Querying an LDAP

@KenL Almost got me there. I also had to set the AuthenticationType of the DirectoryEntry to get it to work. Also, pay attention to how you are using wildcards (Kleene Stars).

DirectoryEntry rootEntry = new DirectoryEntry("LDAP://some.ldap.server.com");
rootEntry.AuthenticationType = AuthenticationTypes.None; //Or whatever it need be
DirectorySearcher searcher = new DirectorySearcher(rootEntry);
var queryFormat = "(&(objectClass=user)(objectCategory=person)(|(SAMAccountName=*{0}*)(cn=*{0}*)(gn=*{0}*)(sn=*{0}*)(email=*{0}*)))";
searcher.Filter = string.Format(queryFormat, searchString);
foreach(SearchResult result in searcher.FindAll()) 
{
    Console.WriteLine("account name: {0}", result.Properties["samaccountname"].Count > 0 ? result.Properties["samaccountname"][0] : string.Empty);
    Console.WriteLine("common name: {0}", result.Properties["cn"].Count > 0 ? result.Properties["cn"][0] : string.Empty);
}

First Element of response, using ADSI (old fashion)

How to do Almost everything (with ADSI) on Active Directory with C#

Second Element of response, begining .NET 3.5 Microsoft introduce 'Principal' and 'AccountManagement'.

How to do Almost everything (with AccountManagement) on Active Directory with C#

Third Element of response, you can use low level (native LDAP) protocol with System.DirectoryServices.Protocols (S.DS.P).

Remark : If you are interested in how to interrogate active directory from native code, you may have a look to LDAP C-Binding API as discribed in RFC 1823 specifies, Microsoft support it, see MS Strategy for Lightweight Directory Access Protocol (LDAP). You'll find the using and reference manuals of the Microsoft API in Lightweight Directory Access Protocol.

code wise it is a lot simpler than you think. You need to create a connection to the directory, set up a searcher, then search by property name.

DirectoryEntry entry = new DirectoryEntry("LDAP://MyDomain.com");
DirectorySearcher dSearch = new DirectorySearcher(entry);
dSearch.Filter = "(&(objectCategory=person)(objectClass=user)(" + SType + "=" + Name + "))";

SType is the name type, and Name is the actual user name