First you need to System.DirectoryServices namespace, to check if the user is Authenticated in LDAP (active directory) then you can use below snippet
public bool IsAuthenticated(string domain, string username, string pwd)
{
string domainAndUsername = domain + @"\" + username;
DirectoryEntry entry = new DirectoryEntry(_path, domainAndUsername, pwd);
try
{
object obj = entry.NativeObject;
DirectorySearcher search = new DirectorySearcher(entry);
search.Filter = "(SAMAccountName=" + username + ")";
search.PropertiesToLoad.Add("cn");
SearchResult result = search.FindOne();
if(null == result)
{
return false;
}
_path = result.Path;
_filterAttribute = (string)result.Properties["cn"][0];
}
catch (Exception ex)
{
throw new Exception("Error authenticating user. " + ex.Message);
}
return true;
}