public sealed class EnrollCommand : ICommand
{
    public long StudentId { get; }
    public string Course { get; }
    public string Grade { get; }

    public EnrollCommand(long studentId, string course, string grade)
    {
        if (course == null || grade == null)
            throw ArgumentException();

        Id = id;
        Course = course;
        Grade = grade;
    }
}