'Required' error even after adding the required namespace

Question

Look at the following code, I am using the required(System.ComponentModel.DataAnnotations) namespace but it keeps showing me red squiggly with the error that 'type or namespace name 'required' could not be found(are you missing a using directive.....)

using System.ComponentModel.DataAnnotations;

namespace SportsStore.Domain.Entities {

public class ShippingDetails {
    [Required(ErrorMessage = "Please enter a name")]
    public string Name { get; set; }

    [Required(ErrorMessage = "Please enter the first address line")]
    public string Line1 { get; set; }
    public string Line2 { get; set; }
    public string Line3 { get; set; }

    [Required(ErrorMessage = "Please enter a city name")]
    public string City { get; set; }

    [Required(ErrorMessage = "Please enter a state name")]
    public string State { get; set; }

    public string Zip { get; set; }

    [Required(ErrorMessage = "Please enter a country name")]
    public string Country { get; set; }

    public bool GiftWrap { get; set; }
}
}

Answer 1

Right click on project references. Select Add Reference.

In the .Net tab select System.ComponentModel.DataAnnotations

Answer 2

Select the "Required" keyword and press Ctrl+">". It will show you the corresponding namespace name through intellisense. If the namespace is not found or not available, it will show you - "Generate Class ...".