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How to discover mutability of fields through reflection
f#

I would like to discover through Reflection whether a given field of a class is mutable or not. It would seem that the IsInitOnly property should be true for immutable fields. However, that seems not to be the case:

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type TestType(immutableParam: int) =
    let mutable mutableField = immutableParam
    member t.Param = immutableParam
    
let t = TestType 10
t.GetType().GetField("immutableParam", System.Reflection.BindingFlags.NonPublic ||| System.Reflection.BindingFlags.Instance).IsInitOnly
|> printf "%A\n"
t.GetType().GetField("mutableField", System.Reflection.BindingFlags.NonPublic ||| System.Reflection.BindingFlags.Instance).IsInitOnly
|> printf "%A\n"

This returns false for both the mutable and the immutable field. Is there something I'm doing wrong or is that a problem with the F# compiler? Is there another way to achieve what I want?

.

I got a response from Luke at Microsoft:

You

are right that F# doesn’t code-gen the IsInitOnly flag. It would certainly be

possible to emit it more often, but in general F# needs the flexibility to have

different notions of immutability than those of the IsInitOnly flag in the CLR.

So you won’t be able to treat “F# immutable” as equivalent to “IsInitOnly is

true”. Of course, the result is that .NET reflection cannot discover F#

immutability. You could take a look at FSharp.PowerPack.Metadata in the F#

PowerPack, which provides some functionality for reading the F# metadata

directly.

By on 7/10/2009 3:01 PM ()

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