[c#] DirectorySearcher Filter

When I run this query

// Next row is used to login to AD
DirectoryEntry entry = GetEntry(domain, adminUser, adminPassword);
// Here starts the query
DirectorySearcher search = new DirectorySearcher(entry)
{
    SearchScope = SearchScope.Subtree,
    Filter = "(&" +
        "(objectClass=user)" +
        // "(distinguishedname=*OU=Ingegneria*)" +
        "(givenname=s*)" +
        "(samaccountname=*100)" +
    ")"
};
search.PropertiesToLoad.Add("distinguishedname");
SearchResultCollection result = search.FindAll();

I get six entries and that's correct.
All records, if I use record.GetDirectoryEntry() have

distinguishedname: CN=xxx,OU=Utenti,OU=Ingegneria,DC=xxx,DC=xxx

Anyway if I remove comment on distinguishedname part of the filter, I get zero entries!!
I also tried to use search.PropertiesToLoad.Add("distinguishedname"); without luck.
How can I search distinguishedname in filter?

UPDATE:
If I try to use "(distinguishedname=*)" + in filter , I still get six records, so I think I can search on distinguishedname...
UPDATE2:
I also tried to use code in Search Active Directory for an OU using a partial path to the OU:

Filter = "(&(objectClass=user)(ou=Ingegneria))";

but I have zero entries (I got two if I remove (objectClass=user) part)

This question is related to c# active-directory

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