Show Authentication dialog in C# for windows Vista/7

I managed to implement a solution that is working for me.

Here is the source code:

    [DllImport("ole32.dll")]
    public static extern void CoTaskMemFree(IntPtr ptr);

    [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
    private struct CREDUI_INFO
    {
        public int cbSize;
        public IntPtr hwndParent;
        public string pszMessageText;
        public string pszCaptionText;
        public IntPtr hbmBanner;
    }  


    [DllImport("credui.dll", CharSet = CharSet.Auto)]
    private static extern bool CredUnPackAuthenticationBuffer(int dwFlags,
                                                               IntPtr pAuthBuffer,
                                                               uint cbAuthBuffer,
                                                               StringBuilder pszUserName,
                                                               ref int pcchMaxUserName,
                                                               StringBuilder pszDomainName,
                                                               ref int pcchMaxDomainame,
                                                               StringBuilder pszPassword,
                                                               ref int pcchMaxPassword);

    [DllImport("credui.dll", CharSet = CharSet.Auto)]
    private static extern int CredUIPromptForWindowsCredentials(ref CREDUI_INFO notUsedHere,
                                                                 int authError,
                                                                 ref uint authPackage,
                                                                 IntPtr InAuthBuffer,
                                                                 uint InAuthBufferSize,
                                                                 out IntPtr refOutAuthBuffer,
                                                                 out uint refOutAuthBufferSize,
                                                                 ref bool fSave,
                                                                 int flags);



    public static void GetCredentialsVistaAndUp(string serverName, out NetworkCredential networkCredential)
    {
        CREDUI_INFO credui = new CREDUI_INFO();
        credui.pszCaptionText = "Please enter the credentails for " + serverName;
        credui.pszMessageText = "DisplayedMessage";
        credui.cbSize = Marshal.SizeOf(credui);
        uint authPackage = 0;
        IntPtr outCredBuffer = new IntPtr();
        uint outCredSize;
        bool save = false;
        int result = CredUIPromptForWindowsCredentials(ref credui,
                                                       0,
                                                       ref authPackage,
                                                       IntPtr.Zero,
                                                       0,
                                                       out outCredBuffer,
                                                       out outCredSize,
                                                       ref save,
                                                       1 /* Generic */);

        var usernameBuf = new StringBuilder(100);
        var passwordBuf  = new StringBuilder(100);
        var domainBuf = new StringBuilder(100);

        int maxUserName = 100;
        int maxDomain = 100;
        int maxPassword = 100;
        if (result == 0)
        {
            if (CredUnPackAuthenticationBuffer(0, outCredBuffer, outCredSize, usernameBuf, ref maxUserName,
                                               domainBuf, ref maxDomain, passwordBuf, ref maxPassword))
            {
                //TODO: ms documentation says we should call this but i can't get it to work
                //SecureZeroMem(outCredBuffer, outCredSize);

                //clear the memory allocated by CredUIPromptForWindowsCredentials 
                CoTaskMemFree(outCredBuffer);
                networkCredential = new NetworkCredential()
                                        {
                                            UserName = usernameBuf.ToString(),
                                            Password = passwordBuf.ToString(),
                                            Domain = domainBuf.ToString()
                                        };
                return;
            }
        }

        networkCredential = null;
    }

I still need to work out the fine details such as how to remember the last credentials that were entered etc…

But the major part works.