Create a Class with the properties as
public class StudAbstract
{
public string FirstName { get; set; }
public string Email { get; set; }
}
and change your code as
public List<studabsttract> GetStuCosineSimilarity()
{
List<studabstract> lst = new List<studabstract>();
for (int i = 0; i < array.Length; i++)
{
int a = array[i];
var studUserId = (from s in DB.Students where s.StudentId == a select s.UserId).FirstOrDefault();
var studIDAbstract = (from u in DB.Users
join stud in DB.Students on u.UserId equals stud.UserId
where u.UserId == studUserId
select new StudAbstract()
{
FirstName = u.FirstName,
Email = u.Email
}).ToList();
lst.AddRange(studIDAbstract);
}
return lst;
}