H7 Multithreading and Asynchronous Processing

Question 1

Name at least 3 threadpool methodes:

Answer 1

QueueUserWorkItem
GetAvailableThreads
GetMaxThreads
GetMinThreads
SetMaxThreads
SetMinThreads

Question 2

What is the most importend methode of the Threadpool.

Answer 2

public static bool QueueUserWorkItem(WaitCallback callBack)

public static bool QueueUserWorkItem(WaitCallback callBack, Object state)

public delegate void WaitCallback(Object state)

ThreadPool.QueueUserWorkItem((x) => result += ReadDataFromIO());

Question 3

Is there a join methode in a threadpool?

Answer 3

No, you have to wait until it is finished.

Question 4

Which steps do you take for a backgroundworker thread?

Answer 4

DoWork += DoWorkEventHandler methode

when work is done assign the result to : DoWorkEventArgs.Result

RunWorkerCompleted += RunWorkerCompletedEventHandler methode

Check RunWorkerCompletedEventArgs.Error

Optionaly :

WorkerReportsProgress = true
ProgressChanged += ProgressChangedEventHandler methode

start : BackgroundWorkerInstance.RunWorkerAsync()

Question 5

Give an example of Invoke.

Answer 5

Invoke means : put this methode pointer + (optional argument) in the queue of the thread of the e.g. UI.

void worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e) {

if (this.InvokeRequired) {
this.Invoke(
new Action(UpdateLabel),
e.Result.ToString());
//this.Invoke(Delegate methode, arguments that are passed to the delegate methode.)

}

else {
UpdateLabel(e.Result.ToString());
}

}

private void UpdateLabel(string text) {
lblResult.Text = text;
}

Question 6

How do you tell that you are done with the thread?

Answer 6

Thread.Sleep(0);

Question 7

What is the difference between a foreground and a background thread and how do you make a background thread?

Answer 7

A forground thread blocks the main application thread for exiting until it is finished. A Background thread gets killed if the main application finishes.

MyThreadInstance.IsBackground = true;

Question 8

What delegate do you use for an instance of a Thread?

Answer 8

Thread myThread = new Thread(NewTreadStart(MethodeName));

Question 9

How can we give thread controll up for e.g. 200 ms in a loop?

Answer 9

while(true)
{

Task.Sleep(200)
…
}

Question 10

What delegate do you use for an instance of a Thread with parameters?

Answer 10

private void MytestMethode(object myValue)
{

    }

    private void button1_Click(object sender, EventArgs e)
    {

        Thread t = new Thread(new ParameterizedThreadStart(MytestMethode));

        t.Start("Peter");
    }

Question 11

How do you make a static field per thread? So 5 threads each having an individual static (non shared) field?

Answer 11

[ThreadStatic]

public static int mystaticvar;

Question 12

What is actualy a task?

Answer 12

It is a managed threadpool thread.

Question 13

How can you start a task?

Answer 13

Task.Run(delegate);

Question 14

How do you tell a task that it uses a action delegate when giving the methode name?

Answer 14

Task t = Task.Run(action:MyMethodeName);

Question 15

What is a schedular?

Answer 15

The component that places a task in the thread queue.

Question 16

How do you put a task in the thread queue of the UI?

Answer 16

Task.Factory.StartNew(UpdateLabel,
CancellationToken.None,
TaskCreationOptions.None,
TaskScheduler.FromCurrentSynchronizationContext());

Question 17

Give an example of a async/await methode.

Answer 17

public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            myAsyncMethode();
        }

        public async void myAsyncMethode()
        {
            await ReadDataFromIOAsync();
        }

    public Task ReadDataFromIOAsync()
    {
        return Task.Run(new Func<double(ReadDataFromIO));
    }

        public double ReadDataFromIO()
        {
            // We are simulating an I/O by putting the current thread to sleep.
            Thread.Sleep(20000);
            return 10d;
        }

Question 18

Give an example of a async/await methode with a direct await task…

Answer 18

myAsyncMethode();

public async void myAsyncMethode()
        {
            //await ReadDataFromIOAsync();
            await (Task)Task.Run(new Func(ReadDataFromIO));
        }

        public Task ReadDataFromIOAsync()
        {
            return Task.Run(new Func(ReadDataFromIO));
        }

Question 19

What is the “EventWaitHandle” how does it work?

Answer 19

Create instance of the EventWaitHandle class and use it in the body of a seperate thread execution.

// We use this event to signal when the thread is don executing.
EventWaitHandle calculationDone =
new EventWaitHandle(false, EventResetMode.ManualReset);

// Create a work item to read from I/O
ThreadPool.QueueUserWorkItem((x) => {
result += ReadDataFromIO();
calculationDone.Set();
});

//In the main thread:
// Wait for the thread to finish
calculationDone.WaitOne();

Question 20

What 2 version of EventWaitHandle are there and what do they do?

Answer 20

EventWaitHandle: AutoResetEvent

Sets the mode to auto reset

EventWaitHandle: ManualResetEvent.

Sets the mode to manual reset.

Question 21

What is an alternative to EventWaitHandle class with usage of WaitHandle.WaitAll

Answer 21

CoundownEvent Class
.NET 4 introduced a new class called CoundownEvent, defined in the System.Threading
namespace. The usage scenario is straightforward: You need to wait for a predefined number of
threads to finish their work.

Question 22

How can you implement a thread barrier?

Answer 22

Use the “Barrier” class:

var participants = 5;

Barrier barrier = new Barrier(participants + 1,
// We add one for the main thread.
b => { // This method is only called when all the paricipants arrived.
Console.WriteLine(“{0} paricipants are at rendez-vous point {1}.”,
b.ParticipantCount -1, // We substract the main thread.
b.CurrentPhaseNumber);
});

for (int i = 0; i < participants; i++) {
var localCopy = i;
Task.Run(() => {
Console.WriteLine("Task {0} left point A!", localCopy);
Thread.Sleep(1000 * localCopy + 1); // Do some "work"
if (localCopy % 2 == 0) {
Console.WriteLine("Task {0} arrived at point B!", localCopy);
->barrier.SignalAndWait();barrier.RemoveParticipant(););
}

Question 23

What lock type starts with M… give an example.

Answer 23

object syncObject = new object();
Monitor.Enter(syncObject);
// Code updating some shared data
Monitor.Exit(syncObject);

Question 24

What is a big risk when using the Monitor class?

Answer 24

Deadlock.

If an exception is thrown after the Monitor.Enter methode then the Monitor.Exit methode is not called, leaving you with a dead lock:

To prevent this:

object syncObject = new object();
Monitor.Enter(syncObject);
try {
// Code updating some shared data
}
finally {
Monitor.Exit(syncObject);
}
...

Question 25

What is a c# code snipped for Moni…

Answer 25

object syncObject = new object();
lock (syncObject) {
// Code updating some shared data
}

Here there will not be a dead lock if an exception is thrown.

Question 26

what is wrong with lock(this)?

Answer 26

It could cause a deadlock.

public class LockThisBadSample {
public void OneMethod() {
lock (this) {
// Do Something here
}
}
}
public class UsingTheLockedObject {
public void AnotherMethod(){
LockThisBadSample lockObject = new LockThisBadSample();
lock (lockObject) {
// Do something else
}
}
}

Question 27

How can you make certain opperations atomic therefore removing the need for lock (which is resource intensive).

Answer 27

Use of the Interlocked class.

Question 28

How can you create a cancelation?

Answer 28

Use :

CancellationTokenSource class
and a struct: CancellationToken

You can use a cancelationToken in a methode in a seperate thread.

var tokenSource = new CancellationTokenSource();
CancellationToken myToken = tokenSource .Token;

Question 29

Can the cancelation token force a thread to stop?

Answer 29

No, the code in the thread can choose to stop (cooperative cancellations).