How to compare equality of f# discriminated union instances?

Question

A month ago I asked that question about how to fulfill business rules with F# types.

The given answer worked well for me but when working with the type I struggled by comparing two instances of that type.

Here´s the implementation again:

    [<AutoOpen>]
    module JobId =
        open System
        type JobId = private JobId of string with
            static member Create() = JobId(Guid.NewGuid().ToString("N"))

            static member Create(id:Guid) =
                if id = Guid.Empty then None
                else Some(JobId(id.ToString("N")))

            static member Create(id:string) =
                try JobId.Create(Guid(id))
                with :? FormatException -> None

        [<Extension>]
        type Extensions =
            [<Extension>]
            static member GetValue(jobId : JobId) : string =
                match jobId with
                | JobId.JobId id -> id

Because it´s used as an identifier I always have to compare two instances.
code in C#

IEnumerable<JobId> jobIds = await GetAllJobids();
JobId selectedJobId = GetSelectedJobId();

// that´s what I would like to do
if (jobIds.Any(id => id == selectedJobId))
{
    ...
}

// now I always have to compare the strings
if (jobIds.Any(id => id.GetValue() == selectedJobId.GetValue()))
{
    ...
}

I tried to override == operators but i couldn´t do it because the internal value is not just a simple field.
Is it possible to get to my wanted behavior? Thanks for your help!

Answer 1

F# unions are .NET sealed classes with implemented structural compare by default, but they don't overrite the '==' operators.

Check the representation of the F# classes in C# . You can see how the .Equal method is implemented.

What you can do is

if (jobIds.Any(id => id.Equals(selectedJobId))) {}