c#v2.0 扩展特性 翻译(2)。
　　Generic type instantiations
范型实例化

Similar to a non-generic type, the compiled representation of a generic type is intermediate language (IL) instructions and metadata. The representation of the generic type of course also encodes the existence and use of type parameters.

和非泛型类似，泛型被编译后表示成中间代码指令和元数据。泛型的表示当然也是将已有的和使用的类型参数编码。



The first time an application creates an instance of a constructed generic type, such as Stack<int>, the just-in-time (JIT) compiler of the .NET Common Language Runtime converts the generic IL and metadata to native code, substituting actual types for type parameters in the process. Subsequent references to that constructed generic type then use the same native code. The process of creating a specific constructed type from a generic type is known as a generic type instantiation.

当应用程序第一次创建一个新的被构造的泛型，例如Stack<int>，.Net公共运行时的JIT将泛型的中间代码和元数据转化成本地代码，在进程中用真实类型取代类型参数。后来引用已经被构建的泛型就运行本地代码。根据指定的构建类型来创建泛型被称作泛型的实例化。



The .NET Common Language Runtime creates a specialized copy of the native code for each generic type instantiation with a value type, but shares a single copy of the native code for all reference types (since, at the native code level, references are just pointers with the same representation).

.Net 公共语言运行时为每个值类型的泛型创建一个专门的本地代码拷贝。但会为所有引用类型共享一个单独的本地代码拷贝。（因为，在本地代码层次，引用和指针就是同一表示）



19.1.2 Constraints
约束



Commonly, a generic class will do more than just store data based on a type parameter. Often, the generic class will want to invoke methods on objects whose type is given by a type parameter. For example, an Add method in a Dictionary<K,V> class might need to compare keys using a CompareTo method:

一般来说，一个泛型类不仅可以存储建立在类型参数上的数据，还能做更多。通常，泛型类会尝试调用被指定类型对象上的方法。举例说，在Dictionary<K,V>类中一个Add方法可能需要通过CompareTo方法比较关键字。

public class Dictionary<K,V>
{
public void Add(K key, V value)
{
...

if (key.CompareTo(x) < 0) {...} // Error, no CompareTo method
...
}
}

Since the type argument specified for K could be any type, the only members that can be assumed to exist on the key parameter are those declared by type object, such as Equals, GetHashCode, and ToString; a compile-time error therefore occurs in the example above. It is of course possible to cast the key parameter to a type that contains a CompareTo method. For example, the key parameter could be cast to IComparable:

然而类型参数K可能是任何类型，被假定存在于Key参数的唯一成员变量是那些object类型所声明的，比如说 Equal，GetHashCode和ToString ；上面的代码将引发一个编译时错误。当然也可以把Key

参数转化成一个包含CompareTo方法的类型。例如，Key参数可能被转化成支持IComparable接口。

public class Dictionary<K,V>
{
public void Add(K key, V value)
{
...

if (((IComparable)key).CompareTo(x) < 0) {...}
...
}
}

While this solution works, it requires a dynamic type check at run-time, which adds overhead. It furthermore defers error reporting to run-time, throwing an InvalidCastException if a key doesn’t implement IComparable.

当以上解决方案运行时，加在上面的代码要求进行一个运行时的动态类型检查。而且它在运行时才报告错误，并在当key不支持IComparable接口时会抛出一个InvalidCastException.



To provide stronger compile-time type checking and reduce type casts, C# permits an optional list of constraints to be supplied for each type parameter. A type parameter constraint specifies a requirement that a type must fulfill in order to be used as an argument for that type parameter. Constraints are declared using the word where, followed by the name of a type parameter, followed by a list o

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