Entity Framework Basics

Question:
In Course.cs add data annotation attributes to the Title property in order to make the following model refinements:

    A value will be required and the associated database column will not allow null values
    The string value will not exceed 200 characters in length and the associated database column data type will be nvarchar(200)

Code Given:
Course.cs
using System.Collections.Generic;

namespace Treehouse.CodeChallenges
{
    public class Course
    {
        public Course()
        {
            Students = new List<CourseStudent>();
        }

        public int Id { get; set; }
        public int TeacherId { get; set; }
        public string Title { get; set; }
        public string Description { get; set; }
        public int Length { get; set; }

        public Teacher Teacher { get; set; }
        public ICollection<CourseStudent> Students { get; set; }
    }
}

Error: 
Bummer! Did you add the 'using' directive for the System.ComponentModel.DataAnnotations namespace?

My Answer:
using System.Collections.Generic;

namespace Treehouse.CodeChallenges
{
    public class Course
    {
        public Course()
        {
            Students = new List<CourseStudent>();
        }

        public int Id { get; set; }
        public int TeacherId { get; set; }
        public string Title { get; set; }
        [Required, StringLength(200)]
        public string Description { get; set; }
        public int Length { get; set; }

        public Teacher Teacher { get; set; }
        public ICollection<CourseStudent> Students { get; set; }
    }
}