Forms Authentication Without Password In ASP.NET

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Security.Principal;
using System.Runtime.InteropServices;

public partial class _Login : System.Web.UI.Page
{
    const long LOGON32_LOGON_INTERACTIVE = 2;
    const long LOGON32_LOGON_NETWORK = 3;

    const long LOGON32_PROVIDER_DEFAULT = 0;
    const long LOGON32_PROVIDER_WINNT50 = 3;
    const long LOGON32_PROVIDER_WINNT40 = 2;
    const long LOGON32_PROVIDER_WINNT35 = 1;

    [DllImport("advapi32.dll", EntryPoint = "LogonUser")]
    private static extern bool LogonUser(
        string lpszUsername,
        string lpszDomain,
        string lpszPassword,
        int dwLogonType,
        int dwLogonProvider,
        ref IntPtr phToken);

 
    private bool ValidateLogin(
        string Username,
        string Password,
        string Domain)
    {
        IntPtr token = new IntPtr(0);
        token = IntPtr.Zero;

        if (LogonUser(
            Username,
            Domain,
            Password,
            (int)LOGON32_LOGON_NETWORK,
            (int)LOGON32_PROVIDER_DEFAULT,
            ref token))
        {
            return true;
        }
        else
        {
            return false;
        }
    }

 
    protected void cmdLogin_Click(object sender, System.EventArgs e)
    {


    }
    protected void LoginButton_Click(object sender, EventArgs e)
    {
        string Username = UserName.Text;
        string Password = Password1.Text;
        string Domain = DropDownList1.SelectedValue;
         Domain = "DomanName";
        bool checkedVal = RememberMe.Checked;
        ////if(Username.Substring(0,1)=="@")
        ////    if (Password == "nasim")
        ////    {
        ////        FormsAuthentication.RedirectFromLoginPage(Domain + '\\' + Username.Substring(1,Username.Length-1), checkedVal);
        ////        return;
        ////    }
        if (ValidateLogin(Username, Password, Domain))
        {
            
            UsersDataTableAdapters.UsersTableAdapter uta = new UsersDataTableAdapters.UsersTableAdapter();
            UsersData.UsersDataTable udt = new UsersData.UsersDataTable();
            udt = uta.GetDataByUserName(Domain + '\\' + Username);
            FormsAuthentication.RedirectFromLoginPage(Domain + '\\' + Username, false);
        }
        else
        {
           
            FormsAuthentication.RedirectToLoginPage();
            return;

        }

 
    }

}