的 @克雷格的回答是正确的。要解决，将其更改为：

@Craig's answer is correct. To resolve, change it to:

IEnumerable<T> MakeStuff()

编辑：至于为什么，看的的IEnumerable< T>接口​​>：

public interface IEnumerable<out T> : IEnumerable

注意的&IList的LT; T>接口​​>没有out关键字。方差支持在接口和委托，而不是类泛型类型参数，所以它并不适用于列出< T>

Note that the IList<T> Interface does not have the out keyword. Variance is supported for generic type parameters in interfaces and delegates, not classes, so it doesn't apply to List<T>.