类型转换为EntityCollection <Object>

Question

I have a method taking a parameter EntityCollection

Say:

DisplayItems(EntityCollection<Object> items);

Why is it not possible to call?

EntityCollection<Student> students;

DisplayItems((EntityCollection<Object>) students); //type casting here

How can I achieve this?

Please help

Answer 1

Your problem occurs because of Covariance and Contravariance , imagine that worked as you want it to, you could then do this :

public void DisplayItems(EntityCollection<Object> items)
{
        //Probably not called add but you get the idea...
        items.Add(new AnyObjectILike());
        items.Add(new System.Windows.Form());
}

EntityCollection<Student> students;  
DisplayItems((EntityCollection<Object>)  students); //type casting here 

Clearly adding instances that are not of type Student to the EntityCollection<Student> causes a massive problem which is one of the reasons this is not allowed, you can alter this behaviour for interfaces using the In and Out keywords.

Answer 2

Similar answer can be found here:

convert or cast a List<t> to EntityCollection<T>

You need to loop and cast each element individually, adding them to a new EntityCollection.

Another option would be to use the Cast() method, but I'm not sure that will work with an EntityCollection. It works with a List:

List<Object> myItems = students.Cast<Object>().ToList();

I'm unable to check this would work with an EntityCollection but worth a try?