F#: DataContractJsonSerializer.WriteObject method - json

Question

I am new to programming and F# is my first language.
Here are the relevant parts of my code:
let internal saveJsonToFile<'t> (someObject:'t) (filePath: string) =
use fileStream = new FileStream(filePath, FileMode.OpenOrCreate)
(new DataContractJsonSerializer(typeof<'t>)).WriteObject(fileStream, someObject)
let dummyFighter1 = { id = 1; name = "Dummy1"; location = "Here"; nationality = "Somalia"; heightInMeters = 2.0; weightInKgs = 220.0; weightClass = "Too fat"}
let dummyFighter2 = { id = 2; name = "Dummy2"; location = "There"; nationality = "Afghanistan"; heightInMeters = 1.8; weightInKgs = 80.0; weightClass = "Just Nice"}
let filePath = #"G:\User\Fighters.json"
saveJsonToFile dummyFighter1 filePath
saveJsonToFile dummyFighter2 filePath
When I run "saveJsonToFile dummyFighter1 filePath", the information is successfully saved. My problem is this: Once I run "saveJsonToFile dummyFighter2 filePath", it immediately replaces all the contents that are already in the file, i.e., all the information about dummyFighter1.
What changes should I make so that information about dummyFighter2 is appended to the file, instead of replacing information about dummyFighter1?

Answer 1

Change the way you open a file setting FileMode.OpenOrCreate to FileMode.Append. Append means "create or append" :
use fileStream = new FileStream(filePath, FileMode.Append)
From MSDN (https://msdn.microsoft.com/fr-fr/library/system.io.filemode%28v=vs.110%29.aspx) :
FileMode.Append opens the file if it exists and seeks to the end of the file, or
creates a new file. This requires FileIOPermissionAccess.Append
permission. FileMode.Append can be used only in conjunction with
FileAccess.Write. Trying to seek to a position before the end of the
file throws an IOException exception, and any attempt to read fails
and throws a NotSupportedException exception.