How to match Nullable Date parameter in f#

Question

Learning F#, can't find the answer to this... I'd like to handle case where a Nullable parameter (DateTime? in the original c#) is null, but I get the error "Nullable does not have null as a proper value". What is the correct way to do this?

let addIfNotNull(ht:Hashtable, key:string, value:Nullable<DateTime>) = 
        match value with 
        | null -> ()
        | _ -> ht.Add(key,value)
        ht  

Answer 1

从https://docs.microsoft.com/zh-cn/dotnet/fsharp/language-reference/symbol-and-operator-reference/nullable-operators：

可以System.Nullable<'T>通过使用Value属性从对象获得实际值，并且可以System.Nullable<'T>通过调用HasValue方法确定对象是否具有值。

所以代替match你

if value.HasValue then ht.Add(key, value.Value)

你可以用

match Option.ofNullable value with ...

或声明一些活动模式以提供帮助。