Unknown username or bad password, LDAP Active Directory - asp.net

Question

I'm trying to authenticate against AD using application mode (ADAM), but keep getting unknown username or bad password. If I test the login in LDP.exe it logs in no problem, on simple bind. I've trawled through all similar posts with the same issue, but have not resolved it, any suggestions what I should be checking for?
private bool ValidateActiveDirectoryLogin(string Username, string Password)
{
bool Success = false;
System.DirectoryServices.DirectoryEntry Entry = new System.DirectoryServices.DirectoryEntry("LDAP://localhost:389/OU=Users,O=TestDirectory", Username, Password);
System.DirectoryServices.DirectorySearcher Searcher = new System.DirectoryServices.DirectorySearcher(Entry);
Searcher.SearchScope = System.DirectoryServices.SearchScope.Subtree;
try
{
System.DirectoryServices.SearchResult Results = Searcher.FindOne();
Success = (Results != null);
}
catch (Exception ex)
{
Success = false;
throw;
}
return Success;
}

Answer 1

Determine what context your application is hitting AD with. If your ASP.NET application pool identity is one that is low privileged, it won't have enough permissions to query active directory. If you don't want to create a custom user to run the app pool as with appropriate permissions - you could use the LogonUser API to make your ValidateActiveDirectoryLogin call under the security context of that account.
Finally, you should consider using System.DirectoryServices.AccountManagement if you are using .NET 3.5 or above.
You can use code like
bool validCreds = false;
using (PrincipalContext context = new PrincipalContext(ContextType.Domain))
{
validCreds = context.ValidateCredentials( username, password );
}