为什么我不能返回IEnumerable <IGrouping<A,B> &gt;作为IEnumerable <IEnumerable<B> &gt;

Question

I've used GroupBy() , and produced an IEnumerable<IGrouping<TKey,TValue>> .

And I can foreach over it, and pass each element (each IGrouping<TKey,TValue> ) into a method that accepts IEnumerable<TValue>

This is unsurprising - IGrouping<TKey,TValue> implements IEnumerable<TValue>

But if I define a method that accepts IEnumerable<IEnumerable<B>> , then I can't directly pass in the output of GroupBy() , even though it is an IEnumerable<> and every element inside it is an IEnumerable<B>

Why?

Answer 1

正如几个人所指出的（确实，正如我在脑海中所知道的那样），这仅仅是一个协方差问题，并且仅在.NET 4之前的环境中才出现。