C# Inline initialization of an IGrouping [duplicate]

Question

This question already has answers here:

Is it possible to create some IGrouping object (7 answers)

Closed 9 years ago.

If I want to initialize a list of groupings, can I do this inline?

Update with context

I already know I cannot initialize an interface, but is there already an implementation built into dotnet?

I don't want to create my own implementation, because I am trying to refactor an existing private method into its own class which is public, thus I need to pass parameters in for unit testing.

void MyMethod(IList<IGrouping<int, MyObject>> objGroupings)

Currently I have resorted to initializing a list, then grouping by a key:

var fooList = new List<MyObject>
{
     new MyObject{ foo = 5 },
     new MyObject{ foo = 5 },
     new MyObject{ foo = 5 },
     new MyObject{ foo = 2 },
     new MyObject{ foo = 2 }
};

var fooGrouping = fooList.GroupBy(o => o.foo).ToList();

Answer 1

IGrouping is just an interface and cannot be initialised. If you want to use the inline initialisation, you need you initialise an object that implements IGrouping

Answer 2

I agree with @David Pilkington's answer. Here's a class I've written/used for this purpose in the past:

sealed class Grouping<TKey, TElement> : IGrouping<TKey, TElement>
{
    private readonly TKey m_key;
    private readonly IEnumerable<TElement> m_elements;

    public Grouping(TKey key, IEnumerable<TElement> elements)
    {
        if (elements == null)
            throw new ArgumentNullException("elements");

        m_key = key;
        m_elements = elements;
    }

    public TKey Key
    {
        get { return m_key; }
    }

    public IEnumerator<TElement> GetEnumerator()
    {
        return m_elements.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }
}

Answer 3

You can initialize a dictionary instead:

var dic = new Dictionary<string, int>()
{
    { "foo", 1 },
    { "bar", 2 },
};

If you concern of multiple values with the same key, use dictionary of list or values (choose the list's type which be more efficient for your code)