关于委托怎么匹配的问题，以及逆变和协变

Question

看了一下msdn里面关于逆变和协变的章节，在委托 那一章里面又一个疑问

代码如下：

public class First { }
public class Second : First { }
public delegate First SampleDelegate(Second a);
public delegate R SampleGenericDelegate<A, R>(A a);


// Matching signature.
public static First ASecondRFirst(Second first)
{ return new First(); }

// The return type is more derived.
public static Second ASecondRSecond(Second second)
{ return new Second(); }

// The argument type is less derived.
public static First AFirstRFirst(First first)
{ return new First(); }

// The return type is more derived 
// and the argument type is less derived.
public static Second AFirstRSecond(First first)
{ return new Second(); }
// Assigning a method with a matching signature 
// to a non-generic delegate. No conversion is necessary.
SampleDelegate dNonGeneric = ASecondRFirst;
// Assigning a method with a more derived return type 
// and less derived argument type to a non-generic delegate.
// The implicit conversion is used.
SampleDelegate dNonGenericConversion = AFirstRSecond;    //         <-这一块有问题

// Assigning a method with a matching signature to a generic delegate.
// No conversion is necessary.
SampleGenericDelegate<Second, First> dGeneric = ASecondRFirst;
// Assigning a method with a more derived return type 
// and less derived argument type to a generic delegate.
// The implicit conversion is used.
SampleGenericDelegate<Second, First> dGenericConversion = AFirstRSecond;

如题那块上面的注释所说，分派了一个方法，它的返回值类型是原类型的derived輩，它的参数类型是是原类型的base輩。  然后发生了隐式转换和delegate相匹配。

1.这是否是逆变和协变。如果是逆变和协变是否是默认可以的？因为子类转成父类可以理解(也就是协变)，但是父类转为子类也可以吗？

2.我尝试了一下其他的

定义了委托和方法

public delegate First FFDelegate(First f);
public static Second RSecondASecond(Second s)
{	return new Second();	}
public static First RFirstASecond(Second s)
{	return new First();	}
public static Second RSecondAFirst(First f)
{	return new Second();	}

发现

FFDelegate FFssDele = RSecondASecond;   //error报错，为什么呢，他不是会发生自动转换的吗?
FFDelegate FFFSDele = RFirstASecond;	//error 
FFDelegate FFSFDele = RSecondAFirst;	//但是这个却成功了，难道是返回值可以自动转换，而参数不可以吗？