How do you create F# anonymous records in C#?

Question

I can see that if I create a new anonymous record, eg.

let myRecord = {| SomeInteger = 5 |}

then if it's exposed to C# then I can dot into it with

var someInteger = myRecord.SomeInteger;

What about the other way round, if I have an F# function, say:

let unwrap (record : {| SomeInteger : int |}) = record.SomeInteger

and it's exposed to C#, how can I instantiate an argument for this function from C# and call it? I tried naively just placing a C# anonymous type there, ie.

var unwrapped = unwrap(new { SomeInteger = 5 });

but this didn't compile. I note in the RFC for the feature it's said that "The feature must achieve compatibility with C# anonymous objects (from C# 3.0)" but it's not specifically mentioned in which ways. Is this supported?

Answer 1

Unfortunately, it seems to be basically impossible at this. We can use the awesome tool https://sharplab.io/ to have a look what is going on and how will the API look like.

You can have a look at the example at: https://sharplab.io/#v2:EYLgxg9gTgpgtADwGwBYA0AbEAzAzgHwFgAoDGAFwAIBbATwCUZIoATSgXkoG99KBlCNRgBJAHbkYAcxhQOlAKyV8AXxJkqAV1EB3KAEMADgB4A5HoB8lABSxmbEN14ChYidNkOzS5QEo5t6BYAOmcRcSkZEiA==

I have pasted following F# into it and let it compile and then decompile as C# code:

let createRecord () = {| SomeInteger = 5 |}
let unwrap (record : {| SomeInteger : int |}) = record.SomeInteger

We can see that the F# compiler generated class named <>f__AnonymousType2453178905 . The name is the major problem, as you can't reference it in C# :/ (AFAIK). Btw, it interesting that the type of SomeInteger is generic, so you can write the unwrap function generic and it will still work:

let unwrap<'a> (record : {| SomeInteger : 'a |}) = record.SomeInteger

The translated function look like this

public static <>f__AnonymousType2453178905<int> createRecord()
{
    return new <>f__AnonymousType2453178905<int>(5);
}
public static int unwrap(<>f__AnonymousType2453178905<int> record)
{
    return record.SomeInteger;
}

That means:

• I'd avoid using anonymous records in public API
• If you really want to use them, you'll have to provide factory functions for them.
  • they can be generic in all parameters, for example let createMyRecord a = {| SomeInteger = a |} let createMyRecord a = {| SomeInteger = a |} will work well
  • while it will be usable from C#, the seeing that the parameter should be of type <>f__AnonymousType3239938913<<A>j__TPar, <B>j__TPar, <SomeInteger>j__TPar> will not be very helpful
• If somebody else has built the API and now you have to use, you can still create the instance using reflection
  • you can get the type from a MethodInfo by calling method.GetParameters()[0].ParameterType or by Type.GetType("namespace.typename numberOfGenericArgs")
  • the type has a constructor, the properties are there in alphabetic order. You can use Activator.CreateInstance(...) to call it.