Question

Using C# 4 you can utilize lazy initialization for MEF. See http://msdn.microsoft.com/en-us/library/dd986615.aspx

// So I have this member, populated through MEF.
private Lazy<MyItem, ISomeInterface> item;
public Lazy<MyItem, ISomeInterface> Item
{
    get
    {
        return item;
    }
    set
    {
        item = value;
    }
}

Now, what if I have a MyItem instance that I would like to assign to this lazy member variable? This does not work:

var myItem = new MyItem(); // Implements ISomeInterface
o.Item = myItem; // Cannot convert type...

UPDATE: I simplified my sample a bit too much. The problem here is that I have lazy-evaluated items (coming from a MEF plugin manager) in terms of Lazy<MyItem, ISomeInterface>. Sometimes these items are already instantiated which asks for a construct as follows:

var item = new Lazy<MyItem, ISomeInterface>(obj);

However, that causes a MissingMemberException:
“The lazily-initialized type does not have a public, parameterless constructor.”

Q: How do I assign a Lazy<T, U> variable with an instance of T (that implements U)?

Answer 1

The solution is to use the ToLazy method that MEF uses:
https://mefcontrib.svn.codeplex.com/svn/trunk/src/MefContrib.Models.Provider/ComposableMember.cs