implicit operator using interfaces

I have a generic class that I'm trying to implement implicit type casting for. While it mostly works, it won't work for interface casting. Upon further investigation, I found that there is a compiler error: "User-defined conversion from interface" that applies. While I understand that this should be enforced in some cases, what I'm trying to do does seem like a legitimate case.

Here's an example:

public class Foo<T> where T : IBar
{
    private readonly T instance;

    public Foo(T instance)
    {
        this.instance = instance;
    }
    public T Instance
    {
        get { return instance; }
    }
    public static implicit operator Foo<T>(T instance)
    {
        return new Foo<T>(instance);
    }
}

Code to use it:

var concreteReferenceToBar = new ConcreteBar();
IBar intefaceReferenceToBar = concreteReferenceToBar;
Foo<ConcreteBar> concreteFooFromConcreteBar = concreteReferenceToBar;
Foo<IBar> fooFromConcreteBar = concreteReferenceToBar;
Foo<IBar> fooFromInterfaceBar = intefaceReferenceToBar; // doesn't work

Does anyone know a workaround, or can anyone explain in a satisfactory way why I shuouldn't be able to cast interfaceReferenceToBar implicitly to Foo<IBar>, since in my case it is not being converted, but only contained within Foo?

EDIT: It looks like covariance might offer salvation. Let's hope the C# 4.0 specification allows for implicit casting of interface types using covariance.

The reason you can't do this is because it is specifically forbidden in the C# language specification:

Source: ECMA-334 Section 15.10.4

> A class or struct is permitted to > declare a conversion from a source > type S to a target type T provided all > of the following are true: > > - ... > - Neither S nor T is object or an interface-type.

and

> User-defined conversions are not > allowed to convert from or to > interface-types. In particular, this > restriction ensures that no > user-defined transformations occur > when converting to an interface-type, > and that a conversion to an > interface-type succeeds only if the > object being converted actually > implements the specified > interface-type.