CODEWITHSUNDEEP
c#meflazy-evaluation
l33t
l33t

Reputation: 19937

Assign Lazy<T, TMetaData> with some instance of T

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)?

Upvotes: 0

Views: 1050

Answers (3)

l33t
l33t

Reputation: 19937

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

Upvotes: 0

Tobias Nilsson
Tobias Nilsson

Reputation: 512

Or:

Item = new Lazy<Foo>
(
    ()=> 
    { 
        Foo fobj = new Foo() { ID = 99 }; 
        return fobj; 
    }
 );

Source

Upvotes: 2

Paul Ruane
Paul Ruane

Reputation: 38590

I know it's called Lazy but you could at least read the docs! :)

o.Item = new Lazy<MyItem>(InitFunction);

Upvotes: 0

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