Active Directory/LDAP :: Error "Invalid Dn Syntax Specified" When Try To Use Authentication Form With LDAP

I tried to use Authentication form with LDAP but when I run my web application I received error: Invalid dn syntax specified. This is my code:

public bool IsAuthenticated(string domain, string username, string password)
{
// string domainAndUsername = domain + @"" + username;
string domainAndUsername = "http://localhost:389
jsmith";
// DirectoryEntry entry = new DirectoryEntry(_path, domainAndUsername, password);// _path = LDAP://localhost:389/dc=example,dc=com
DirectoryEntry entry = new DirectoryEntry("LDAP://localhost:389",domainAndUsername,"rJsmitH");
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 (System.Exception ex)
{
throw new Exception("Error authenticating user. " + ex.Message);
}
return true;
}

I got exception at line Object obj = entry.NativeObject; and error message is: Invalid dn syntax specified. how can i take a right dn syntax? I tried some domain name such as: LDAP://localhost:389, [URL], ... but I still get this error