A call to PInvoke function has unbalanced the stack

A call to PInvoke function has unbalanced the stack

Making a function call to .NET 4 to native code is resulting in the
following exception:
A call to PInvoke function has unbalanced the stack. This is likely
because the managed PInvoke signature does not match the unmanaged target
signature. Check that the calling convention and parameters of the PInvoke
signature match the target unmanaged signature.
Here is my definition in Native Code:
typedef void ( CALLBACK* CB_DOWNLOADING )
(
ULONG, // secs elapsed
LPARAM, // custom callback param
LPBOOL // abort?
);
FETCH_API HttpFetchW
(
LPCWSTR url,
IStream** retval,
LPCWSTR usrname = NULL,
LPCWSTR pwd = NULL,
BOOL unzip = TRUE,
CB_DOWNLOADING cb = NULL,
LPARAM cb_param = 0,
LPWSTR ctype = NULL,
ULONG ctypelen = 0
);
Here is the .NET counterpart:
[DllImport(dllPath, CharSet = CharSet.Unicode, SetLastError = true,
CallingConvention = CallingConvention.Cdecl)]
internal static extern FETCH HttpFetchW
(
[MarshalAs(UnmanagedType.LPWStr)]
string url,
out System.Runtime.InteropServices.ComTypes.IStream retval,
[MarshalAs(UnmanagedType.LPWStr)]
string usrname,
[MarshalAs(UnmanagedType.LPWStr)]
string pwd,
bool unzip,
dlgDownloadingCB cb,
IntPtr cb_param,
[MarshalAs(UnmanagedType.LPWStr)]
string ctype,
ulong ctypelen
);
Where FETCH is an enum.
[UnmanagedFunctionPointerAttribute(CallingConvention.Cdecl)]
internal delegate void dlgDownloadingCB(ulong elapsedSec, IntPtr
lParam, bool abort);
internal static void DownloadingCB(ulong elapsedSec, IntPtr lParam, bool
abort)
{
// Console.WriteLine("elapsedSec = " + elapsedSec.ToString());
}
Can anyone suggest an alternative within the .NET 4?
Thank you very much for your help.