单调的协方差编译异常

我正在尝试在MSDN（https://msdn.microsoft.com/en-us/library/ee207183.aspx）中编译协方差/反方差示例。

// Assignment compatibility.  
string str = "test";
// An object of a more derived type is assigned to an object of a less derived type.  
object obj = str;

// Covariance. 
IEnumerable<string> strings = new List<string>();
// An object that is instantiated with a more derived type argument  
// is assigned to an object instantiated with a less derived type argument.  
// Assignment compatibility is preserved. 
IEnumerable<object> objects = strings;

// Contravariance.            
// Assume that the following method is in the class:  
// static void SetObject(object o) { } 
Action<object> actObject = SetObject;
// An object that is instantiated with a less derived type argument  
// is assigned to an object instantiated with a more derived type argument.  
// Assignment compatibility is reversed. 
Action<string> actString = actObject;

问题是我遇到了两个错误：

Example1.cs（22,39）：

错误CS0266：无法将类型隐式转换System.Collections.Generic.IEnumerable<string>为System.Collections.Generic.IEnumerable<object>。存在显式转换（您是否缺少演员表？）

Example1.cs（40,36）：

错误CS0029：无法将类型隐式转换System.Action<object>为System.Action<string>

我可以通过强制转换来解决第一个问题：“ IEnumerable objects =（IEnumerable）strings;”，但是我不确定如何解决第二个问题。基本上，我不知道为什么从MSDN代码的编译中会出错。

我使用单声道Mono C# compiler version 3.12.0.0进行编译。可能是什么问题？