在F#中测试空引用

问题描述

给出以下内容:

[<DataContract>]
type TweetUser = {
    [<field:DataMember(Name="followers_count")>] Followers:int
    [<field:DataMember(Name="screen_name")>] Name:string
    [<field:DataMember(Name="id_str")>] Id:int
    [<field:DataMember(Name="location")>] Location:string}

[<DataContract>]
type Tweet = {
    [<field:DataMember(Name="id_str")>] Id:string
    [<field:DataMember(Name="text")>] Text:string
    [<field:DataMember(Name="retweeted")>] IsRetweeted:bool
    [<field:DataMember(Name="created_at")>] DateStr:string
    [<field:DataMember(Name="user", IsRequired=false)>] User:TweetUser
    [<field:DataMember(Name="sender", IsRequired=false)>] Sender:TweetUser
    [<field:DataMember(Name="source")>] Source:string}

使用DataContractJsonSerializer(typeof<Tweet[]>)反序列化将导致用户"或发件人"字段为空(至少这是调试器告诉我的内容).

Deserializing with DataContractJsonSerializer(typeof<Tweet[]>) will result in either the User or Sender field being null (at least that's what the debugger is telling me).

如果我尝试编写以下内容:

If I try to write the following:

    let name = if tweet.User <> null
                  then tweet.User.Name
                  else tweet.Sender.Name

编译器发出错误:类型'TweetUser'没有适当的'null'"

the compiler emits the error: "The type 'TweetUser' does not have 'null' as a proper value"

在这种情况下如何测试空值?

How do I test null values in this case?

推荐答案

要循环扩展@Tomas的答案;-]

To cyclically expand on @Tomas' answer ;-]

let name = if not <| obj.ReferenceEquals (tweet.User, null)
              then tweet.User.Name
              else tweet.Sender.Name

或

let inline isNull (x:^T when ^T : not struct) = obj.ReferenceEquals (x, null)

Unchecked.defaultof<_>在做正确的事情，并为您的记录类型生成null.问题是默认的相等运算符使用泛型结构比较，它期望您在使用F#类型时始终按照F#的规则进行操作.无论如何，空检查实际上只需要首先进行参照比较.

Unchecked.defaultof<_> is doing the right thing and producing nulls for your record types; the issue is that the default equality operator uses generic structural comparison, which expects you to always play by F#'s rules when using F# types. In any case, a null-check really only warrants referential comparison in the first place.

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