C# Collection/List - Unique ID

Question

In C# I'm trying to create a list of objects and when a new thing is added to the list, it is checked to make sure the same ID isn't used. I have the solution in Linq but I'm trying to do it without linq.

public void AddStudent(Student student)
        {
            if (students == null)                               
            {
                students.Add(student);                          
            }
            else
            {
                if ((students.Count(s => s.Id == student.Id)) == 1)   

                  // LINQ query, student id is unique
            {
                throw new ArgumentException("Error student " 
                  + student.Name + " is already in the class");
            }
            else
            {
                students.Add(student);
            }
        }
    }

Answer 1

Another approach would be to use a HashSet instead of a List.

The Student class:

public class Student
{
    private int id;

    public override int GetHashCode()
    {
        return this.id;
    }
    public override bool Equals(object obj)
    {
        Student otherStudent = obj as Student;
        if (otherStudent !=null)
        {
            return this.id.Equals(otherStudent.id);
        }
        else
        {
            throw new ArgumentException();
        }

    }

    public int Id
    {
        get { return id; }
        set { id = value; }
    }

}

Then you can add stuff like this

    HashSet<Student> hashSetOfStudents = new HashSet<Student>();
    Student s1 = new Student() { Id = 1 };
    Student s2 = new Student() { Id = 2 };
    Student s3 = new Student() { Id = 2 };

    hashSetOfStudents.Add(s1);
    hashSetOfStudents.Add(s2);
    hashSetOfStudents.Add(s3);

The addition of s3 will fail because it has the same Id as s2.

Answer 2

You can override Student.Equals() and Student.GetHashCode() to check if the Student.Id is equal. If the student list inherits from List<Student>, you can just use the default Add() method. It will only add students with different Ids.

public class Student
{
    // ...

    public override bool Equals(object obj)
    {
        // Check for null values and compare run-time types.
        if (obj == null || GetType() != obj.GetType()) 
            return false;

        Student other = obj as Student;
        return this.Id == other.Id;
    }

    public override int GetHashCode()
    {
        return this.Id.GetHashCode();
    }
}

public class StudentList : List<Student> { }

// Usage:

var students = new StudentList();
students.Add(student);