObjectCollection UserSearch =
            new ManagementObjectSearcher("Select * From Win32_Account Where Name = '" + Account + "'").Get();
        try
        {
            foreach(ManagementObject UserObject in UserSearch)
            {
                IntPtr SID_ptr=new IntPtr(0);
                ConvertStringSidToSid(UserObject["SID"].ToString(), out SID_ptr);
                int size = GetLengthSid(SID_ptr);
                SID = new byte[size];
                Marshal.Copy(SID_ptr, SID, 0, size);
                Marshal.FreeHGlobal(SID_ptr);
            }
        }
        catch (Exception se)
        {
            Trace.WriteLine(se.Message);
        }
        ManagementObject Trustee = new System.Management.ManagementClass("Win32_Trustee").CreateInstance();
        Trustee["SID"] = SID;
        return Trustee;
    }

}

Alternative Managed API:

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