Code Listings

Chapter 21: Threading

Getting started

class ThreadTest
{
  static void Main()
  {
    Thread t = new Thread (WriteY);          // Kick off a new thread
    t.Start();                               // running WriteY()

    // Simultaneously, do something on the main thread.
    for (int i = 0; i < 1000; i++) Console.Write ("x");
  }

  static void WriteY()
  {
    for (int i = 0; i < 1000; i++) Console.Write ("y");
  }
}

Calling Join:

static void Main()
{
  Thread t = new Thread (Go);
  t.Start();
  t.Join();
  Console.WriteLine ("Thread t has ended!");
}

static void Go() { for (int i = 0; i < 1000; i++) Console.Write ("y"); }

Thread.Sleep

Thread.Sleep (TimeSpan.FromHours (1));  // sleep for 1 hour
Thread.Sleep (500);                     // sleep for 500 milliseconds

Passing data to a thread - with a lambda expression

static void Main()
{
  Thread t = new Thread ( () => Print ("Hello from t!") );
  t.Start();
}

static void Print (string message) { Console.WriteLine (message); }

new Thread (() =>
{
  Console.WriteLine ("I'm running on another thread!");
  Console.WriteLine ("This is so easy!");
}).Start()

ParameterizedThreadStart

static void Main()
{
  Thread t = new Thread (Print);
  t.Start ("Hello from t!");
}

static void Print (object messageObj)
{
  string message = (string) messageObj;   // We need to cast here
  Console.WriteLine (message);
}

Lambda expressions and captured variables

for (int i = 0; i < 10; i++)
  new Thread (() => Console.Write (i)).Start();

Fix:

for (int i = 0; i < 10; i++)
{
  int temp = i;
  new Thread (() => Console.Write (temp)).Start();
}

Sharing data between threads - each stack is isolated:

static void Main()
{
  new Thread (Go).Start();      // Call Go() on a new thread
  Go();                         // Call Go() on the main thread
}

static void Go()
{
  // Declare and use a local variable - 'cycles'
  for (int cycles = 0; cycles < 5; cycles++) Console.Write (cycles);
}

// OUTPUT:  0123401234

Sharing via a common reference

static void Main()
{
  Introducer intro = new Introducer();
  intro.Message = "Hello";

  var t = new Thread (intro.Run);
  t.Start(); t.Join();

  Console.WriteLine (intro.Reply);
}

class Introducer
{
  public string Message;
  public string Reply;

  public void Run()
  {
    Console.WriteLine (Message);
    Reply = "Hi right back!";
  }
}

Foreground and background threads

class PriorityTest
{
  static void Main (string[] args)
  {
    Thread worker = new Thread ( () => Console.ReadLine() );
    if (args.Length > 0) worker.IsBackground = true;
    worker.Start();
  }
}

Exception handling

public static void Main()
{
   new Thread (Go).Start();
}

static void Go()
{
  try
  {
    ...
    throw null;    // The NullReferenceException will get caught below
    ...
  }
  catch (Exception ex)
  {
    Typically log the exception, and/or signal another thread
    that we've come unstuck
    ...
  }
}

Thread pooling - via TPL

static void Main()    // The Task class is in System.Threading.Tasks
{
  Task.Factory.StartNew (Go);
}

static void Go()
{
  Console.WriteLine ("Hello from the thread pool!");
}

Task<TResult>

static void Main()
{
  // Start the task executing:
  Task<string> task = Task.Factory.StartNew<string>
    ( () => DownloadString ("http://www.linqpad.net") );

  // We can do other work here and it will execute in parallel:
  RunSomeOtherMethod();

  // When we need the task's return value, we query its Result property:
  // If it's still executing, the current thread will now block (wait)
  // until the task finishes:
  string result = task.Result;
}

static string DownloadString (string uri)
{
  using (var wc = new System.Net.WebClient())
    return wc.DownloadString (uri);
}

Entering thread pool with QueueUserWorkItem

static void Main()
{
  ThreadPool.QueueUserWorkItem (Go);
  ThreadPool.QueueUserWorkItem (Go, 123);
  Console.ReadLine();
}

static void Go (object data)   // data will be null with the first call.
{
  Console.WriteLine ("Hello from the thread pool! " + data);
}

Thread pool with async delegates

static void Main()
{
  Func<string, int> method = Work;
  IAsyncResult cookie = method.BeginInvoke ("test", null, null);
  //
  // ... here's where we can do other work in parallel...
  //
  int result = method.EndInvoke (cookie);
  Console.WriteLine ("String length is: " + result);
}

static int Work (string s) { return s.Length; }

Async delegates with callback

static void Main()
{
  Func<string, int> method = Work;
  method.BeginInvoke ("test", Done, method);
  // ...
  //
}

static int Work (string s) { return s.Length; }

static void Done (IAsyncResult cookie)
{
  var target = (Func<string, int>) cookie.AsyncState;
  int result = target.EndInvoke (cookie);
  Console.WriteLine ("String length is: " + result);
}

Locking - unsafe

class ThreadUnsafe
{
  static int _val1 = 1, _val2 = 1;

  static void Go()
  {
    if (_val2 != 0) Console.WriteLine (_val1 / _val2);
    _val2 = 0;
  }
}

With a lock:

class ThreadSafe
{
  static readonly object _locker = new object();
  static int _val1, _val2;

  static void Go()
  {
    lock (_locker)
    {
      if (_val2 != 0) Console.WriteLine (_val1 / _val2);
      _val2 = 0;
    }
  }
}

Monitor.Enter and Monitor.Exit

Monitor.Enter (_locker);
try
{
  if (_val2 != 0) Console.WriteLine (_val1 / _val2);
  _val2 = 0;
}
finally { Monitor.Exit (_locker); }

Monitor.Enter - lockTaken overloads

bool lockTaken = false;
try
{
  Monitor.Enter (_locker, ref lockTaken);
  // Do your stuff...
}
finally { if (lockTaken) Monitor.Exit (_locker); }

Nested (reentrant) locking

static readonly object _locker = new object();

static void Main()
{
  lock (_locker)
  {
     AnotherMethod();
     // We still have the lock - because locks are reentrant.
  }
}

static void AnotherMethod()
{
  lock (_locker) { Console.WriteLine ("Another method"); }
}

Deadlocks

object locker1 = new object();
object locker2 = new object();

new Thread (() => {
                    lock (locker1)
                    {
                      Thread.Sleep (1000);
                      lock (locker2);      // Deadlock
                    }
                  }).Start();
lock (locker2)
{
  Thread.Sleep (1000);
  lock (locker1);                          // Deadlock
}

Mutex - processor-wide

class OneAtATimePlease
{
  static void Main()
  {
    // Naming a Mutex makes it available computer-wide. Use a name that's
    // unique to your company and application (e.g., include your URL).

    using (var mutex = new Mutex (false, "oreilly.com OneAtATimeDemo"))
    {
      // Wait a few seconds if contended, in case another instance
      // of the program is still in the process of shutting down.

      if (!mutex.WaitOne (TimeSpan.FromSeconds (3), false))
      {
        Console.WriteLine ("Another instance of the app is running. Bye!");
        return;
      }
      RunProgram();
    }
  }

  static void RunProgram()
  {
    Console.WriteLine ("Running. Press Enter to exit");
    Console.ReadLine();
  }
}

Semaphore

class TheClub      // No door lists!
{
  static SemaphoreSlim _sem = new SemaphoreSlim (3);    // Capacity of 3
 
  static void Main()
  {
    for (int i = 1; i <= 5; i++) new Thread (Enter).Start (i);
  }

  static void Enter (object id)
  {
    Console.WriteLine (id + " wants to enter");
    _sem.Wait();
    Console.WriteLine (id + " is in!");           // Only three threads
    Thread.Sleep (1000 * (int) id);               // can be here at
    Console.WriteLine (id + " is leaving");       // a time.
    _sem.Release();
  }
}

Locking around thread-unsafe types

class ThreadSafe
{
  static List <string> _list = new List <string>();

  static void Main()
  {
    new Thread (AddItem).Start();
    new Thread (AddItem).Start();
  }

  static void AddItem()
  {
    lock (_list) _list.Add ("Item " + _list.Count);

    string[] items;
    lock (_list) items = _list.ToArray();
    foreach (string s in items) Console.WriteLine (s);
  }
}

Thread safety in application servers

static class UserCache
{
  static Dictionary <int, User> _users = new Dictionary <int, User>();

  internal static User GetUser (int id)
  {
    User u = null;

    lock (_users)
      if (_users.TryGetValue (id, out u))
        return u;

    u = RetrieveUser (id);           // Method to retrieve from database;
    lock (_users) _users [id] = u;
    return u;
  }
}

Rich client applications and thread safety

public partial class MyWindow : Window
{
  public MyWindow()
  {
    InitializeComponent();
    new Thread (Work).Start();
  }

  void Work()
  {
    Thread.Sleep (5000);           // Simulate time-consuming task
    UpdateMessage ("The answer");
  }

  void UpdateMessage (string message)
  {
    Action action = () => txtMessage.Text = message;
    Dispatcher.Invoke (action);
  }
}

Immutable classes for thread safety

class ProgressStatus    // Represents progress of some activity
{
  public readonly int PercentComplete;
  public readonly string StatusMessage;

  // This class might have many more fields...

  public ProgressStatus (int percentComplete, string statusMessage)
  {
    PercentComplete = percentComplete;
    StatusMessage = statusMessage;
  }
}

Memory barriers and volatility - unsafe

class Foo
{
  int _answer;
  bool _complete;

  void A()
  {
    _answer = 123;
    _complete = true;
  }

  void B()
  {
    if (_complete) Console.WriteLine (_answer);
  }
}

Full fences

class Foo
{
  int _answer;
  bool _complete;

  void A()
  {
    _answer = 123;
    Thread.MemoryBarrier();    // Barrier 1
    _complete = true;
    Thread.MemoryBarrier();    // Barrier 2
  }

  void B()
  {
    Thread.MemoryBarrier();    // Barrier 3
    if (_complete)
    {
      Thread.MemoryBarrier();       // Barrier 4
      Console.WriteLine (_answer);
    }
  }
}

class Foo
{
  int _answer1, _answer2, _answer3;
  bool _complete;

  void A()
  {
    _answer1 = 1; _answer2 = 2; _answer3 = 3;
    Thread.MemoryBarrier();
    _complete = true;
    Thread.MemoryBarrier();
  }

  void B()
  {
    Thread.MemoryBarrier();
    if (_complete)
    {
      Thread.MemoryBarrier();
      Console.WriteLine (_answer1 + _answer2 + _answer3);
    }
  }
}

Do we really need locks and barriers?

static void Main()
{
  bool complete = false; 
  var t = new Thread (() =>
  {
    bool toggle = false;
    while (!complete) toggle = !toggle;
  });
  t.Start();
  Thread.Sleep (1000);
  complete = true;
  t.Join();        // Blocks indefinitely
}

Traps with volatile

class IfYouThinkYouUnderstandVolatile
{
  volatile int x, y;
 
  void Test1()        // Executed on one thread
  {
    x = 1;            // Volatile write (release-fence)
    int a = y;        // Volatile read (acquire-fence)
    ...
  }
 
  void Test2()         // Executed on another thread
  {
    y = 1;            // Volatile write (release-fence)
    int b = x;        // Volatile read (acquire-fence)
    ...
  }
}

Interlocked

class Atomicity
{
  static int _x, _y;
  static long _z;

  static void Test()
  {
    long myLocal;
    _x = 3;             // Atomic
    _z = 3;             // Nonatomic on 32-bit environs (_z is 64 bits)
    myLocal = _z;       // Nonatomic on 32-bit environs (_z is 64 bits)
    _y += _x;           // Nonatomic (read AND write operation)
    _x++;               // Nonatomic (read AND write operation)
  }
}

class Program
{
  static long _sum;
 
  static void Main()
  {                                                             // _sum
    // Simple increment/decrement operations:
    Interlocked.Increment (ref _sum);                              // 1
    Interlocked.Decrement (ref _sum);                              // 0

    // Add/subtract a value:
    Interlocked.Add (ref _sum, 3);                                 // 3

    // Read a 64-bit field:
    Console.WriteLine (Interlocked.Read (ref _sum));               // 3

    // Write a 64-bit field while reading previous value:
    // (This prints "3" while updating _sum to 10)
    Console.WriteLine (Interlocked.Exchange (ref _sum, 10));       // 10

    // Update a field only if it matches a certain value (10):
    Console.WriteLine (Interlocked.CompareExchange (ref _sum,
                                                    123, 10);      // 123
  }
}

AutoResetEvent

class BasicWaitHandle
{
  static EventWaitHandle _waitHandle = new AutoResetEvent (false);

  static void Main()
  {
    new Thread (Waiter).Start();
    Thread.Sleep (1000);                  // Pause for a second...
    _waitHandle.Set();                    // Wake up the Waiter.
  }

  static void Waiter()
  {
    Console.WriteLine ("Waiting...");
    _waitHandle.WaitOne();                // Wait for notification
    Console.WriteLine ("Notified");
  }
}

AutoResetEvent - 2-way signaling

class TwoWaySignaling
{
  static EventWaitHandle _ready = new AutoResetEvent (false);
  static EventWaitHandle _go = new AutoResetEvent (false);
  static readonly object _locker = new object();
  static string _message;

  static void Main()
  {
    new Thread (Work).Start();

    _ready.WaitOne();                  // First wait until worker is ready
    lock (_locker) _message = "ooo";
    _go.Set();                         // Tell worker to go

    _ready.WaitOne();
    lock (_locker) _message = "ahhh";  // Give the worker another message
    _go.Set();

    _ready.WaitOne();
    lock (_locker) _message = null;    // Signal the worker to exit
    _go.Set();
  }

  static void Work()
  {
    while (true)
    {
      _ready.Set();                          // Indicate that we're ready
      _go.WaitOne();                         // Wait to be kicked off...
      lock (_locker)
      {
        if (_message == null) return;          // Gracefully exit
        Console.WriteLine (_message);
      }
    }
  }
}

CountdownEvent

static CountdownEvent _countdown = new CountdownEvent (3);

static void Main()
{
  new Thread (SaySomething).Start ("I am thread 1");
  new Thread (SaySomething).Start ("I am thread 2");
  new Thread (SaySomething).Start ("I am thread 3");
  _countdown.Wait();   // Blocks until Signal has been called 3 times
  Console.WriteLine ("All threads have finished speaking!");
}

static void SaySomething (object thing)
{
  Thread.Sleep (1000);
  Console.WriteLine (thing);
  _countdown.Signal();
}

Wait handles and the thread pool

static ManualResetEvent _starter = new ManualResetEvent (false);

public static void Main()
{
  RegisteredWaitHandle reg = ThreadPool.RegisterWaitForSingleObject
   (_starter, Go, "Some Data", -1, true);
  Thread.Sleep (5000);
  Console.WriteLine ("Signaling worker...");
  _starter.Set();
  Console.ReadLine();
  reg.Unregister (_starter);    // Clean up when we’re done.
}

public static void Go (object data, bool timedOut)
{
  Console.WriteLine ("Started - " + data);
  // Perform task...
}

Signaling with wait and pulse - simple example

class SimpleWaitPulse
{
  static readonly object _locker = new object();
  static bool _go;

  static void Main()
  {                                // The new thread will block
    new Thread (Work).Start();     // because _go==false.

    Console.ReadLine();            // Wait for user to hit Enter

    lock (_locker)                 // Let's now wake up the thread by
    {                              // setting _go=true and pulsing.
      _go = true;
      Monitor.Pulse (_locker);
    }
  }

  static void Work()
  {
    lock (_locker)
      while (!_go)
        Monitor.Wait (_locker);    // Lock is released while we’re waiting

    Console.WriteLine ("Woken!!!");
  }
}

Wait and Pulse - producer/consumer queue

using System;
using System.Threading;
using System.Collections.Generic;

public class PCQueue : IDisposable
{
  readonly object _locker = new object();
  Thread[] _workers;
  Queue<Action> _itemQ = new Queue<Action>();

  public PCQueue (int workerCount)
  {
    _workers = new Thread [workerCount];

    // Create and start a separate thread for each worker
    for (int i = 0; i < workerCount; i++)
      (_workers [i] = new Thread (Consume)).Start();
  }

  public void Dispose()
  {
    // Enqueue one null item per worker to make each exit.
    foreach (Thread worker in _workers) EnqueueItem (null);
  }

  public void EnqueueItem (Action item)
  {
    lock (_locker)
    {
      _itemQ.Enqueue (item);           // We must pulse because we're
      Monitor.Pulse (_locker);         // changing a blocking condition.
    }
  }

  void Consume()
  {
    while (true)                        // Keep consuming until
    {                                   // told otherwise.
      Action item;
      lock (_locker)
      {
        while (_itemQ.Count == 0) Monitor.Wait (_locker);
        item = _itemQ.Dequeue();
      }
      if (item == null) return;         // This signals our exit.
      item();                           // Execute item.
    }
  }
}

static void Main()
{
  using (PCQueue q = new PCQueue (2))
  {
    for (int i = 0; i < 10; i++)
    {
      int itemNumber = i;      // To avoid the captured variable trap
      q.EnqueueItem (() =>
      {
        Thread.Sleep (1000);          // Simulate time-consuming work
        Console.Write (" Task" + itemNumber);
      });
    }

    Console.WriteLine ("Enqueued 10 items");
    Console.WriteLine ("Waiting for items to complete...");
  }

  // Exiting the using statement runs PCQueue's Dispose method, which
  // tells the consumers to end when outstanding items are complete.
}

Simulating wait handles

readonly object _locker = new object();
bool _signal;

void WaitOne()
{
  lock (_locker)
  {
    while (!_signal) Monitor.Wait (_locker);
  }
}

void Set()
{
  lock (_locker) { _signal = true; Monitor.PulseAll (_locker); }
}

void Reset() { lock (_locker) _signal = false; }

The Barrier Class

static Barrier _barrier = new Barrier (3);

static void Main()
{
  new Thread (Speak).Start();
  new Thread (Speak).Start();
  new Thread (Speak).Start();
}

static void Speak()
{
  for (int i = 0; i < 5; i++)
  {
    Console.Write (i + " ");
    _barrier.SignalAndWait();
  }
}

Event-based asynchronous pattern:

var wc = new WebClient();
wc.DownloadStringCompleted += (sender, args) =>
{
  if (args.Cancelled)
    Console.WriteLine ("Canceled");
  else if (args.Error != null)
    Console.WriteLine ("Exception: " + args.Error.Message);
  else
  {
    Console.WriteLine (args.Result.Length + " chars were downloaded");
    // We could update the UI from here...
  }
};
wc.DownloadStringAsync (new Uri ("http://www.linqpad.net"));  // Start it

BackgroundWorker

class Program
{
  static BackgroundWorker _bw = new BackgroundWorker();

  static void Main()
  {
    _bw.DoWork += bw_DoWork;
    _bw.RunWorkerAsync ("Message to worker");
    Console.ReadLine();
  }

  static void bw_DoWork (object sender, DoWorkEventArgs e)
  {
    // This is called on the worker thread
    Console.WriteLine (e.Argument);        // writes "Message to worker"
    // Perform time-consuming task...
  }
}

BackgroundWorker - more functionality

using System;
using System.Threading;
using System.ComponentModel;

class Program
{
  static BackgroundWorker _bw;

  static void Main()
  {
    _bw = new BackgroundWorker
    {
      WorkerReportsProgress = true,
      WorkerSupportsCancellation = true
    };
    _bw.DoWork += bw_DoWork;
    _bw.ProgressChanged += bw_ProgressChanged;
    _bw.RunWorkerCompleted += bw_RunWorkerCompleted;

    _bw.RunWorkerAsync ("Hello to worker");

    Console.WriteLine ("Press Enter in the next 5 seconds to cancel");
    Console.ReadLine();
    if (_bw.IsBusy) _bw.CancelAsync();
    Console.ReadLine();
  }

  static void bw_DoWork (object sender, DoWorkEventArgs e)
  {
    for (int i = 0; i <= 100; i += 20)
    {
      if (_bw.CancellationPending) { e.Cancel = true; return; }
      _bw.ReportProgress (i);
      Thread.Sleep (1000);      // Just for the demo... don't go sleeping
    }                           // for real in pooled threads!

    e.Result = 123;    // This gets passed to RunWorkerCompleted
  }

  static void bw_RunWorkerCompleted (object sender,
                                     RunWorkerCompletedEventArgs e)
  {
    if (e.Cancelled)
      Console.WriteLine ("You canceled!");
    else if (e.Error != null)
      Console.WriteLine ("Worker exception: " + e.Error.ToString());
    else
      Console.WriteLine ("Complete: " + e.Result);      // from DoWork
  }

  static void bw_ProgressChanged (object sender,
                                  ProgressChangedEventArgs e)
  {
    Console.WriteLine ("Reached " + e.ProgressPercentage + "%");
  }
}

Subclassing BackgroundWorker

public class Client
{
  public FinancialWorker GetFinancialTotalsBackground (int foo, int bar)
  {
    return new FinancialWorker (foo, bar);
  }
}

public class FinancialWorker : BackgroundWorker
{
  public Dictionary <string,int> Result;   // You can add typed fields.
  public readonly int Foo, Bar;

  public FinancialWorker()
  {
    WorkerReportsProgress = true;
    WorkerSupportsCancellation = true;
  }

  public FinancialWorker (int foo, int bar) : this()
  {
    this.Foo = foo; this.Bar = bar;
  }

  protected override void OnDoWork (DoWorkEventArgs e)
  {
    ReportProgress (0, "Working hard on this report...");

    // Initialize financial report data
    // ...

    while (! finished-report )
    {
      if (CancellationPending) { e.Cancel = true; return; }
      // Perform another calculation step ...
      // ...
      ReportProgress (percentCompleteCalc, "Getting there...");
    }
    ReportProgress (100, "Done!");
    e.Result = Result = completed-report-data;
  }
}

Interrupt

static void Main()
{
  Thread t = new Thread (delegate()
  {
    try { Thread.Sleep (Timeout.Infinite); }
    catch (ThreadInterruptedException) { Console.Write ("Forcibly "); }
    Console.WriteLine ("Woken!");
  });
  t.Start();
  t.Interrupt();
}

Safe cancellation

class RulyCanceler
{
  object _cancelLocker = new object();
  bool _cancelRequest;
  public bool IsCancellationRequested
  {
    get { lock (_cancelLocker) return _cancelRequest; }
  }

  public void Cancel() { lock (_cancelLocker) _cancelRequest = true; } 

  public void ThrowIfCancellationRequested()
  {
    if (IsCancellationRequested) throw new OperationCanceledException();
  }
}

class Test
{
  static void Main()
  {
    var canceler = new RulyCanceler();
    new Thread (() => {
                        try { Work (canceler); }
                        catch (OperationCanceledException)
                        {
                          Console.WriteLine ("Canceled!");
                        }
                      }).Start();
    Thread.Sleep (1000);
    canceler.Cancel();               // Safely cancel worker.
  }

  static void Work (RulyCanceler c)
  {
    while (true)
    {
      c.ThrowIfCancellationRequested();
      // ...
      try      { OtherMethod (c); }
      finally  { /* any required cleanup */ }
    }
  }

  static void OtherMethod (RulyCanceler c)
  {
    // Do stuff...
    c.ThrowIfCancellationRequested();
  }
}

Lazy initialization - manual approach

Expensive _expensive;
readonly object _expenseLock = new object();

public Expensive Expensive
{
  get
  {
    lock (_expenseLock)
    {
      if (_expensive == null) _expensive = new Foo();
      return _expensive;
    }
  }
}

Lazy<T>

Lazy<Expensive> _expensive = new Lazy<Expensive>
  (() => new Expensive(), true);

public Expensive Expensive { get { return _expensive.Value; } }

LazyInitializer

Expensive _expensive;
public Expensive Expensive
{ 
  get          // Implement double-checked locking
  { 
    LazyInitializer.EnsureInitialized (ref _expensive,
                                      () => new Expensive());
    return _expensive;
  }
}

Double-checked locking pattern implementation

volatile Expensive _expensive;
public Expensive Expensive
{
  get
  {
    if (_expensive == null)
    {
      var expensive = new Expensive();
      lock (_expenseLock) if (_expensive == null) _expensive = expensive;
    }
    return _expensive;
  }
}

Race-to-initialize pattern

volatile Expensive _expensive;
public Expensive Expensive
{
  get
  {
    if (_expensive == null)
    {
      var instance = new Expensive();
      Interlocked.CompareExchange (ref _expensive, instance, null);
    }
    return _expensive;
  }
}

ThreadLocal

var localRandom = new ThreadLocal<Random>(() => new Random());
Console.WriteLine (localRandom.Value.Next());

var localRandom = new ThreadLocal<Random>
 ( () => new Random (Guid.NewGuid().GetHashCode()) );

GetData and SetData

class Test
{
  // The same LocalDataStoreSlot object can be used across all threads.
  LocalDataStoreSlot _secSlot = Thread.GetNamedDataSlot ("securityLevel");

  // This property has a separate value on each thread.
  int SecurityLevel
  {
    get
    {
      object data = Thread.GetData (_secSlot);
      return data == null ? 0 : (int) data;    // null == uninitialized
    }
    set { Thread.SetData (_secSlot, value); }
  }
  ...

Reader/Writer locks

class SlimDemo
{
  static ReaderWriterLockSlim _rw = new ReaderWriterLockSlim();
  static List<int> _items = new List<int>();
  static Random _rand = new Random();

  static void Main()
  {
    new Thread (Read).Start();
    new Thread (Read).Start();
    new Thread (Read).Start();

    new Thread (Write).Start ("A");
    new Thread (Write).Start ("B");
  }

  static void Read()
  {
    while (true)
    {
      _rw.EnterReadLock();
      foreach (int i in _items) Thread.Sleep (10);
      _rw.ExitReadLock();
    }
  }

  static void Write (object threadID)
  {
    while (true)
    {
      int newNumber = GetRandNum (100);
      _rw.EnterWriteLock();
      _items.Add (newNumber);
      _rw.ExitWriteLock();
      Console.WriteLine ("Thread " + threadID + " added " + newNumber);
      Thread.Sleep (100);
    }
  }

  static int GetRandNum (int max) { lock (_rand) return _rand.Next(max); }
}

Upgradeable locks

while (true)
{
  int newNumber = GetRandNum (100);
  _rw.EnterUpgradeableReadLock();
  if (!_items.Contains (newNumber))
  {
    _rw.EnterWriteLock();
    _items.Add (newNumber);
    _rw.ExitWriteLock();
    Console.WriteLine ("Thread " + threadID + " added " + newNumber);
  }
  _rw.ExitUpgradeableReadLock();
  Thread.Sleep (100);
}

Lock recursion

rw.EnterWriteLock();
rw.EnterReadLock();
Console.WriteLine (rw.IsReadLockHeld);     // True
Console.WriteLine (rw.IsWriteLockHeld);    // True
rw.ExitReadLock();
rw.ExitWriteLock();

Timers - Threading timer

using System;
using System.Threading;

class Program
{
  static void Main()
  {
    // First interval = 5000ms; subsequent intervals = 1000ms
    Timer tmr = new Timer (Tick, "tick...", 5000, 1000);
    Console.ReadLine();
    tmr.Dispose();         // This both stops the timer and cleans up.
  }

  static void Tick (object data)
  {
    // This runs on a pooled thread
    Console.WriteLine (data);          // Writes "tick..."
  }
}

System.Timer

using System;
using System.Timers;   // Timers namespace rather than Threading

class SystemTimer
{
  static void Main()
  {
    Timer tmr = new Timer();       // Doesn't require any args
    tmr.Interval = 500;
    tmr.Elapsed += tmr_Elapsed;    // Uses an event instead of a delegate
    tmr.Start();                   // Start the timer
    Console.ReadLine();
    tmr.Stop();                    // Stop the timer
    Console.ReadLine();
    tmr.Start();                   // Restart the timer
    Console.ReadLine();
    tmr.Dispose();                 // Permanently stop the timer
  }

  static void tmr_Elapsed (object sender, EventArgs e)
  {
    Console.WriteLine ("Tick");
  }
}