为什么这个片段的输出 System.Int32的而不是可空＆LT;的Int32＆GT;

 诠释？ X = 5;
Console.WriteLine（x.GetType（））;
 

解决方案

的GetType（）是一个方法对象。照片 要调用它，在可空＆LT; T＆GT; 结构必须进行装箱

您可以看到在IL code：

  // INT？ X = 5;
IL_0000：ldloca.s 00
IL_0002：ldc.i4.5
IL_0003：拨打System.Nullable＆LT; System.Int32的＆GT; ..男星

//Console.WriteLine（x.GetType（））;
IL_0008：ldloc.0
IL_0009：箱System.Nullable＆LT; System.Int32的＆GT;
IL_000E：callvirt System.Object.GetType
IL_0013：打电话的System.Console.WriteLine
 

可空类型是由CLR特殊处理;它是不可能有一个可空类型的装箱实例。 相反，拳击可空类型将导致空引用（如果 hasVa​​lue的是假的），或装箱值（如果有值）。

因此​​，中System.Nullable＆LT; System.Int32的＆GT; 指令导致盒装的Int32 ，而不是盒装可空＆LT;的Int32＆GT;

因此​​，这是不可能的的GetType（）来的永远的回报可空＆LT; T＆GT; 。

要看到这更清楚，请看下面的code：

 静态无效的主要（）
{
    诠释？ X = 5;
    打印类型（X）;
}
静态无效的打印类型＆LT; T＆GT;（T VAL）{
    Console.WriteLine（编译时类型：+ typeof运算（T））;
    Console.WriteLine（运行时类型：+ val.GetType（））;
}
 

这版画

编译时类型：System.Nullable`1 [System.Int32的]   运行时类型：System.Int32

Why is the output of this snippet System.Int32 instead of Nullable<Int32>?

int? x = 5;
Console.WriteLine(x.GetType());

GetType() is a method of object. To call it, the Nullable<T> struct must be boxed.

You can see this in the IL code:

//int? x = 5;
IL_0000:  ldloca.s    00 
IL_0002:  ldc.i4.5    
IL_0003:  call        System.Nullable<System.Int32>..ctor

//Console.WriteLine(x.GetType());
IL_0008:  ldloc.0     
IL_0009:  box         System.Nullable<System.Int32>
IL_000E:  callvirt    System.Object.GetType
IL_0013:  call        System.Console.WriteLine

Nullable types are treated specially by CLR; it is impossible to have a boxed instance of a nullable type. Instead, boxing a nullable type will result in a null reference (if HasValue is false), or the boxed value (if there is a value).

Therefore, the box System.Nullable<System.Int32> instruction results in a boxed Int32, not a boxed Nullable<Int32>.

Therefore, it is impossible for GetType() to ever return Nullable<T>.

To see this more clearly, look at the following code:

static void Main()
{
    int? x = 5;
    PrintType(x);   
}
static void PrintType<T>(T val) {
    Console.WriteLine("Compile-time type: " + typeof(T));
    Console.WriteLine("Run-time type: " + val.GetType());
}

This prints

Compile-time type: System.Nullable`1[System.Int32] Run-time type: System.Int32