If an OU contains 3000 users, how to use DirectorySearcher to find all of them?

I use this code:

DirectoryEntry objEntry;
DirectorySearcher objSearchEntry;
SearchResultCollection objSearchResult;
string strFilter = "(&(objectCategory=User))";
objEntry = new DirectoryEntry(conOUPath, conUser, conPwd, AuthenticationTypes.Secure);
objEntry.RefreshCache();
objSearchEntry = new DirectorySearcher(objEntry);
objSearchEntry.Filter=strFilter;
objSearchEntry.SearchScope=SearchScope.Subtree;
objSearchEntry.CacheResults=false;
objSearchResult=objSearchEntry.FindAll();

Each time, it only return 1000 users, but there are 3000 users in that OU.

How can i find all of them ?