1. CNS 3260 C# .NET Software Development
C# Threading
CNS 3260
C# .NET Software Development
C# Threading

2. Timers Timers execute a method at a given interval
Three to choose from:
System.Timers.Timer
System.Threading.Timer
System.Windows.Forms.Timer
C# Threading

3. System.Timers.Timer Constructors: Members: Timer()
Timer(double interval)
Members:
bool AutoReset
bool Enabled
double Interval
in milliseconds
Close()
Start()
Stop()
Elapsed
System.Timers.ElapsedEventHandler delegate
C# Threading

4. System.Threading.Timer
Constructor:
Timer(TimerCallBack, object, long, long)
Members:
Change(long, long)
TimerCallBack
System.Threading.TimerCallBack(object state)
cannot be changed once set
C# Threading

5. System.Windows.Forms.Timer
Has to be used with Windows Forms
Members:
bool Enabled
int Interval
Start()
Stop()
Tick
System.EventHandler delegate
(See Timers Demo)
C# Threading

6. Threading: Advantages and Dangers
Main
Thread
Reasons to use Threading
Timers
UI Responsiveness
Multiple processors
Dangers
Race Conditions
Deadlock
Child
Thread
C# Threading

7. Spawning a new Thread IO
Delegates
System.Thread
C# Threading

8. Threading IO
FileStream.BeginRead(byte[] buffer, int offset, int numBytes, ASyncCallBack userCallBack, object stateObject)
returns IAsyncResult object
userCallBack
delegate void ASyncCallBack(IAsyncResult ar)
gets executed when the operation is finished
FileStream.EndRead(IAsyncResult ar)
Waits (blocks until the read is complete)
returns (int) the number of bytes read
C# Threading

9. Threading a Delegate (See DelegateThreading Demo)
BeginInvoke(AsyncCallBack userCallBack, object stateObject)
Returns an IAsyncResult object
executes userCallBack when finished
EndInvoke(IAsyncResult ar)
Blocks until the delegate thread is finished
Use AsyncResult object unless you have specific needs
(See DelegateThreading Demo)
C# Threading

10. IAsyncResult Members: AsyncResult object AsyncState AsynchWaitHandle
User Object passed in
AsynchWaitHandle
Gets a thread handle that can be used to block
CompletedSynchronously
Indicates that the operation was fast enough that the system didn’t actually spawn a thread
Use with IO operations
IsCompleted
Indicates whether the operation has completed
AsyncResult object
extend if you need specialized behavior
C# Threading

11. System.Threading.Thread
For more control over your threading situation
Members:
static CurrentThread
static Sleep()
Constructor
Thread(ThreadStart start)
IsBackground
Priority
ThreadState
Abort()
Interrupt()
Join()
Resume()
Start()
Suspend()
C# Threading

12. Thread Static Members CurrentThread Sleep(int ticks)
Returns a thread object for the current thread
Sleep(int ticks)
1 tick == 1 ms
Sleep(0) just releases the CPU
Never use a busy-wait
C# Threading

13. Thread Constructor Takes a ThreadStart delegate
ThreadStart(void () target)
Takes a method which returns void and has no params:
void MyMethod()
Thread begins execution when Start() is called
Thread executes method passed to ThreadStart
C# Threading

14. Thread Properties IsBackground Priority ThreadState get or set
Once all foreground threads are finished, the runtime calls Abort on all background threads
Default is false (or foreground)
Priority
ThreadPriority Enum
Lowest, BelowNormal, Normal, AboveNormal, Highest
ThreadState
New Thread: ThreadState.Unstarted
Started Thread: ThreadState.Running
Sleep called: ThreadState.WaitSleepJoin
Suspend called: ThreadState.Suspended
See ThreadState Enumeration
C# Threading

15. Thread Methods Start() Suspend() Resume() Interrupt()
Begins execution of the thread
Once a thread is finished, it cannot be restarted
Suspend()
Suspends the thread
If the thread is already suspended, there is no effect
Resume()
Resumes a suspended thread
Interrupt()
Resumes a thread that is in a WaitSleepJoin state
If the thread is not in WaitSleepJoin state it will be interrupted next time it is blocked
C# Threading

16. Thread Methods (Continued)
Abort()
Attempts to abort the thread
Throws a ThreadAbortException
Join()
Blocks the calling thread until the owning thread terminates
C# Threading

17. Exceptions Between threads
Must be handled in the thread it was thrown in
Exceptions not handled in the thread are considered unhandled and will terminate that thread
C# Threading

18. Threading Dangers volatile Fields Race Conditions Deadlock
The CPU often stores variables in registers for optimization
Other threads accessing that variable may not get the true value
Race Conditions
When multiple threads access the same memory
A thread is interrupted while updating memory
Solution: “lock” memory in a mutex, monitor, etc.
Deadlock
Two or more threads waiting on each other to release resources
C# Threading

19. volatile Fields volatile fields may not be cached
Types that may be volatile are:
Any reference type
Any pointer (unsafe code)
sbyte, byte, short, ushort, int, uint
An enum with one of the above base types
Example:
The Thread class also contains 2 static methods:
VolatileRead()
VolatileWrite()
public volatile int myChangingInt = 0;
C# Threading

20. Race Conditions Covered thoroughly in CNS 3060
Solved using a Mutex or Monitor
Called Synchronization
C# Threading

21. Synchronization Classes and Constructs
lock keyword
Mutex class
Monitor class
Interlocked class
ReaderWriterLock class
C# Threading

22. The lock Keyword Marks a statement as a critical section
Is a Monitor underneath
Example:
lockObject must be a reference-type instance
Typically you will lock on:
‘this’
locks the current instance
typeof(MyClass)
global lock for the given type
A collection instance
locks access to a specific collection
lock(lockObject) {
// critical section }
C# Threading

23. The lock Mutex lock defines one Mutex for each object locked on
Blocks until the current thread is finished
(See Lock Demo)
C# Threading

24. Mutex Class Stands for Mutual-Exclusion
Blocking synchronization object
WaitOne()
Begins the critical section
ReleaseMutex()
Ends critical section
Call ReleaseMutex() in a finally block
C# Threading

25. Monitor Class (static)
Enter(object obj)
Begins a critical section
Blocks if another thread has the same lock
Exit(object obj)
Ends a critical section
Releases the lock
TryEnter(object obj)
Returns true if it obtains the lock
Returns false if it can’t obtain the lock
Avoids blocking if you can’t obtain the lock
Wait(object obj)
Releases the lock on an object and blocks until it reaquires the lock
Pulse(object obj)
Signals the next waiting thread that the lock may be free
PulseAll(object obj)
Signals all waiting threads that the lock may be free
C# Threading

26. Interlocked Class (static)
Provides atomic operations for variables
CompareExchange(ref int dest, int source, int compare)
Replaces dest with source if dest == compare
Overloaded
Exchange(ref int dest, int source)
places source into dest
returns the original value of dest
Increment(ref int value)
increments value
Decrement(ref int value)
decrements value
C# Threading

27. ReaderWriterLock Class
Allows a single writer or multiple readers
Members:
IsReaderLockHeld
IsWriterLockHeld
WriterSeqNum
AcquireReaderLock()
Increments the reader count
AcquireWriterLock()
Blocks until the lock is obtained
ReleaseReaderLock()
Decrements the reader count
ReleaseWriterLock()
Releases the writer lock
ReleaseLock()
Unconditionally releases the lock
C# Threading

28. Deadlock Your job to avoid it object1 waiting for has Resource1
C# Threading

29. Windows Threading
Windows disallows updating many GUI components across threads
To update from the worker thread to the GUI thread
InvokeRequired()
Invoke(System.Delegate)
(See Invoke Demo)
C# Threading

30. Mediator Pattern Specialization
GUI Thread
Mediator
Worker Thread
subscribes
Exposed Event
fires when
needed
public FireEvent()
C# Threading