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Asked: 2026-05-16T16:47:43+00:00

This is my custom collection declaration. public interface IMenuCollection<T> : ICollection<T> public class HTMLMenuCollection:

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This is my custom collection declaration.

public interface IMenuCollection<T> : ICollection<T>
public class HTMLMenuCollection: IMenuCollection<HTMLMenu>

I am trying to cast to it from another collection IList<T>.

IList<HTMLMenu> htmlMenuList = new List<HTMLMenu>();
...
HTMLMenuCollection tempColl = (HTMLMenuCollection)htmlMenuList;

I have no idea why this won’t work. If I cast IList<T> to ICollection<T> it works just fine, but with this I get an invalid cast exception. What am I doing wrong?

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1 Answer

  1. Editorial Team

    Think of inheritance as a ‘is a’ relationship.

    Then you can think of it as being because HTMLMenuCollection is a List<HTMLMenu> but not every List<HTMLMenu> is a HTMLMenuCollection (since List<HTMLMenu> doesn’t inherit from HTMLMenuCollection).

    The reason that you can cast from a List to IList to ICollection is the following:

    public class List<T> : IList<T>, ICollection<T> // and more

    and

    public interface IList<T> : ICollection<T> // and more
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