How to make IEnumerable readonly?

Question

Why are the lists list1Instance and p in the Main method of the below code pointing to the same collection?

class Person
         


        

Answer 1

This code returns a derived class, so as requested the return type hasn't changed.

It does throw an error if you try and change a field (via property) so is 'read only'. If you did want to be able to change values without affecting the original the clone answer above is better.

class  Person
{
    public virtual string FirstName { get; set; }
    public virtual string LastName { get; set; }


    public Person(string firstName, string lastName) {
        this.FirstName = firstName;
        this.LastName = lastName;
    }

}

class PersonReadOnly : Person
{
    public override string FirstName { get { return base.FirstName; } set { throw new Exception("setting a readonly field"); } }
    public override string LastName { get { return base.LastName; } set { throw new Exception("setting a readonly field"); } }

    public PersonReadOnly(string firstName, string lastName) : base(firstName, lastName)
    {
    }
    public PersonReadOnly(Person p) : base(p.FirstName, p.LastName)
    {

    }

}

class List1
{
    public List l1 = new List();

    public List1()
    {
        l1.Add(new Person("f1", "l1"));
        l1.Add(new Person("f2", "l2"));
        l1.Add(new Person("f3", "l3"));
        l1.Add(new Person("f4", "l4"));
        l1.Add(new Person("f5", "l5"));
    }
    public IEnumerable Get()
    {
        foreach (Person p in l1)
        {
            yield return new PersonReadOnly(p);
        }
        //return l1.AsReadOnly(); 
    }

}  
class Program
{

    static void Main(string[] args)
    {
        List1 list1Instance = new List1();

        List p = new List(list1Instance.Get());           

        UpdatePersons(p);

        bool sameFirstName = (list1Instance.l1[0].FirstName == p[0].FirstName);
    }

    private static void UpdatePersons(List list)
    {
        // readonly message thrown
        list[0].FirstName = "uf1";
    }