Sorry for the bad title, I am not really sure what my issue is with as I dont understand it. I have a task that I have to complete where I need to get a unit test to pass (below).

[Test]
    public void TheJarShouldContain30SweetsWhenCreated()
    {
        IJarOfSweetsCreator jarOfSweetsCreator = new JarOfSweetsCreator();
        IJarOfSweets jarOfSweets = jarOfSweetsCreator.Create();
        const int expectedNumberOfSweets = 30;
        int numberOfSweets = jarOfSweets.Count;

        Assert.AreEqual(expectedNumberOfSweets, numberOfSweets);
    }

Although it is not much to go on, JarOfSweetsCreator has this code:

public class JarOfSweetsCreator : IJarOfSweetsCreator
{
    public IJarOfSweets Create()
    {

        //throw new NotImplementedException();
    }
}

and IJarOfSweets has this:

public interface IJarOfSweets : IReadOnlyCollection<ISweet>
{
    void Shuffle ();
    ISweet TakeSweetFromJar ();
}

So the idea is that I have to make the test pass from creating and counting 30 from IJarOfSweets and I cannot modify the test code. I dont understand how to create and count 30 instances of an interface if you can only have one but I know that sounds daft. I am assuming it has something to do with the IReadOnlyCollection<ISweet> part of the IJarOfSweets interface but I dont know how to use it. Is IJarOfSweets used like a collection or would I create a collection within this interface? If it is a read only collection, how am I supposed to make 30 of them? If it really isn't possible to get the test to pass in this way then help with correcting it would also help.