为什么Nullable< T>考虑一个结构，而不是一个类？ [英] Why is Nullable<T> considered a struct and not a class?

问题描述

我试图定义一个泛型类，它接受任何可以设置为null的类型：

  public abstract class BundledValue_Classes< T> 
其中T：class 
 {
 private Tuple< T，object> _bundle = new Tuple< T，object>（）; 
 public T Value 
 {
 get {return _bundle == null？ null：_bundle.Item1; } 
 set {_bundle = new Tuple< T，object>（value，obj）; } 
} 
 // ... 
  

这很好。我也想创建一个可以接受任何基本类型（int，double等）的派生类，并简单地将其解析为与上述类相同的类，但是其中的基元被包装在Nullable $ b $中b

  public abstract class BundledValue_Structs< T> 
：BundledValue_Classes< T？> 
其中T：struct 
 {
 // ... 
} 
  

但由于某种原因，这不起作用。我得到的错误是：类型'T？'必须是引用类型才能在通用类型或方法'BundledValue_Classes< T>'中使用它作为参数'T'

是否清楚我在这里要做什么？我不想复制粘贴整个 BundledValue_Classes< T> 类的内容来处理包含在 Nullable< T> 。

也许有一些方法可以在 BundledValue_Classes< T> ，这样它不仅可以接受类，而且还可以为任何类型分配一个空值（包括 Nullable< T> 类型，这似乎是唯一的）

解决方案

可为空< T> 是一个结构体，根据文档。它根本不是一个班级。它是一个值类型，在字段方面看起来有点像这样：

  public struct Nullable< T> 
 {
私人只读T值; 
 private readonly bool hasValue; 
 
  

null 值对于 Nullable< T> 类型不是空引用，它只是 hasValue 字段为false的值。

请注意， Nullable< T> 不满足约束条件其中TFoo：struct 然而，因为它实际上是一个不可空值值类型约束。基本上 Nullable< T> 不满足类或 struct 约束。 不存在的限制，只允许可为空的类型（引用或可为空< T> ）。

I'm trying to define a generic class that takes any type that can be set to null:

public abstract class BundledValue_Classes<T> 
    where T : class
{
    private Tuple<T, object> _bundle= new Tuple<T, object>();
    public T Value
    { 
        get { return _bundle == null ? null : _bundle.Item1; }
        set { _bundle = new Tuple<T, object>(value, obj); }
    }
    //...

This works fine. I also want to create a derived class that can take any primitive type (int, double, etc), and simply resolve that into the same as the above class, but where the primitive is wrapped in a Nullable

public abstract class BundledValue_Structs<T> 
    : BundledValue_Classes<T?> 
    where T : struct
{
   //...
}

But for some reason, this doesn't work. I get the error: The type 'T?' must be a reference type in order to use it as parameter 'T' in the generic type or method 'BundledValue_Classes<T>'

Is it clear what I'm trying to do here? I don't want to have to copy paste the contents of the whole BundledValue_Classes<T> class just to handle the case of a primitive wrapped in a Nullable<T>.

Perhaps there's some way to set the type constraint on BundledValue_Classes<T> so that it will not only accept classes, but any type that can be assigned a null value (including Nullable<T> types, which appear to be the only exception to the rule.)

解决方案

Nullable<T> is a struct, as per the documentation. It simply isn't a class. It's a value type, which looks a bit like this in terms of fields:

public struct Nullable<T>
{
    private readonly T value;
    private readonly bool hasValue;
}

The null value for a Nullable<T> type isn't a null reference, it's just a value where the hasValue field is false.

Note that Nullable<T> doesn't satisfy a condition for a constraint of where TFoo : struct however, as that's actually a non-nullable value type constraint. Basically Nullable<T> doesn't satisfy either the class or the struct constraint. There's no constraint which does only allow for nullable types (reference or Nullable<T>).

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