C# and active directory

[Scatman]

Hi,
I wrote a program in C# that add user into the active directory. The program works fine. I would like to do two things.
First how can I join this tree rows to one?

DirectoryEntry ent = new DirectoryEntry();
DirectoryEntry ou2 = ent.Children.Find("OU=first");
DirectoryEntry ou = ou2.Children.Find("OU=second");
 

And second. How can I check if username already exists in AD in the specific container? I found this on internet but I dont understand if search whole AD structure or only specific container. If it's for whole structure how do I changes for specific container?

public bool UserExists(string UserName){
DirectoryEntry de = GetDirectoryEntry();
DirectorySearcher deSearch = new DirectorySearcher();
deSearch.SearchRoot =de;
deSearch.Filter = "(&(objectClass=user) (cn=" + UserName +"))"; 
SearchResultCollection results = deSearch.FindAll();
if(results.Count == 0){
return false;
}
else{
return true;
}
}