C# 泛型委托的可逆变不能协变的问题？
//定义两个泛型委托实现
static void SetObject(object o) { }

static void SetString(string s) { }

Action<object> actObject = SetObject;
Action<string> actString = actObject; //ok

Action<string> actString1 = SetString;
Action<object> actObject1 = actString1;//编译不通过 why？？