Why IEnumerable needs to implement Add but no other methods?

Question

I have this class:

class Salaries : IEnumerable
{
    List<double> salaries = new List<double>();

    public Salaries()
    {
    }

    public void Insert(double salary)
    {
        salaries.Add(salary);
    }

    public IEnumerator GetEnumerator()
    {
        return salaries.GetEnumerator();
    }
}

In the program I try to add some values like this:

Salaries mySalaries = new Salaries() { 1000, 2000, 3000 };

I get this error:

'Salaries' does not contain a definition for 'Add' and no extension method 'Add' accepting a first argument of type 'Salaries' could be found (are you missing a using directive or an assembly reference?)

Why do I get this error?

Where can I find (using VS) that I need to implement Add function?

If I look in the List definition, I can see more functions, so why the compiler wants me to implement Add?

(I know that if I change the method in the class from Insert to Add it's OK. But the question is why only add, and where does it come from?)

Possible duplicate of [How can I add an item to a IEnumerable collection?](http://stackoverflow.com/questions/1210295/how-can-i-add-an-item-to-a-ienumerablet-collection) — Idos, Feb 21 '16 at 11:18
Why don't you just rename Insert to Add and get it over with? It is a misleading name anyway since it doesn't actually "insert", it only appends. — Hans Passant, Feb 21 '16 at 12:31
Please read my comment again. Especially the last part. The only thing you can do wrong is not trying it. — Hans Passant, Feb 21 '16 at 13:02

score 5 · Answer 1 · answered Feb 21 '16 at 11:18

5

Because you are calling the Add method when you use a collection initializer.

Salaries mySalaries = new Salaries() { 1000, 2000, 3000 };

is translated by the compiler to:

Salaries mySalaries = new Salaries();
mySalaries.Add(1000);
mySalaries.Add(2000);
mySalaries.Add(3000);

answered Feb 21 '16 at 11:18

Dennis_E

8,751
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1

See also the documentation on MSDN on [Collection initializers](https://msdn.microsoft.com/en-us/library/bb384062.aspx#Anchor_2) which explains this – Rhumborl Feb 21 '16 at 11:21
Or the C# spec, for the definitive answer. – Rawling Feb 21 '16 at 12:16
So why don't I get the same error when I do this: List numbers = new List() { 1, 2, 3, 4 }; – Noam B. Feb 21 '16 at 12:19
2

@NoamB. Because `List` has an Add method. Your Salaries class does not. – Dennis_E Feb 21 '16 at 12:31

Why IEnumerable needs to implement Add but no other methods?

1 Answers1