FileCache.cs

    class FileCache : TokenCache
    {
        private const DataProtectionScope DpScope = DataProtectionScope.CurrentUser;
        private static readonly byte[] DpEntropy = Convert.FromBase64String("SmuJ3fxsOxitDcR+51332A==");
        private const string FileLockPrefix = "9384fef8-0a3e-4bad-8c5e-8d1001a80aba";
        private readonly object _fileLock;
        private readonly FileInfo _file;

        public FileCache(string filePath = @".\TokenCache.dat")
        {
            _file = new FileInfo(filePath);
            AfterAccess = AfterAccessNotification;
            BeforeAccess = BeforeAccessNotification;
            _fileLock = string.Intern(FileLockPrefix + _file.FullName.ToLowerInvariant());

            lock (_fileLock)
            {
                ReadFile();
            }
        }

        public override void Clear()
        {
            lock (_fileLock)
            {
                _file.Delete();
            }
        }

        void BeforeAccessNotification(TokenCacheNotificationArgs args)
        {
            lock (_fileLock)
            {
                ReadFile();
            }
        }

        void AfterAccessNotification(TokenCacheNotificationArgs args)
        {
            if (!HasStateChanged)
                return;

            lock (_fileLock)
            {
                if (!HasStateChanged)
                    return;

                WriteFile();
                HasStateChanged = false;
            }
        }

        private void ReadFile()
        {
            try
            {
                var cipherText = ReadAllBytes(_file.FullName);
                var clearText = Unprotect(cipherText, DpEntropy, DpScope);
                Deserialize(clearText);
            }
            catch
            {
                Deserialize(null);
            }
        }

        private void WriteFile()
        {
            var clearText = Serialize();
            var cipherText = Protect(clearText, DpEntropy, DpScope);
            WriteAllBytes(_file.FullName, cipherText);
        }
    }

Improvements:

• Lock scoped at file path (pluralsight example used a static lock accessed by instance methods)
• Lock contended for at surface level of call stack (this pattern leads to deadlocks less often)
• Double-checked locking at call site for writing to file
• DPAPI entropy provided.
• Removed file exists race condition.
• Lock when deleting file.

oops forgot to include usings:

using Microsoft.IdentityModel.Clients.ActiveDirectory;
using System;
using System.IO;
using System.Security.Cryptography;
using static System.IO.File;
using static System.Security.Cryptography.ProtectedData;