How to access Abstract Class Method

As Murali said, abstract classes can't be instantiated and it's good practice to make any constructor protected rather than public so you can't try and do this by mistake. If you don't supply a constructor for an abstract class, then the system provides a default protected, parameterless constructor which does nothing apart from calling its base class constructor.

Although you need to create an instance of a class which derives from clscon in order to open the connection created in the abstract class, you don't need to redeclare the protected field 'con' which will give you a compiler error saying that you're hiding the base class member and, worse still, will reset it to a new SqlConnection object with no associated connection string. So I'd do instead:

namespace nsDal
{
    public abstract class clscon
    {
        protected SqlConnection con = new SqlConnection();
        protected clscon()
        {
            con.ConnectionString = ConfigurationManager.ConnectionStrings["cn"].ConnectionString;
        }
    }


    public  class clscon1 : clscon
    {
       public void OpenConnection()
       {
          con.Open();
       }
    }
}


// code to open connection

clscon1 con1 = new clscon1();
con1.OpenConnection();