Why use readonly IEnumerable<T>

Question

Why would you declare an IEnumerable<T> readonly?

From the this article on async & await we have the following code.

class OrderHandler
{
    private readonly IEnumerable<Order> _orders;

    public OrderHandler()
    {
        // Set orders.
    }
    public IEnumerable<Order> GetAllOrders()
    {
        return _orders;
    }
}

IEnumerable<T> is immutable. How is this different from the readonly keyword?

Answer 1

The readonly keyword here applies to the field _orders. It simply means that the field can not be assigned a different value during the lifetime of the object. For example, this is not possible:

class OrderHandler
{
    private readonly IEnumerable<Order> _orders;

    public OrderHandler()
    {
        // Set orders.
    }

    void SomeMethod()
    {
        _orders = new Order[0];
    }
}

You will receive this compiler error:

A readonly field cannot be assigned to (except in a constructor or a variable initializer)

This does not make the collection read-only. For example, you could still do this:

class OrderHandler
{
    public readonly IEnumerable<Order> Orders;

    public OrderHandler()
    {
        Orders = new List<Order>();
    }
}

((List<Order>)OrderHandler.Orders).Add(new Order());

Which would probably violate the thread-safety of the class. See Tigran's answer for information on immutable collections.

Further Reading

• readonly (C# Reference)