Question

I’m using a Dictionary<int, MyType> in a class. That class implements a interface that requires an IList<MyType> to be returned. Is there a simple way to to cast the one to the other (without copying the entire thing)?

My current solution follows:

private IList<MyType> ConvertToList(Dictionary<int, MyType>.ValueCollection valueCollection)
{
    List<MyType> list = new List<MyType>();
    list.AddRange(valueCollection);
    return list;
}

Answer 1

You’ll need to do a copy, but this is probably a good thing. In C# 2, your current code is almost the cleanest you can make. It would be improved by directly constructing your list off your values (List<MyType> list = new List<MyType>(valueCollection);), but a copy will still be required.

Using LINQ with C# 3, however, you would be able to do:

myDictionary.Values.ToList();

That being said, I would not (probably) try to avoid the copy. Returning a copy of your values tends to be safer, since it prevents the caller from causing problems if they attempt to modify your collection. By returning a copy, the caller can do list.Add(...) or list.Remove(...) without causing your class problems.

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Edit: Given your comment below, if all you want is an IEnumerable<T> with a Count, you can just return ICollection<T>. This is directly implemented by ValueCollection, which means you can just return your dictionary’s values directly, with no copying:

private ICollection<MyType> ConvertToList(Dictionary<int, MyType>.ValueCollection valueCollection)
{
    return valueCollection;
}

(Granted, this method becomes really useless in this case – but I wanted to demonstrate it for you…)