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C#泛型运作原理 C#泛型运作原理的深入理解

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前言#

我们都知道泛型在C#的重要性,泛型是OOP语言中三大特征的多态的最重要的体现,几乎泛型撑起了整个.NET框架,在讲泛型之前,我们可以抛出一个问题,我们现在需要一个可扩容的数组类,且满足所有类型,不管是值类型还是引用类型,那么在没有用泛型方法实现,如何实现?

一.泛型之前的故事#

我们肯定会想到用object来作为类型参数,因为在C#中,所有类型都是基于Object类型的。因此Object是所有类型的最基类,那么我们的可扩容数组类如下:

 public class ArrayExpandable
 {
 private object?[] _items = null;

 private int _defaultCapacity = 4;

 private int _size;

 public object? this[int index]
 {
 get
 {
 if (index < 0 || index >= _size) 
  throw new ArgumentOutOfRangeException(nameof(index));
 return _items[index];
 }
 set
 {
 if (index < 0 || index >= _size) 
  throw new ArgumentOutOfRangeException(nameof(index));
 _items[index] = value;
 }
 }

 public int Capacity
 {
 get => _items.Length;
 set
 {
 if (value < _size)
 {
  throw new ArgumentOutOfRangeException(nameof(value));
 }
 if (value != _items.Length)
 {
  if (value > 0)
  {
  object[] newItems = new object[value];
  if (_size > 0)
  {
  Array.Copy(_items, newItems, _size);
  }
  _items = newItems;
  }
  else
  {
  _items = new object[_defaultCapacity];
  }
 }
 }
 }

 public int Count => _size;


 public ArrayExpandable()
 {
 _items = new object?[0];
 }

 public ArrayExpandable(int capacity)
 {
 _items = new object?[capacity];
 }

 public void Add(object? value)
 {
 //数组元素为0或者数组元素容量满
 if (_size == _items.Length) EnsuresCapacity(_size + 1);
 _items[_size] = value;
 _size++;
 }

 private void EnsuresCapacity(int size)
 {
 if (_items.Length < size)
 {
 int newCapacity = _items.Length == 0 ? _defaultCapacity : _items.Length * 2;
 if (newCapacity < size) newCapacity = size;
 Capacity = newCapacity;
 }
 }

然后我们来验证下:

var arrayStr = new ArrayExpandable();
var strs = new string[] { "ryzen", "reed", "wymen" };
for (int i = 0; i < strs.Length; i++)
{
 arrayStr.Add(strs[i]);
 string value = (string)arrayStr[i];//改为int value = (int)arrayStr[i] 运行时报错
 Console.WriteLine(value);
}
Console.WriteLine($"Now {nameof(arrayStr)} Capacity:{arrayStr.Capacity}");

var array = new ArrayExpandable();
for (int i = 0; i < 5; i++)
{
 array.Add(i);
 int value = (int)array[i];
 Console.WriteLine(value);
}
Console.WriteLine($"Now {nameof(array)} Capacity:{array.Capacity}");

输出:

Copy
ryzen
reed
wymen
gavin
Now arrayStr Capacity:4
0
1
2
3
4
Now array Capacity:8

貌似输出结果是正确的,能够动态进行扩容,同样的支持值类型Struct的int32和引用类型的字符串,但是其实这里会发现一些问题,那就是

  1. 引用类型string进行了类型转换的验证
  2. 值类型int32进行了装箱和拆箱操作,同时进行类型转换类型的检验
  3. 发生的这一切都是在运行时的,假如类型转换错误,得在运行时才能报错

大致执行模型如下:

引用类型:

值类型:

 那么有没有一种方法能够避免上面遇到的三种问题呢?在借鉴了cpp的模板和java的泛型经验,在C#2.0的时候推出了更适合.NET体系下的泛型

二.用泛型实现#

public class ArrayExpandable<T>
{
 private T[] _items;

 private int _defaultCapacity = 4;

 private int _size;

 public T this[int index]
 {
 get
 {
 if (index < 0 || index >= _size) 
  throw new ArgumentOutOfRangeException(nameof(index));
 return _items[index];
 }
 set
 {
 if (index < 0 || index >= _size) 
  throw new ArgumentOutOfRangeException(nameof(index));
 _items[index] = value;
 }
 }

 public int Capacity
 {
 get => _items.Length;
 set
 {
 if (value < _size)
 {
  throw new ArgumentOutOfRangeException(nameof(value));
 }
 if (value != _items.Length)
 {
  if (value > 0)
  {
  T[] newItems = new T[value];
  if (_size > 0)
  {
  Array.Copy(_items, newItems, _size);
  }
  _items = newItems;
  }
  else
  {
  _items = new T[_defaultCapacity];
  }
 }
 }
 }

 public int Count => _size;


 public ArrayExpandable()
 {
 _items = new T[0];
 }

 public ArrayExpandable(int capacity)
 {
 _items = new T[capacity];
 }
 public void Add(T value)
 {
 //数组元素为0或者数组元素容量满
 if (_size == _items.Length) EnsuresCapacity(_size + 1);
 _items[_size] = value;
 _size++;
 }

 private void EnsuresCapacity(int size)
 {
 if (_items.Length < size)
 {
 int newCapacity = _items.Length == 0 ? _defaultCapacity : _items.Length * 2;
 if (newCapacity < size) newCapacity = size;
 Capacity = newCapacity;
 }
 }
 }

那么测试代码则改写为如下:

var arrayStr = new ArrayExpandable<string>();
var strs = new string[] { "ryzen", "reed", "wymen", "gavin" };
for (int i = 0; i < strs.Length; i++)
{
 arrayStr.Add(strs[i]);
 string value = arrayStr[i];//改为int value = arrayStr[i] 编译报错
 Console.WriteLine(value);
}
Console.WriteLine($"Now {nameof(arrayStr)} Capacity:{arrayStr.Capacity}");

var array = new ArrayExpandable<int>();
for (int i = 0; i < 5; i++)
{
 array.Add(i);
 int value = array[i];
 Console.WriteLine(value);
}
Console.WriteLine($"Now {nameof(array)} Capacity:{array.Capacity}");

输出:

Copy
ryzen
reed
wymen
gavin
Now arrayStr Capacity:4
0
1
2
3
4
Now array Capacity:8

我们通过截取部分ArrayExpandable<T>的IL查看其本质是个啥:

//声明类
.class public auto ansi beforefieldinit MetaTest.ArrayExpandable`1<T>
 extends [System.Runtime]System.Object
{
 .custom instance void [System.Runtime]System.Reflection.DefaultMemberAttribute::.ctor(string) = ( 01 00 04 49 74 65 6D 00 00 )   
} 


//Add方法
.method public hidebysig instance void Add(!T 'value') cil managed
{
 // 代码大小 69 (0x45)
 .maxstack 3
 .locals init (bool V_0)
 IL_0000: nop
 IL_0001: ldarg.0
 IL_0002: ldfld int32 class MetaTest.ArrayExpandable`1<!T>::_size
 IL_0007: ldarg.0
 IL_0008: ldfld !0[] class MetaTest.ArrayExpandable`1<!T>::_items
 IL_000d: ldlen
 IL_000e: conv.i4
 IL_000f: ceq
 IL_0011: stloc.0
 IL_0012: ldloc.0
 IL_0013: brfalse.s IL_0024
 IL_0015: ldarg.0
 IL_0016: ldarg.0
 IL_0017: ldfld int32 class MetaTest.ArrayExpandable`1<!T>::_size
 IL_001c: ldc.i4.1
 IL_001d: add
 IL_001e: call instance void class MetaTest.ArrayExpandable`1<!T>::EnsuresCapacity(int32)
 IL_0023: nop
 IL_0024: ldarg.0
 IL_0025: ldfld !0[] class MetaTest.ArrayExpandable`1<!T>::_items
 IL_002a: ldarg.0
 IL_002b: ldfld int32 class MetaTest.ArrayExpandable`1<!T>::_size
 IL_0030: ldarg.1
 IL_0031: stelem !T
 IL_0036: ldarg.0
 IL_0037: ldarg.0
 IL_0038: ldfld int32 class MetaTest.ArrayExpandable`1<!T>::_size
 IL_003d: ldc.i4.1
 IL_003e: add
 IL_003f: stfld int32 class MetaTest.ArrayExpandable`1<!T>::_size
 IL_0044: ret
} // end of method ArrayExpandable`1::Add


 原来定义的时候就是用了个T作为占位符,起一个模板的作用,我们对其实例化类型参数的时候,补足那个占位符,我们可以在编译期就知道了其类型,且不用在运行时进行类型检测,而我们也可以对比ArrayExpandable和ArrayExpandable<T>在类型为值类型中的IL,查看是否进行拆箱和装箱操作,以下为IL截取部分:

ArrayExpandable:

 IL_0084: newobj instance void GenericSample.ArrayExpandable::.ctor()
 IL_0089: stloc.2
 IL_008a: ldc.i4.0
 IL_008b: stloc.s V_6
 IL_008d: br.s IL_00bc
 IL_008f: nop
 IL_0090: ldloc.2
 IL_0091: ldloc.s V_6
 IL_0093: box [System.Runtime]System.Int32 //box为装箱操作
 IL_0098: callvirt instance void GenericSample.ArrayExpandable::Add(object)
 IL_009d: nop
 IL_009e: ldloc.2
 IL_009f: ldloc.s V_6
 IL_00a1: callvirt instance object GenericSample.ArrayExpandable::get_Item(int32)
 IL_00a6: unbox.any [System.Runtime]System.Int32 //unbox为拆箱操作

ArrayExpandable:

 IL_007f: newobj instance void class GenericSample.ArrayExpandable`1<int32>::.ctor()
 IL_0084: stloc.2
 IL_0085: ldc.i4.0
 IL_0086: stloc.s V_6
 IL_0088: br.s IL_00ad
 IL_008a: nop
 IL_008b: ldloc.2
 IL_008c: ldloc.s V_6
 IL_008e: callvirt instance void class GenericSample.ArrayExpandable`1<int32>::Add(!0)
 IL_0093: nop
 IL_0094: ldloc.2
 IL_0095: ldloc.s V_6
 IL_0097: callvirt instance !0 class GenericSample.ArrayExpandable`1<int32>::get_Item(int32)

 我们从IL也能看的出来,ArrayExpandable<T>的T作为一个类型参数,在编译后在IL已经确定了其类型,因此当然也就不存在装拆箱的情况,在编译期的时候IDE能够检测类型,因此也就不用在运行时进行类型检测,但并不代表不能通过运行时检测类型(可通过is和as),还能通过反射体现出泛型的灵活性,后面会讲到

 其实有了解ArrayList和List的朋友就知道,ArrayExpandable和ArrayExpandable<T>其实现大致就是和它们一样,只是简化了很多的版本,我们这里可以通过 BenchmarkDotNet 来测试其性能对比,代码如下:

 [SimpleJob(RuntimeMoniker.NetCoreApp31,baseline:true)]
 [SimpleJob(RuntimeMoniker.NetCoreApp50)]
 [MemoryDiagnoser]
 public class TestClass
 {

 [Benchmark]
 public void EnumAE_ValueType()
 {
  ArrayExpandable array = new ArrayExpandable();
  for (int i = 0; i < 10000; i++)
  {
  array.Add(i);//装箱
  int value = (int)array[i];//拆箱
  }
  array = null;//确保进行垃圾回收
 }

 [Benchmark]
 public void EnumAE_RefType()
 {
  ArrayExpandable array = new ArrayExpandable();
  for (int i = 0; i < 10000; i++)
  {
  array.Add("r");
  string value = (string)array[i];
  }
  array = null;//确保进行垃圾回收
 }

 [Benchmark]
 public void EnumAE_Gen_ValueType()
 {
  ArrayExpandable<int> array = new ArrayExpandable<int>();
  for (int i = 0; i < 10000; i++)
  {
  array.Add(i);
  int value = array[i];
  }
  array = null;//确保进行垃圾回收;
 }

 [Benchmark]
 public void EnumAE_Gen_RefType()
 {
  ArrayExpandable<string> array = new ArrayExpandable<string>();
  for (int i = 0; i < 10000; i++)
  {
  array.Add("r");
  string value = array[i];
  }
  array = null;//确保进行垃圾回收;
 }

 [Benchmark]
 public void EnumList_ValueType()
 {
  List<int> array = new List<int>();
  for (int i = 0; i < 10000; i++)
  {
  array.Add(i);
  int value = array[i];
  }
  array = null;//确保进行垃圾回收;
 }


 [Benchmark]
 public void EnumList_RefType()
 {
  List<string> array = new List<string>();
  for (int i = 0; i < 10000; i++)
  {
  array.Add("r");
  string value = array[i];
  }
  array = null;//确保进行垃圾回收;
 }

 [Benchmark(Baseline =true)]
 public void EnumAraayList_valueType()
 {
  ArrayList array = new ArrayList();
  for (int i = 0; i < 10000; i++)
  {
  array.Add(i);
  int value = (int)array[i];
  }
  array = null;//确保进行垃圾回收;
 }


 [Benchmark]
 public void EnumAraayList_RefType()
 {
  ArrayList array = new ArrayList();
  for (int i = 0; i < 10000; i++)
  {
  array.Add("r");
  string value = (string)array[i];
  }
  array = null;//确保进行垃圾回收;
 }
 }

 我还加入了.NETCore3.1和.NET5的对比,且以.NETCore3.1的EnumAraayList_valueType方法为基准,性能测试结果如下:

用更直观的柱形图来呈现:

 我们能看到在这里List的性能在引用类型和值类型中都是所以当中是最好的,不管是执行时间、GC次数,分配的内存空间大小,都是最优的,同时.NET5在几乎所有的方法中性能都是优于.NETCore3.1,这里还提一句,我实现的ArrayExpandable和ArrayExpandable<T>性能都差于ArrayList和List,我还没实现IList和各种方法,只能说句dotnet基金会牛逼

三.泛型的多态性#

多态的声明#

类、结构、接口、方法、和委托可以声明一个或者多个类型参数,我们直接看代码:

interface IFoo<InterfaceT>
{
 void InterfaceMenthod(InterfaceT interfaceT);
}

class Foo<ClassT, ClassT1>: IFoo<StringBuilder>
{
 public ClassT1 Field;
 
 public delegate void MyDelegate<DelegateT>(DelegateT delegateT);

 public void DelegateMenthod<DelegateT>(DelegateT delegateT, MyDelegate<DelegateT> myDelegate)
 {
 myDelegate(delegateT);
 }

 public static string operator +(Foo<ClassT, ClassT1> foo,string s)
 {
 return $"{s}:{foo.GetType().Name}";
 }


 public List<ClassT> Property{ get; set; }
 public ClassT1 Property1 { get; set; }

 public ClassT this[int index] => Property[index];//没判断越界


 public Foo(List<ClassT> classT, ClassT1 classT1)
 {
 Property = classT;
 Property1 = classT1;
 Field = classT1;
 Console.WriteLine($"构造函数:parameter1 type:{Property.GetType().Name},parameter2 type:{Property1.GetType().Name}");
 }

 //方法声明了多个新的类型参数
 public void Method<MenthodT, MenthodT1>(MenthodT menthodT, MenthodT1 menthodT1)
 {
 Console.WriteLine($"Method<MenthodT, MenthodT1>:{(menthodT.GetType().Name)}:{menthodT.ToString()}," +
 $"{menthodT1.GetType().Name}:{menthodT1.ToString()}");
 }

 public void Method(ClassT classT)
 {
 Console.WriteLine($"{nameof(Method)}:{classT.GetType().Name}:classT?.ToString()");
 }

 public void InterfaceMenthod(StringBuilder interfaceT)
 {
  Console.WriteLine(interfaceT.ToString());
 }
}

控制台测试代码:

static void Main(string[] args)
{
 Test();
 Console.ReadLine();
}

static void Test()
{
 var list = new List<int>() { 1, 2, 3, 4 };
 var foo = new Foo<int, string>(list, "ryzen");

 var index = 0;
 Console.WriteLine($"索引:索引{index}的值:{foo[index]}");
 
 Console.WriteLine($"Filed:{foo.Field}");

 foo.Method(2333);

 foo.Method<DateTime, long>(DateTime.Now, 2021);

 foo.DelegateMenthod<string>("this is a delegate", DelegateMenthod);

 foo.InterfaceMenthod(new StringBuilder().Append("InterfaceMenthod:this is a interfaceMthod"));

 Console.WriteLine(foo+"重载+运算符");
}

static void DelegateMenthod(string str)
{
 Console.WriteLine($"{nameof(DelegateMenthod)}:{str}");
}

输出如下:

构造函数:parameter1 type:List`1,parameter2 type:String
索引:索引0的值:1
Filed:ryzen
Method:Int32:classT?.ToString()
Method<MenthodT, MenthodT1>:DateTime:2021/03/02 11:45:40,Int64:2021
DelegateMenthod:this is a delegate
InterfaceMenthod:this is a interfaceMthod
重载+运算符:Foo`2

我们通过例子可以看到的是:

多态的继承#

父类和实现类或接口的接口都可以是实例化类型,直接看代码:

interface IFooBase<IBaseT>{}

interface IFoo<InterfaceT>: IFooBase<string>
{
 void InterfaceMenthod(InterfaceT interfaceT);
}

class FooBase<ClassT>
{

}

class Foo<ClassT, ClassT1>: FooBase<ClassT>,IFoo<StringBuilder>{}

我们可以通过例子看出:

多态的递归#

我们定义如下一个类和一个方法,且不会报错:

 class D<T> { }
 class C<T> : D<C<C<T>>> 
 { 
 void Foo()
 {
  var foo = new C<C<T>>();
  Console.WriteLine(foo.ToString());
 }
 }

因为T能在实例化的时候确定其类型,因此也支持这种循环套用自己的类和方法的定义

四.泛型的约束#

where的约束#

我们先上代码:

 class FooBase{ }

 class Foo : FooBase 
 {
  
 }
 
 class someClass<T,K> where T:struct where K :FooBase,new()
 {

 }

 static void TestConstraint()
 {
  var someClass = new someClass<int, Foo>();//通过编译
  //var someClass = new someClass<string, Foo>();//编译失败,string不是struct类型
  //var someClass = new someClass<string, long>();//编译失败,long不是FooBase类型
 }

 

再改动下Foo类:

class Foo : FooBase 
{
 public Foo(string str)
 {

 }
}

static void TestConstraint()
{
 var someClass = new someClass<int, Foo>();//编译失败,因为new()约束必须类含有一个无参构造器,可以再给Foo类加上个无参构造器就能编译通过
}

 我们可以看到,通过where语句,可以对类型参数进行约束,而且一个类型参数支持多个约束条件(例如K),使其在实例化类型参数的时候,必须按照约束的条件对应实例符合条件的类型,而where条件约束的作用就是起在编译期约束类型参数的作用

out和in的约束#

 说到out和in之前,我们可以说下协变和逆变,在C#中,只有泛型接口和泛型委托可以支持协变和逆变

协变#

我们先看下代码:

class FooBase{ }

class Foo : FooBase 
{

}

interface IBar<T> 
{
 T GetValue(T t);
}

class Bar<T> : IBar<T>
{
 public T GetValue(T t)
 {
  return t;
 }
}

static void Test()
{
 var foo = new Foo();
 FooBase fooBase = foo;//编译成功

 IBar<Foo> bar = new Bar<Foo>();
 IBar<FooBase> bar1 = bar;//编译失败
 }

 这时候你可能会有点奇怪,为啥那段代码会编译失败,明明Foo类可以隐式转为FooBase,但作为泛型接口类型参数实例化却并不能呢?使用out约束泛型接口IBar的T,那段代码就会编译正常,但是会引出另外一段编译报错:

interface IBar<out T> 
{
 T GetValue(string str);//编译成功
 //T GetValue(T t);//编译失败 T不能作为形参输入,用out约束T支持协变,T可以作为返回值输出
 
}

IBar<Foo> bar = new Bar<Foo>();
IBar<FooBase> bar1 = bar;//编译正常

因此我们可以得出以下结论:

而支持迭代的泛型接口IEnumerable也是这么定义的:

 public interface IEnumerable<out T> : IEnumerable
 {
  new IEnumerator<T> GetEnumerator();
 }

逆变#

我们将上面代码改下:

class FooBase{ }

class Foo : FooBase 
{

}

interface IBar<T> 
{
 T GetValue(T t);
}

class Bar<T> : IBar<T>
{
 public T GetValue(T t)
 {
  return t;
 }
}

static void Test1()
{
 var fooBase = new FooBase();
 Foo foo = (Foo)fooBase;//编译通过,运行时报错

 IBar<FooBase> bar = new Bar<FooBase>();
 IBar<Foo> bar1 = (IBar<Foo>)bar;//编译通过,运行时报错
}

我们再改动下IBar,发现出现另外一处编译失败

interface IBar<in T> 
{
 void GetValue(T t);//编译成功
 //T GetValue(T t);//编译失败 T不能作为返回值输出,用in约束T支持逆变,T可以作为返回值输出
}

 IBar<FooBase> bar = new Bar<FooBase>();
 IBar<Foo> bar1 = (IBar<Foo>)bar;//编译通过,运行时不报错
 IBar<Foo> bar1 = bar;//编译通过,运行时不报错

因此我们可以得出以下结论:

同样的泛型委托Action就是个逆变的例子:

public delegate void Action<in T>(T obj);

五.泛型的反射#

我们先来看看以下代码:

static void Main(string[] args)
{
 var lsInt = new ArrayExpandable<int>();
 lsInt.Add(1);
 var lsStr = new ArrayExpandable<string>();
 lsStr.Add("ryzen");
 var lsStr1 = new ArrayExpandable<string>();
 lsStr.Add("ryzen");
}

然后通过ildasm查看其IL,开启视图-》显示标记值,查看Main方法:

void Main(string[] args) cil managed
{
 .entrypoint
 // 代码大小  52 (0x34)
 .maxstack 2
 .locals /*11000001*/ init (class MetaTest.ArrayExpandable`1/*02000003*/<int32> V_0,
   class MetaTest.ArrayExpandable`1/*02000003*/<string> V_1,
   class MetaTest.ArrayExpandable`1/*02000003*/<string> V_2)
 IL_0000: nop
 IL_0001: newobj  instance void class MetaTest.ArrayExpandable`1/*02000003*/<int32>/*1B000001*/::.ctor() /* 0A00000C */
 IL_0006: stloc.0
 IL_0007: ldloc.0
 IL_0008: ldc.i4.1
 IL_0009: callvirt instance void class MetaTest.ArrayExpandable`1/*02000003*/<int32>/*1B000001*/::Add(!0) /* 0A00000D */
 IL_000e: nop
 IL_000f: newobj  instance void class MetaTest.ArrayExpandable`1/*02000003*/<string>/*1B000002*/::.ctor() /* 0A00000E */
 IL_0014: stloc.1
 IL_0015: ldloc.1
 IL_0016: ldstr  "ryzen" /* 70000001 */
 IL_001b: callvirt instance void class MetaTest.ArrayExpandable`1/*02000003*/<string>/*1B000002*/::Add(!0) /* 0A00000F */
 IL_0020: nop
 IL_0021: newobj  instance void class MetaTest.ArrayExpandable`1/*02000003*/<string>/*1B000002*/::.ctor() /* 0A00000E */
 IL_0026: stloc.2
 IL_0027: ldloc.1
 IL_0028: ldstr  "ryzen" /* 70000001 */
 IL_002d: callvirt instance void class MetaTest.ArrayExpandable`1/*02000003*/<string>/*1B000002*/::Add(!0) /* 0A00000F */
 IL_0032: nop
 IL_0033: ret
} // end of method Program::Main

打开元数据表将上面所涉及到的元数据定义表和类型规格表列出:

metainfo:

-----------定义部分
TypeDef #2 (02000003)
-------------------------------------------------------
	TypDefName: MetaTest.ArrayExpandable`1 (02000003)
	Flags  : [Public] [AutoLayout] [Class] [AnsiClass] [BeforeFieldInit] (00100001)
	Extends : 0100000C [TypeRef] System.Object
	1 Generic Parameters
		(0) GenericParamToken : (2a000001) Name : T flags: 00000000 Owner: 02000003
	
	Method #8 (0600000a) 
	-------------------------------------------------------
		MethodName: Add (0600000A)
		Flags  : [Public] [HideBySig] [ReuseSlot] (00000086)
		RVA  : 0x000021f4
		ImplFlags : [IL] [Managed] (00000000)
		CallCnvntn: [DEFAULT]
		hasThis 
		ReturnType: Void
		1 Arguments
			Argument #1: Var!0
		1 Parameters
		(1) ParamToken : (08000007) Name : value flags: [none] (00000000)
		

------类型规格部分
TypeSpec #1 (1b000001)
-------------------------------------------------------
	TypeSpec : GenericInst Class MetaTest.ArrayExpandable`1< I4> //14代表int32
	MemberRef #1 (0a00000c)
	-------------------------------------------------------
		Member: (0a00000c) .ctor: 
		CallCnvntn: [DEFAULT]
		hasThis 
		ReturnType: Void
		No arguments.
	MemberRef #2 (0a00000d)
	-------------------------------------------------------
		Member: (0a00000d) Add: 
		CallCnvntn: [DEFAULT]
		hasThis 
		ReturnType: Void
		1 Arguments
			Argument #1: Var!0

TypeSpec #2 (1b000002)
-------------------------------------------------------
	TypeSpec : GenericInst Class MetaTest.ArrayExpandable`1< String>
	MemberRef #1 (0a00000e)
	-------------------------------------------------------
		Member: (0a00000e) .ctor: 
		CallCnvntn: [DEFAULT]
		hasThis 
		ReturnType: Void
		No arguments.
	MemberRef #2 (0a00000f)
	-------------------------------------------------------
		Member: (0a00000f) Add: 
		CallCnvntn: [DEFAULT]
		hasThis 
		ReturnType: Void
		1 Arguments
		Argument #1: Var!0

 这时候我们就可以看出,元数据为泛型类ArrayExpandable<T>定义一份定义表,生成两份规格,也就是当你实例化类型参数为int和string的时候,分别生成了两份规格代码,同时还发现以下的现象:

var lsInt = new ArrayExpandable<int>();//引用的是类型规格1b000001的成员0a00000c .ctor构造
lsInt.Add(1);//引用的是类型规格1b000001的成员0a00000d Add
 
var lsStr = new ArrayExpandable<string>();//引用的是类型规格1b000002的成员0a00000e .ctor构造
lsStr.Add("ryzen");//引用的是类型规格1b000002的成员0a00000f Add
var lsStr1 = new ArrayExpandable<string>();//和lsStr一样
lsStr.Add("ryzen");//和lsStr一样

 非常妙的是,当你实例化两个一样的类型参数string,是共享一份类型规格的,也就是同享一份本地代码,因此上面的代码在线程堆栈和托管堆的大致是这样的:

由于泛型也有元数据的存在,因此可以对其做反射:

Console.WriteLine($"-----------{nameof(lsInt)}---------------");
Console.WriteLine($"{nameof(lsInt)} is generic?:{lsInt.GetType().IsGenericType}");
Console.WriteLine($"Generic type:{lsInt.GetType().GetGenericArguments()[0].Name}");
Console.WriteLine("---------Menthods:");
foreach (var method in lsInt.GetType().GetMethods())
{
  Console.WriteLine(method.Name);
}
Console.WriteLine("---------Properties:");
foreach (var property in lsInt.GetType().GetProperties())
{
  Console.WriteLine($"{property.PropertyType.ToString()}:{property.Name}");
}


Console.WriteLine($"\n-----------{nameof(lsStr)}---------------");
Console.WriteLine($"{nameof(lsStr)} is generic?:{lsStr.GetType().IsGenericType}");
Console.WriteLine($"Generic type:{lsStr.GetType().GetGenericArguments()[0].Name}");
Console.WriteLine("---------Menthods:");
foreach (var method in lsStr.GetType().GetMethods())
{
  Console.WriteLine(method.Name);
}
Console.WriteLine("---------Properties:");
foreach (var property in lsStr.GetType().GetProperties())
{
  Console.WriteLine($"{property.PropertyType.ToString()}:{property.Name}");
}

输出:

-----------lsInt---------------
lsInt is generic?:True
Generic type:Int32
---------Menthods:
get_Item
set_Item
get_Capacity
set_Capacity
get_Count
Add
GetType
ToString
Equals
GetHashCode
---------Properties:
System.Int32:Item
System.Int32:Capacity
System.Int32:Count


-----------lsStr---------------
lsStr is generic?:True
Generic type:String
---------Menthods:
get_Item
set_Item
get_Capacity
set_Capacity
get_Count
Add
GetType
ToString
Equals
GetHashCode
---------Properties:
System.String:Item
System.Int32:Capacity
System.Int32:Count

六.总结#

 泛型编程作为.NET体系中一个很重要的编程思想,主要有以下亮点:

参考#
Design and Implementation of Generics for the .NET Common Language Runtime

https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/generics/

《CLR Via C# 第四版》

《你必须知道的.NET(第二版)》

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