判断泛型的值是否为default(T)

   1: public static bool IsDefault<T>(this T value) 

   2: {

   3:       //...

   4: }

1. ==的问题

    如果用==直接判断(default(T) == value)，编译时会提示错误：Error CS0019: 运算符“==”无法应用于“T”和“T”类型的操作数 (CS0019)。

 

2. object.Equals的问题

    object提供了一个静态方法，可用于比较两个对象是否相等：

   1: public static bool Equals(object objA, object objB)

   2: {

   3:     if (objA == objB)

   4:     {

   5:         return true;

   6:     }

   7:     if (objA == null || objB == null)

   8:     {

   9:         return false;

  10:     }

  11:     return objA.Equals(objB);

  12: }

   但是该方法接收的是引用类型的实例，如果传入的是值类型(譬如int、enum、struct等)，则会对值类型进行装箱(boxing)。
   

3. EqualityComparer<T>.Default.Equals

    最给力的方法，还是用EqualityComparer<T>.Default.Equals，

   1: public static bool IsDefault<T>(this T value) 

   2: {

   3:      return EqualityComparer<T>.Default.Equals(value, default(T)); 

   4: }

   其实现了一个默认的比较器，可以比较Byte、Nullable<T>、Enum、Object、以及实现了IEquatable<T>接口(譬如int、bool、自定义类型等)的所有类型，可以用ILSpy来查看该类型的具体实现：

   1: private static EqualityComparer<T> defaultComparer;

   2: public static EqualityComparer<T> Default

   3: {

   4:     [TargetedPatchingOptOut("Performance critical to inline across NGen image boundaries"), SecuritySafeCritical]

   5:     get

   6:     {

   7:         EqualityComparer<T> equalityComparer = EqualityComparer<T>.defaultComparer;

   8:         if (equalityComparer == null)

   9:         {

  10:             equalityComparer = EqualityComparer<T>.CreateComparer();

  11:             EqualityComparer<T>.defaultComparer = equalityComparer;

  12:         }

  13:         return equalityComparer;

  14:     }

  15: }

  16: [SecuritySafeCritical]

  17: private static EqualityComparer<T> CreateComparer()

  18: {

  19:     RuntimeType runtimeType = (RuntimeType)typeof(T);

  20:     if (runtimeType == typeof(byte))//其实byte也实现了IEquatable<Byte>，不知道为神马要把byte单独提出来？？？

  21:     {

  22:         return (EqualityComparer<T>)new ByteEqualityComparer();

  23:     }

  24:     if (typeof(IEquatable<T>).IsAssignableFrom(runtimeType))//如果类型实现了IEquatable<T>，则在运行时构造GenericEqualityComparer<T>，调用类型自身提供(重写)的equals方法。

  25:     {

  26:         return (EqualityComparer<T>)RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(GenericEqualityComparer<int>), runtimeType);

  27:     }

  28:     if (runtimeType.IsGenericType && runtimeType.GetGenericTypeDefinition() == typeof(Nullable))

  29:     {

  30:         RuntimeType runtimeType2 = (RuntimeType)runtimeType.GetGenericArguments()[0];

  31:         if (typeof(IEquatable).MakeGenericType(new Type[]

  32:         {

  33:             runtimeType2

  34:         }).IsAssignableFrom(runtimeType2))

  35:         {

  36:             return (EqualityComparer<T>)RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(NullableEqualityComparer<int>), runtimeType2);

  37:         }

  38:     }

  39:     if (runtimeType.IsEnum && Enum.GetUnderlyingType(runtimeType) == typeof(int))

  40:     {

  41:         return (EqualityComparer<T>)RuntimeTypeHandle.CreateInstanceForAnotherGenericParameter((RuntimeType)typeof(EnumEqualityComparer<int>), runtimeType);

  42:     }

  43:     return new ObjectEqualityComparer<T>();

  44: }

 

4. ILSpy比JustDecompile强大呀

4.1. ObjectEqualityComparer<T>.Equals方法的比较

 

4.2. EqualityComparer<T>.CreateComparer()方法：

各种杯具。。。

 

标签: default(T),IEquatable<T>,EqualityCompare