open System

open Microsoft.FSharp.Reflection

​

type op = A | B | C | D

​

let Construct<'T> (caseInfo: UnionCaseInfo) =

    let fields = caseInfo.GetFields()

    FSharpValue.MakeUnion(caseInfo, Array.zeroCreate fields.Length) :?> 'T

let GetUnionCaseInfoAndInstance<'T> (caseInfo: UnionCaseInfo) = (caseInfo, Construct<'T> caseInfo)

​

let AllCases<'T> = 

    FSharpType.GetUnionCases(typeof<'T>)

    |> Seq.map GetUnionCaseInfoAndInstance<'T>

​

let dict = [ for (_, instance) in AllCases<op> do yield instance ]

​

printfn "DUs: %A" dict

​