Performing Queries Against Active Directory Domain Services

One of the missions that needed my attention lately was to check whether a user exists in an enterprise Active Directory.
The article will show exactly how to perform such a query.

The DirectoryEntry Class

The DirectoryEntry class represent an entry in Active Directory.
That entry live in memory when created and changes that you perform on it won’t be submitted to Active Directory unless you call the CommitChnages method. That class can be found in System.DirectoryServices namespace.
The following code shows how to create a DirectoryEntry object using a the path to the LDAP, username and password:

var entry = new DirectoryEntry(path, username, password);

The DirectorySearcher Class

The DirectorySearcher class enable us to perform queries against Active Directory. Once you have a DirectoryEntry in hand you can pass it to the DirectorySearcher and then commit queries to your Active Directory. When you create the DirectorySearcher you also supply the optional list of properties that you want to retrieve. As the DirectoryEntry, it is also available in the System.DirectoryServices namespace.
The following code shows how to create a DirectorySearcher with a given DirectoryEntry:

var searcher = new DirectorySearcher(entry);

How to Perform a Query Against Active Directory Domain Service

The following code snippet shows a simple method that perform a query against Active Directory:

private SearchResult SearchLDAPById(string userId, string path, string username, string password)
{
    var entry = new DirectoryEntry(path, username, password);
    var search = new DirectorySearcher(entry);
    search.Filter = string.Format("({0}={1})", "SAMAccountName", userId);
    search.PropertiesToLoad.Add("displayName");
    return search.FindOne();
}

The query returns the display name for a logon name of a user which is saved in Active Directory as SAMAccountName. As you can see we get back a SearchResult object which we can investigate for our retrieved display name.

Summary

In the article I showed how you can perform a query against Active Directory domain service. I also introduced the DirectoryEntry and DirectorySearcher classes. I hope you will find this information useful.