如果你在.NET 3.5和和谈论的Active Directory，那么你应该看看 System.DirectoryServices.AccountManagement （S.DS.AM）命名空间。阅读所有关于它的：

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:

• 在.NET Framework管理目录安全主体3.5
• 上System.DirectoryServices.AccountManagement MSDN文档

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

基本上，你可以定义域范围内，并很容易地找到在AD用户和/或组：

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;
}

新S.DS.AM使得它可以很容易地玩弄用户和组AD！

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

更新：如果您通过域的所有用户需要循环 - 试试这个：

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

您可以使用 PrincipalSearcher 和查询通过例如主要做你的搜索：

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;
    }
}

更新＃2：如果您不能（或不愿）使用 S.DS.AM 办法 - 这是最简单的，对于Active Directory，迄今为止 - 那么你需要回退到的System.DirectoryServices 类和方法：

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();
   }
}