单声道的逆变编译异常

Question

我正在尝试编译 MSDN (https://msdn.microsoft.com/en-us/library/ee207183.aspx) 中的 covariance/contravariance 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):

error CS0266: Cannot implicitly convert type System.Collections.Generic.IEnumerable<string> to System.Collections.Generic.IEnumerable<object>. An explicit conversion exists (are you missing a cast?)

Example1.cs(40,36):

error CS0029: Cannot implicitly convert type System.Action<object>to System.Action<string>

我可以通过转换解决第一个问题：'IEnumerable objects = (IEnumerable)strings;'，但我不确定如何解决第二个问题。基本上，我不知道为什么我从 MSDN 代码的编译中得到错误。

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

Answer 1

问题是我使用了 gmcs 编译器，当我使用 dmcs 或 mcs 编译代码时，代码编译正常。

来自单色页面 (http://www.mono-project.com/docs/about-mono/languages/csharp/)

gmcs: compiler to target the 2.0 mscorlib.
smcs: compiler to target the 2.1 mscorlib, to build Moonlight applications.
dmcs: compiler to target the 4.0 mscorlib.

Starting with Mono version 2.11 a new unified compiler mcs is available. 
It replaces all previous runtime specific compilers (gmcs, dmcs, smcs). 
They still exist (as scripts only) to ease the migration path to mcs 
but we strongly recommend to use mcs.