Question

    public static List<T> GetColumnValuesByHeader<T>(string docName)
    {
      //  How can i get T as Customer or Employee in here?    
    }

List<Customer> customers = GetColumnValuesByHeader<Customer>("Customers.dat")
List<Employee> employees= GetColumnValuesByHeader<Employee>("Employees.dat")

Answer 1

As Jon Skeet said you can use typeof to get the type of T. You can also use the Activator class to create an instance of that class.

Type theType = typeof(T);
object obj = Activator.CreateInstance(theType);