Testing for null reference in F#

f#

Solution

To cyclically expand on @Tomas' answer ;-]

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

or

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

`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.

Problem

Given the following: ``` [<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} ``` 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). If I try to write the following: ``` let name = if tweet.User <> null then tweet.User.Name else tweet.Sender.Name ``` the compiler emits the error: "The type 'TweetUser' does not have 'null' as a proper value" How do I test null values in this case?

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