using(SqlConnection sqlcon = new SqlConnection(constring))
  {
   }

using(SqlConnection sqlcon = new SqlConnection(constring))
  {
   }

Dispose() method permanently removes connection object from memory and the resource no longer exists for any further          processing.

string constring = "Server=(local);Database=my; User Id=sa; Password=sa";
SqlConnection sqlcon = new SqlConnection(constring);
 sqlcon.Open();   // connection is open

try
{
//  code here which will be execute
}
catch
{
     // code  will be execute when  error occurred in try block
}
finally
{
 sqlcon.Dispose();         // close and desroy the  connection object
}