Table of Contents

Getting started

Installing

Important

Install 6.0.0 (netstandard2.0 + net10.0). 5.0.1.268 is the previous TFM set (net45 / netstandard1.6 / netstandard2.0).

dotnet add package FSharp.Interop.Dynamic

<PackageReference Include="FSharp.Interop.Dynamic" Version="6.0.0" />

open FSharp.Interop.Dynamic
open FSharp.Interop.Dynamic.Operators   // optional: ?+?, ?=?, …

open FSharp.Interop.Dynamic brings in the auto-opened ?, ?<-, and !? operators. The Dyn module is always there; you do not need a second open for it.

The three operators you actually use

Operator Meaning
target?Name Get a property, or call a method (the result can be a function you then apply)
target?Name <- value Set a property
!?target Invoke target itself (an F# function boxed as obj, a delegate, a Dynamitey curry) 
open System.Dynamic

let o = ExpandoObject()
o?Name <- "Ada"
let name: string = o?Name

let parts: string[] = "a,b,c"?Split(',')

Annotate the result type when F# cannot infer it. The DLR conversion uses that type.

Why not just use C# dynamic from F#?

You can, with a C# helper, and for a member whose name is a literal in C# that is often simpler. Reach for this library when:

  • The call is in F#. There is no dynamic keyword. ? is the F# spelling.
  • The name is data. o?Name still writes Name in source; Dyn.get name o takes a string.
  • You want F# piping. target |> Dyn.get "Name", target |> Dyn.tryGet "Name".
  • You want an option, not an exception. Dyn.tryGet / Dyn.exists — see tryGet and exists. C# dynamic has no equivalent.

Infer the result, or you get obj

let n: int = "hello"?Length          // 5
let boxed = "hello"?Length           // obj, because nothing asked for int

Methods that return unit (void in CLR) need that annotation too, or the binder asks for a value from a void member and throws:

let items = ResizeArray<string>()
let _: unit = items?Add("x")

Where to go next

  • Why this library — untyped payloads, optional fields, names as data, C# dynamic
  • Operators — ?, ?<-, !? in detail
  • Dyn — the functions behind those operators
  • tryGet and exists — the issue #27 surface
  • Caveats — explicit interface members, AOT, conversion vs missing, 15-arg, optionals, null target
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