using System.DirectoryServices;
DirectoryEntry dirEntry = new DirectoryEntry();
dirEntry.Path = "LDAP://IP/DC=;DC=;DC=;DC=";
dirEntry.Username = txtUser.Text;
dirEntry.Password = txtPassword.Text;
dirEntry.AuthenticationType = AuthenticationTypes.Secure;
DirectorySearcher search = new DirectorySearcher(dirEntry);
search.Filter = "(&(objectClass=user)(User=" + txtUser.Text + "))";
search.PropertiesToLoad.Add("SAMAccountName");
search.SearchScope = SearchScope.Subtree;
SearchResult result = search.FindOne();
using (SearchResultCollection results = search.FindAll())
{session["User"]=""; Response.Redirct();}