F# Dictionary of Options

Question

Why can't I add None to a System.Collections.Generic.Dictionary of Option ? Is it an expected behavior or a bug in Mono?

F# Interactive for F# 3.1 (Open Source Edition)
Freely distributed under the Apache 2.0 Open Source License

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> open System.Collections.Generic;;
> let d1 = new Dictionary<int option, int>();;

val d1 : Dictionary<int option,int> = dict []

> d1.Add(None, 1);;
System.ArgumentNullException: Value cannot be null.
Parameter name: key
   at System.ThrowHelper.ThrowArgumentNullException (ExceptionArgument argument) in <filename unknown>:line 0
   at System.Collections.Generic.Dictionary`2[TKey,TValue].Insert (System.Collections.Generic.TKey key, System.Collections.Generic.TValue value, Boolean add) in <filename unknown>:line 0
   at System.Collections.Generic.Dictionary`2[TKey,TValue].Add (System.Collections.Generic.TKey key, System.Collections.Generic.TValue value) in <filename unknown>:line 0
   at <StartupCode$FSI_0004>.$FSI_0004.main@ () in <filename unknown>:line 0
   at (wrapper managed-to-native) System.Reflection.MonoMethod:InternalInvoke (System.Reflection.MonoMethod,object,object[],System.Exception&)
   at System.Reflection.MonoMethod.Invoke (System.Object obj, BindingFlags invokeAttr, System.Reflection.Binder binder, System.Object[] parameters, System.Globalization.CultureInfo culture) in <filename unknown>:line 0
Stopped due to error
> d1.Add(Some 10, 1);;
val it : unit = ()

I'm using Mono on OS X.

$ mono --version
Mono JIT compiler version 4.2.0 (Stable 4.2.0.179/a224653 Tue Oct  6 11:28:25 PDT 2015)
Copyright (C) 2002-2014 Novell, Inc, Xamarin Inc and Contributors. www.mono-project.com
    TLS:           normal
    SIGSEGV:       altstack
    Notification:  kqueue
    Architecture:  amd64
    Disabled:      none
    Misc:          softdebug
    LLVM:          supported, not enabled.
    GC:            sgen

Answer 1

Its not a bug. Option<'T> uses the attribute [CompilationRepresentation( CompilationRepresentationFlags.UseNullAsTrueValue) which causes None to be represented as null.

So you actually are adding a null as a key to a Dictionary which as you know is not allowed.

For reference:

UseNullAsTrueValue: Permit the use of null as a representation for nullary discriminators in a discriminated union.

More information here Why is None represented as null?