public bool IsUserExistInActiveDirectory(string UserId)
{
bool IsValidLoginName = false;
string domain = "LDAP://" + ConfigurationSettings.AppSettings["Domain"];
System.DirectoryServices.DirectoryEntry entry = 
new DirectoryEntry(domain, ConfigurationSettings.AppSettings["ADServiceAccount"], ConfigurationSettings.AppSettings["ADServiceAccountPassword"],
AuthenticationTypes.Secure);

DirectorySearcher adSearcher = new DirectorySearcher(entry);
adSearcher.SearchScope = SearchScope.Subtree;
adSearcher.Filter = "(&(objectClass=user)(samaccountname=" + UserId + "))"; 
SearchResult oResult = adSearcher.FindOne();
if ( oResult != null)
{
IsValidLoginName = true;
}
return IsValidLoginName;
}
