interface IFolderOrItem<TFolderOrItem> where TFolderOrItem : FolderOrItem {}

abstract class FolderOrItem {}

class Folder : FolderOrItem {}

abstract class Item : FolderOrItem {}

class Document : Item {}

gora estou tentando fazer algo assim:

class Something
{
    IFolderItemOrItem<Item> SelectedItem { get; set; }
    void SomeMagicMethod()
    {
        this.SelectedItem = (IFolderOrItem<Item>)GetMagicDocument();
        // bad bad bad ... ??
    }
    IFolderOrItem<Document> GetMagicDocument()
    {
        return someMagicDocument; // which is of type IFolderOrItem<Document>
    }
}

existe alguma possibilidade de fazer isso funcionar?