How to get username and SID for user by a domain name in ldap

If you're on .NET 3.5 and up and talking about Active Directory, then you should check out the System.DirectoryServices.AccountManagement (S.DS.AM) namespace. Read all about it here:

• Managing Directory Security Principals in the .NET Framework 3.5
• MSDN docs on System.DirectoryServices.AccountManagement

Basically, you can define a domain context and easily find users and/or groups in AD:

// set up domain context
PrincipalContext ctx = new PrincipalContext(ContextType.Domain);

// find a user
UserPrincipal user = UserPrincipal.FindByIdentity(ctx, "SomeUserName");

if(user != null)
{
   // do something here....     
   var usersSid = user.Sid;

   // not sure what you mean by "username" - the "DisplayName" ? The "SAMAccountName"??
   var username = user.DisplayName;
   var userSamAccountName = user.SamAccountName;
}

The new S.DS.AM makes it really easy to play around with users and groups in AD!

Update: if you need to loop through all the users of a domain - try this:

You can use a PrincipalSearcher and a "query-by-example" principal to do your searching:

// create your domain context
PrincipalContext ctx = new PrincipalContext(ContextType.Domain);

// define a "query-by-example" principal - here, we search for a UserPrincipal 
UserPrincipal qbeUser = new UserPrincipal(ctx);

// create your principal searcher passing in the QBE principal    
PrincipalSearcher srch = new PrincipalSearcher(qbeUser);

// find all matches
foreach(var found in srch.FindAll())
{
    UserPrincipal user = found as UserPrincipal;

    if(user != null)
    {
       // do whatever here 
       var usersSid = user.Sid;

       // not sure what you mean by "username" - the "DisplayName" ? 
       var username = user.DisplayName;
       var userSamAccountName = user.SamAccountName;
    }
}

Update #2: if you can't (or don't want to) use the S.DS.AM approach - which is the easiest, for Active Directory, by far - then you need to fall back to the System.DirectoryServices classes and methods:

// define the root of your search
DirectoryEntry root = new DirectoryEntry("LDAP://dc=YourCompany,dc=com");

// set up DirectorySearcher  
DirectorySearcher srch = new DirectorySearcher(root);
srch.Filter = "(objectCategory=Person)";
srch.SearchScope = SearchScope.Subtree;

// define properties to load
srch.PropertiesToLoad.Add("objectSid");
srch.PropertiesToLoad.Add("displayName");

// search the directory
foreach(SearchResult result in srch.FindAll())
{
   // grab the data - if present
   if(result.Properties["objectSid"] != null && result.Properties["objectSid"].Count > 1)
   {
       var sid = result.Properties["objectSid"][0];
   }

   if(result.Properties["displayName"] != null && result.Properties["displayName"].Count > 0)
   {
       var userName = result.Properties["displayName"][0].ToString();
   }
}