interface IFoo
{
     Lazy<Task<int>> MyLazy { get;   set; }

}

class Foo : IFoo
{
    public Lazy<Task<int>> MyLazy { get;    set; }

    public Foo()
    {
        MyLazy = new Lazy<Task<int>>(() => /*heavy operation*/ Task.FromResult(3));
    }

}

public Foo()
{
    ((IFoo)this).MyLazy = new Lazy<Task<int>>(() => /*heavy operation*/ Task.FromResult(3));
}