Timeout expired.  The timeout period elapsed prior to completion of the operation or the server is not responding.

   using SQLDMO;
   using System.Collections.Generic;

   public static List<string> GetSQLServersList2()
   {
       List<string> result = new List<string>();
       ApplicationClass sqlApp = new ApplicationClass();
       NameList lst = sqlApp.ListAvailableSQLServers();
       for (int i = 1; i <= lst.Count; i++)
           result.Add(lst.Item(i));
       lst = null;
       sqlApp = null;

       return result;

   }

using System.Collections.Generic;
using System.Data;
using System.Data.Sql;

public class CListServers
{
   public static List<string> GetSQLServersList()
   {
       List<string> result = new List<string>();

       // Retrieve the enumerator instance and then the data.
       var instance = SqlDataSourceEnumerator.Instance;
       var table = instance.GetDataSources();

       // Display the contents of the table.
       foreach (DataRow row in table.Rows)
       {
           result.Add(string.Format("{0}\\{1}", row[0], row[1]));
       }

       return result;
   }
}

using System.Collections.Generic;
using System.Data;
using Microsoft.SqlServer.Management.Smo;

public class CListServers
{
   public static List<string> GetSQLServersListSMO()
   {
       List<string> result = new List<string>();
       DataTable dt = SmoApplication.EnumAvailableSqlServers(false);
       if (dt.Rows.Count > 0)
       {
           foreach (DataRow dr in dt.Rows)
           {
               result.Add(dr["Name"].ToString());
           }
       }
       return result;
   }
}