本来CLR里所有东西都是class,从system.object下来的。
但是中间插了个ValueType类，CLR又对它特殊对待，而所有Strcuts都继承自ValueType.
也都是传值而不穿引用。
很容易引起误会，把ValueType和Structs等同起来（因为元类型int,double等也是strcuts，也是ValueType)
但是这里enum是一个例外。他是一个传值的东西(valueType）,但是它真不是struct.
Structs are not really a CLR notion — value types are. Part of the confusion
here is that the term “struct” and “value type” are being passed around as
synonomous — which they really aren’t. All C# structs are value types but
not all value types are C# structs.
At the CLR level, there are only classes. In fact, interfaces are actually
classes too but have a specific bit set that denotes them as interfaces and
causes them to be treated differently. A value type is simply a class that
derives from System.ValueType. An enum is a class that derives from System.Enum
(which derives from System.ValueType). The CLR simply treats classes that
derive from System.ValueType in a special way — passing them by value instead
of by reference and allocating them on the stack when declared in a method
body.
Lutz Roeder’s reflector can be a bit misleading if you leave the language
set to C#. Setting it to IL will show the reality of how types are really
declared. Consider this declaration of System.Int32 in IL:
..class public sequential ansi serializable sealed beforefieldinit Int32
extends System.ValueType
implements System.IComparable, System.IFormattable, System.IConvertible,
System.IComparable`1<int32>, System.IEquatable`1<int32>
It is really a class that derives from System.ValueType and implements a
set of specific interfaces. There is nothing remarkably different about how
System.Int32 is declared versus, say, System.StringComparer:
..class public abstract auto ansi serializable beforefieldinit StringComparer
extends object
implements System.Collections.IComparer, System.Collections.IEqualityComparer,
System.Collections.Generic.IComparer`1<string>, System.Collections.Generic.IEqualityComparer`1<str ing>
The only difference is the base type that it derives from. Enums are no different
— consider System.StringComparison:
..class public auto ansi serializable sealed StringComparison
extends System.Enum
Enums *are* classes at the CLR level. Reflection attempts to make distinctions
between different types to clarify them for the client. So, it is not a great
tool to use to try and understand how things are really put together at the
metadata-level.
>System.Enum derives from System.ValueType.
Which does not necessarily make it a structure.
You’re correct, it makes it a value type. Clarification of terminology is
definitely needed here.