Derived List to Base IEnumerable

问题

I have the following code that compiles in .NET Framework version 4.0 and above:

public abstract class MyBase { }
public class MyDerived : MyBase { }

public abstract class MyBaseCollection<T> : IList<T> where T : MyBase
{
    protected readonly IList<T> deriveds = new List<T>();

    public void Test()
    {
        // This line works in .NET versions 4.0 and above, but not in versions below.
        IEnumerable<MyBase> bases = deriveds;
    }

    #region IList members with NotImplementedException
    // ...
    #endregion
}
public class MyDerivedCollection : MyBaseCollection<MyDerived> { }

But in .NET Framework below 4.0 I get a compile error on the following line:

IEnumerable<MyBase> bases = deriveds;

Cannot implicitly convert type 'System.Collections.Generic.IList<T>' to 'System.Collections.Generic.IEnumerable'. An explicit conversion exists (are you missing a cast?)

Question is what has changed (or was introduced) in .NET 4.0 regarding this?
Is there any documentation about this?

回答1:

In .Net 4.0 the IEnumerable<T> interface was changed from:

public interface IEnumerable<T>

To public interface IEnumerable<out T>

Notice that the word out has been added to the generic type parameter. This means that the generic parameter is co-variant which means you can pass in a more derived type.

Covariance Enables you to use a more derived type than originally specified. You can assign an instance of IEnumerable (IEnumerable(Of Derived) in Visual Basic) to a variable of type IEnumerable

See msdn for more information

回答2:

It's about covariance and contravariance of collections. Check the following link to get more information.

Starting with the .NET Framework 4, several generic interfaces have covariant type parameters; for example: IEnumerable, IEnumerator, IQueryable, and IGrouping. All the type parameters of these interfaces are covariant, so the type parameters are used only for the return types of the members.

来源：https://stackoverflow.com/questions/42225616/derived-list-to-base-ienumerable

标签

ienumerable

.net-2.0