当我使用XmlSerialiser反序列化DbNull时为什么不将它变成单身?

问题描述

我一直以为DbNull.value是一个单例.因此，您可以执行以下操作:

I've always assumed that DbNull.value was a singleton. And thus you could do things like this:

VB.NET:

If someObject Is DbNull.Value Then
    ...
End if

C#:

If (someObject == DbNull.Value)
{
    ...
}

但是最近，我使用XmlSerialiser序列化了一个DbNull实例，突然不再是单例了.类型比较操作(例如C#(obj是DBNull))可以正常工作.

But recently, I serialised a DbNull instance using the XmlSerialiser and suddenly it wasn't a singleton any more. Type comparison operations (like C#'s (obj is DBNull)) work OK though.

代码如下:

[Serializable, System.Xml.Serialization.XmlInclude(typeof(DBNull))]
public class SerialiseMe
{
    public SerialiseMe() { }

    public SerialiseMe(object value)
    {
        this.ICanBeDbNull = value;
    }
    public Object ICanBeDbNull { get; set; }
}

public void Foo()
{
    var serialiseDbNull = new SerialiseMe(DBNull.Value);
    var serialiser = new System.Xml.Serialization.XmlSerializer(typeof(SerialiseMe));
    var ms = new System.IO.MemoryStream();
    serialiser.Serialize(ms, serialiseDbNull);
    ms.Seek(0, System.IO.SeekOrigin.Begin);
    var deSerialisedDbNull = (SerialiseMe)serialiser.Deserialize(ms);

    // Is false, WTF!
    var equalsDbNullDeserialised = deSerialisedDbNull.ICanBeDbNull == DBNull.Value;
    // Is false, WTF!
    var refEqualsDbNullDeserialised = object.ReferenceEquals(deSerialisedDbNull.ICanBeDbNull, DBNull.Value);
    // Is true.
    var convertIsDbNullDeserialised = Convert.IsDBNull(deSerialisedDbNull.ICanBeDbNull);
    // Is true.
    var isIsDbNullDeserialised = deSerialisedDbNull.ICanBeDbNull is DBNull;

}

为什么会这样?以及它是如何发生的?并可能在其他任何静态字段中发生吗?

Why is this the case? And how does it happen? And can it possibly happen with any other static fields?

PS:我知道VB代码示例正在进行参考比较，而c#正在调用Object.Equals.两者与DBNull的行为相同.我通常使用VB.

PS: I am aware the VB code sample is doing a reference comparison and c# is calling Object.Equals. Both have the same behaviour with DBNull. I usually work with VB.

推荐答案

尽管 DBNull.Value 是静态只读，并且仅作为单个实例存在...反序列化后，序列化代码将根据流中的数据"创建类 DBNull 的新实例.由于 DBNull.Value 只是一个 DBNull 实例，因此序列化无法知道它是一个特殊"实例.

Although DBNull.Value is a static readonly and only exists as a single instance... when you de-serialize, the serialization code would be creating a new instance of the class DBNull from the 'data' in the stream. Since the DBNull.Value is simply a DBNull instance, there is no way for serialization to know that it is a 'special' instance.

注意:
出于同样的原因，如果使用序列化然后反序列化的单例"实例创建自己的类，则将获得完全相同的行为.尽管反序列化实例将与原始实例没有区别，但它们不会是同一实例.

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