Need help translating a C++ method to C#

[kboek]

I'm trying to use a method defined in the Windows Media Format SDK (wmsdkidl.h)

virtual HRESULT STDMETHODCALLTYPE AcquireCredentials( 
            /* [in] */ WCHAR *pwszRealm,
            /* [in] */ WCHAR *pwszSite,
            /* [size_is][out] */ WCHAR *pwszUser,
            /* [in] */ DWORD cchUser,
            /* [size_is][out] */ WCHAR *pwszPassword,
            /* [in] */ DWORD cchPassword,
            /* [in] */ HRESULT hrStatus,
            /* [out] */ DWORD *pdwFlags) = 0;

In C++ I have some sample code that actually implements this and it's working

HRESULT CNetWrite::AcquireCredentials( WCHAR *pwszRealm, 
                                      WCHAR *pwszSite, 
                                      WCHAR *pwszUser, 
                                      DWORD cchUser, 
                                      WCHAR *pwszPassword, 
                                      DWORD cchPassword, 
                                      HRESULT hrStatus, 
                                      DWORD *pdwFlags )
{
    HRESULT hr = S_OK;
 
    WCHAR *wszUser     = NULL;
    WCHAR *wszPassword = NULL;
 
    hr = ConvertTCharToWChar2( "MyUsername", &wszUser );
    hr = ConvertTCharToWChar2( "MyPassword", &wszPassword );
 
    wcscpy(pwszUser, wszUser);
    wcscpy(pwszPassword, wszPassword);
 
    (*pdwFlags) = 0x00;
 
    SAFE_ARRAYDELETE( wszUser ); 
    SAFE_ARRAYDELETE( wszPassword );
 
    return S_OK;
}

But I cannot get it to work in C#. Until now, I have the following definition

[ComImport]
[Guid("342e0eb7-e651-450c-975b-2ace2c90c48e")]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown  )]
public interface IWMCredentialCallback
{
  void AcquireCredentials(
    [In, MarshalAs(UnmanagedType.LPWStr)] string bstrRealm,
    [In, MarshalAs(UnmanagedType.LPWStr)] string bstrSite,
    [In, Out, MarshalAs(UnmanagedType.LPWStr)] StringBuilder pwszUser,
    [In] uint cchUser,
    [In, Out, MarshalAs(UnmanagedType.LPWStr)] StringBuilder pwszPassword,
    [In] uint cchPassword,
    [In] IntPtr hrStatus,
    [In, Out] uint pdwFlags);
}

with the following implementation

public void AcquireCredentials(string bstrRealm, string bstrSite, StringBuilder pwszUser, uint cchUser, StringBuilder pwszPassword, uint cchPassword, IntPtr hrStatus, uint pdwFlags)
{
  pwszUser     = new StringBuilder("MyUsername");
  pwszPassword = new StringBuilder("MyPassword");
 
  pdwFlags = 0x00;
}

Can somebody help me make this method work in C#?