Naive MSAL TokenCache in F#

CachePersistence.fs

//based on https://github.com/AzureAD/microsoft-authentication-library-for-dotnet/blob/90dca9e8a28102d7eb0d96d70e93350f5a0e095a/samples/desktop/SampleApp/CachePersistence.cs
module MicrosoftIdentityUtil

open Microsoft.Identity.Client
open System.IO

type CachePersistence() = 
    static let fileLock = new obj()
    static let usertokenCache = new TokenCache()
    static let cacheFilePath = 
        System.Reflection.Assembly.GetExecutingAssembly().Location 
        + "msalcache.txt"
    
    static let BeforeAccessNotification = 
        TokenCache.TokenCacheNotification(fun args -> 
            lock fileLock (fun () -> 
                let readContent = 
                    if File.Exists(cacheFilePath) then 
                        File.ReadAllBytes(cacheFilePath)
                    else null
                args.TokenCache.Deserialize(readContent)))

    static let AfterAccessNotification = 
        TokenCache.TokenCacheNotification(fun args ->
            if args.TokenCache.HasStateChanged
            then 
                lock fileLock (fun () -> 
                    File.WriteAllBytes(cacheFilePath, args.TokenCache.Serialize())
                    args.TokenCache.HasStateChanged<- false
                    ))
    
    static member getUserCache() = 
        lock fileLock (fun () -> 
            usertokenCache.SetBeforeAccess(BeforeAccessNotification)
            usertokenCache.SetAfterAccess(AfterAccessNotification)
            usertokenCache)