ToString exception while printing enum

Question

Given I have an enum:

type Cool = A = 'a'
          | B = 'b'

And i try to print it like so

printf "%A" Cool.A

I get the following exception:

ToString exception: The value passed in must be an enum base or an underlying type for an enum, such as an Int32. Parameter name: value

Why does this happen and (if possible) how I can fix this at the enum level?

Answer 1

I can repro this on a machine with F# 2.0 compiler targeting .Net 4.0, but not with F# 2 targeting .Net 3.5 or less, or on a VS2012 (F# 3.0) machine (where .Net 4.5 supersedes 4.0).

I am not sure right now of the cause, but hopefully this info may help you or others narrow it down or find a workaround.

Answer 2

I think it's better not to use char as base value for enum.
Although char seems to be allowed (MSDN article on F# enums), the corresponding article for C# says the opposite. Moreover, an older article says specifically:

base-type (Optional)
The underlying type that specifies the storage allocated for each enumerator. It can be one of the integral types except char. The default is int.

Looking at the source code of System.Enum.ToObject(Type,Object), it is also evident that char is not supported.

So, even if F# performs special handling of enum<char>, better to avoid it, keeping yourself from future integration problems.