How to get all the AD users in different forests in c#

Question

I have MVC2 application which has windows authentication. for the user mapping purpose I have loaded all the AD users in to the drop down.I have new requirement to that take all AD users in different domains to above mentioned drop down.In order i Need to get user's details such as first name, last name etc.Can some one tell is there any possible way to implement this using c#? Below is my code that i am using to take all AD users in single forest.

List<UserMgtModelsMetadata> userList = new List<UserMgtModelsMetadata>();
            string username = ConfigurationManager.AppSettings["ADUsername"].ToString();
            string password = ConfigurationManager.AppSettings["ADPassword"].ToString();
            string path = ConfigurationManager.AppSettings["Path"].ToString();
            string domain = ConfigurationManager.AppSettings["Domain"].ToString();
            DirectoryEntry myLdapConnection = new DirectoryEntry(path, username, password);

            DirectorySearcher search = new DirectorySearcher(myLdapConnection);

            search.PropertiesToLoad.Add("displayname");
            search.PropertiesToLoad.Add("samaccountname");  //Username
            search.PropertiesToLoad.Add("givenname");   //Firstname
            search.PropertiesToLoad.Add("sn");  //Lastname
            search.PropertiesToLoad.Add("userprincipalname");
            search.PropertiesToLoad.Add("memberof");

            SearchResultCollection result = search.FindAll();

            if (result != null)
            {
                for (int i = 0; i < result.Count; i++)
                {
                    ResultPropertyCollection fields = result[i].Properties;

                    if ((fields["displayname"].Count > 0) && (fields["samaccountname"].Count > 0) && (fields["givenname"].Count > 0) && (fields["sn"].Count > 0) && (fields["userprincipalname"].Count > 0))
                    {
                        string mail = fields["userprincipalname"][0].ToString();
                        int index = mail.IndexOf("@") > 0 ? mail.IndexOf("@") : 0;

                        string fullName = (fields["givenname"][0].ToString() +" "+ fields["sn"][0].ToString());
                        if (fullName.Length > 20)
                        {
                            fullName = fullName.Substring(0, 20);
                        }

                        if (!group.Equals(" "))
                        {
                            bool isMember = false;
                            foreach (var item in fields["memberof"])
                            {
                                isMember = item.ToString().Contains("CN=" + group);
                                if (isMember == true)
                                {
                                    break;
                                }
                            }

                            if (isMember == true)
                            {
                                userList.Add(new UserMgtModelsMetadata
                                                    {
                                                        displayName = fullName,
                                                        username = fields["samaccountname"][0].ToString(),
                                                        firstName = fields["givenname"][0].ToString(),
                                                        lastname = fields["sn"][0].ToString(),
                                                        email = mail.Substring(0, index).ToLower() + domain
                                                    });
                            } 
                        }
                        else
                        {
                            userList.Add(new UserMgtModelsMetadata
                                                {
                                                    displayName = fullName,
                                                    username = fields["samaccountname"][0].ToString(),
                                                    firstName = fields["givenname"][0].ToString(),
                                                    lastname = fields["sn"][0].ToString(),
                                                    email = mail.Substring(0, index).ToLower() + domain
                                                }); 
                        }
                    }
                }`