Recently I faced a weird issue where I was not able to run the LDAP queries using Directory Services API of .Net for other domains through legitmate user accounts.
If I was logged into Domain xyz.com, and I was able to run the LDAP queries in xyz.com domain, I was not able to run the queries on abc.com AD from xyz.com.
After digging and running through a few links, here is the code change that helped me fix the issue.

Before

Check the DirectoryEntry Path properties, it's set with direct ad domain.
  1. using (DirectoryEntry rootEntry = new DirectoryEntry())
  2. {
  3. rootEntry.Path = $"LDAP://dc=abc,dc=com";
  4. rootEntry.Username = yourid;
  5. rootEntry.Password = yourpassword;
  7. using (DirectorySearcher _searcher = new DirectorySearcher(rootEntry))
  8. {
  9. _searcher.Filter = filter;
  10. _searcher.PageSize = 1000;
  13. SearchResultCollection _searchResultCollections = _searcher.FindAll();
  14. }
  15. }

Fix

Changing the DirectoryEntry Path to explictly connect to the 389 port fixed the issue.
  1. rootEntry.Path = $"LDAP://abc.com:389/dc=abc,dc=com";
Here is the MSDN article that explains in detail how the lDAP query search works, that gave me some hints.
Happy coding.