FSharp.Interop.Dynamic

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F# operators for the Dynamic Language Runtime. target?Name, target?Name <- value, and !?target are the F# spelling of C# dynamic, with piping and 'T option lookups.

Docs: fsprojects.github.io/FSharp.Interop.Dynamic

This library sits on Dynamitey 3.0.3. It is the F# surface, not Dynamitey itself.

F# has no dynamic keyword. Use this when the member is not known at compile time: Expando/JSON bags, optional fields, method names from config, C# APIs that return dynamic, pythonnet, COM. Use ordinary F# when the type is in your project. Full argument: Why this library.

Version story

6.0.0 is the current package: netstandard2.0 + net10.0, Dyn.tryGet / Dyn.exists. That is a TFM break from 5.0.1.268 (net45 / netstandard1.6 / netstandard2.0). netstandard2.0 still covers current .NET Framework and .NET.

Install (6.0.0)

dotnet add package FSharp.Interop.Dynamic

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

Quick start

open System.Dynamic
open FSharp.Interop.Dynamic

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

let hello: string = "HelloWorld"?Substring(0, 5)

Annotate the result type when F# cannot infer it. Void CLR methods need unit:

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

Check a member without throwing

let present: string option = o |> Dyn.tryGet "Name"     // Some "Ada"
let missing: string option = o |> Dyn.tryGet "NoSuch"   // None
o |> Dyn.exists "Name"    // true
o |> Dyn.exists "NoSuch"  // false

Lookup is Dynamitey InvokeGet. A present null is still present. A present value that cannot convert to 'T still throws — that is not a miss.

Pipe through Dyn

o |> Dyn.set "Name" "Ada"
let name: string = o |> Dyn.get "Name"
let hello: string = "HelloWorld" |> Dyn.invokeMember "Substring" (0, 5)

Target is last so piping works.

Binary operators

open FSharp.Interop.Dynamic.Operators

let n: int = 5 ?+? 4
let f: float = 5 ?+? 3.5
let s: string = "Hello" ?+? " World"
[1; 2; 3; 4] |> List.reduce (?+?)

Direct invoke

let add3: int -> int = !?(+) 3
add3 4  // 7

What ? does

That is why target?Foo(1, 2) is a method call: application forces a function type.

More: Operators, Dyn, tryGet.

Dynamitey

6.0.0 references Dynamitey 3.0.3. tryGet / exists do not wait on Dynamitey 4.0.0. The community continuation of Dynamitey is dynamitey-community/dynamitey; switching this package to it is #29.

You can still open Dynamitey for Build, Dynamic.Curry, DynamicObjects.Dictionary.

Caveats

• The DLR cannot see explicit interface members (same as C# dynamic).
• Not trim-safe or NativeAOT-safe.
• 15 or more arguments throw TypeLoadException (ImpromptuInterface was never shipped). Fourteen work.
• C# optional parameters must all be passed. The DLR does not fill defaults.
• tryGet / exists on a null target throw NullReferenceException (not None / false).
• Do not build member names from untrusted input. SECURITY.md.
• Historical: .NET Core 2.0.0–2.0.2 broke dynamic on nested types inside generics (#11). Current TFMs are fine.

Full list: Caveats.

Every F# snippet above is a test in Tests/ReadmeExamples.fs. pythonnet and SignalR sketches live on the why page and are not compiled here.

Build

Requires the .NET 10 SDK.

dotnet restore
dotnet build -c Release -warnaserror
dotnet test --project Tests/Tests.fsproj -c Release

Docs: dotnet tool install -g docfx --version 2.78.5, then dotnet build FSharp.Interop.Dynamic/FSharp.Interop.Dynamic.fsproj -c Release && docfx docfx/docfx.json.

Release: add the version's section to CHANGELOG.md, then tag v6.0.0 and push. Releasing.

Feedback and contributing

• Bugs and feature requests: open an issue. Include the F# and .NET versions and a minimal snippet.
• Security problems: do not open a public issue. Follow SECURITY.md.
• Code and docs: pull requests are welcome. CONTRIBUTING.md covers the process and what a PR needs before it merges.

Maintainers

• @jbtule, original author
• @AtwoodTM
• @forki

fsprojects default: @fsprojectsgit.