您需要传递一个 delegate instance; Action 可以正常工作：

You need to pass a delegate instance; Action would work fine:

internal void AFoo(string s, Action doOtherThing)
{
    if (something)
    {
        //do something
    }
    else
        doOtherThing();
}

如果 BFoo 无参数，它将以您的示例的方式工作：

If BFoo is parameterless it will work as written in your example:

new ClassA().AFoo("hi", BFoo);

如果需要参数，您需要提供：

If it needs parameters, you'll need to supply them:

new ClassA().AFoo("hi", () => BFoo(123, true, "def"));