Why in C# is Queue scrambling the data in its elements?

Question

I am completely perplexed with how my Queue is function. I am attempting (and failing) to write a small multi-threaded application to collect and display data in C#.
After reading through Albahari's book and using the Consumer/Producer pattern he describes I got most of it to work except my data seems to get scrambled in the queue. Before getting queued the fields in my object have the following values

timeStamp = 6
data[] ={4936, 9845, 24125, 44861}

After being dequeued the data looks like

timeStamp = 6
data[] = {64791, 19466, 47772, 65405}

I don't understand why the values in the data filed are being changed after the dequeue? I am perplexed so I thought I'd throw it out there to see if anyone can point me in the right direction to fixing this or to point me in a different direction to proceed.

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Relevant Code

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Custom Object for data storage

Relevant objects and Fields. The class sensorData is a seperate class use to store my calculations.

public class sensorData
{
    public const int bufSize = 4;
    public UInt16[] data = new UInt16[4];
    public double TimeStamp = 0; 
    public int timeIndex = 0;
}

The following fields are used to setup the queue and signals between the enqueue and dequeue threads.

EventWaitHandle wh = new AutoResetEvent(false);
Queue<sensorData> dataQ = new Queue<sensorData>();
object locker = new object();

Enqueue Method/Thread

This is my worker thread it calculates four Sine curves and queues the result for processing. I also write the results to a file so I know what it has calculated.

private void calculateAndEnqueueData(BackgroundWorker worker, DoWorkEventArgs e)
{
    int j = 0;
    double time = 0;
    double dist;
    UInt16[] intDist = new UInt16[sensorData.bufSize];
    UInt16 uint16Dist;

    // Frequencies of the four Sine curves
    double[] myFrequency = { 1, 2, 5, 10 };

    // Creates the output file.
    StreamWriter sw2 = File.CreateText("c:\\tmp\\QueuedDataTest.txt"); 

    // Main loop to calculate my Sine curves
    while (!worker.CancellationPending)
    {
        // Calculate four Sine curves
        for (int i = 0; i < collectedData.numberOfChannels; i++)
        {
            dist = Math.Abs(Math.Sin(2.0 * Math.PI * myFrequency[i] * time);
            uint16Dist = (UInt16)dist;
            intDist[i] = uint16Dist;
        }

        // Bundle the results and Enqueue them
        sensorData dat = new sensorData();
        dat.data = intDist;
        dat.TimeStamp = time;
        dat.timeIndex = j;

        lock (locker) dataQ.Enqueue(dat);
        wh.Set

        // Output results to file.
        sw2.Write(j.ToString() + ", ");
        foreach (UInt16 dd in dat.intData)
        {
            sw2.Write(dd.ToString() + ", ");
        }
        sw2.WriteLine();

        // Increments time and index.
        j++;
        time += 1 / collectedData.signalFrequency;

        Thread.Sleep(2);
    }
    // Clean up
    sw2.Close();
    lock (locker) dataQ.Enqueue(null);
    wh.Set();
    sw2.Close();
}

Example line in the output file QueuedDataTest.txt

6, 4936, 9845, 24125, 44861,

Dequeue Data Method

This Method dequeues elements from the queue and writes them to a file. Until a null element is found on the queue at which point the job is done.

    private void dequeueDataMethod()
    {
        StreamWriter sw = File.CreateText("C:\\tmp\\DequeueDataTest.txt");

        while (true)
        {
            sensorData data = null;

            // Dequeue available element if any are there.
            lock (locker)
                if (dataQ.Count > 0)
                {
                    data = dataQ.Dequeue();
                    if (data == null)
                    {
                        sw.Close();
                        return;
                    }
                }

            // Check to see if an element was dequeued. If it was write it to file.
            if (data != null)
            {
                sw.Write(data.timeIndex.ToString() + ", ");
                foreach (UInt16 dd in data.data)
                    sw.Write(dd.ToString() + ", ");
                sw.WriteLine();
            }
            else
            {
                wh.WaitOne();
            }
        }

Output result after dequeueing the data and writing it to DequeueDataTest.txt

6, 64791, 19466, 47772, 65405,

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Updates 1:

Location of Locks in current code.

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I have edited the code to place locks around the writing data to the file. So the code blocks I have locks around are as follows.

In the CalculateAndEnqueueData() method I have

lock (locker) dataQ.Enqueue(dat);
wh.Set

lock(locker)
{
  sw2.Write(j.ToString() + ", ");
  foreach (UInt16 dd in dat.intData)
  {
     sw2.Write(dd.ToString() + ", ");
  }
  sw2.WriteLine();
}

In the dequeueDataMethod() I have two areas with locks the first is here

lock(locker) 
    if (dataQ.Count > 0)
    {
       data = dataQ.Dequeue();
       if (data == null)
       {
           sw.Close();
           return;
        }
    }

which I assume locks locker for the code in the if block. The second is where I write the to the file here

lock (locker)
{
    sw.Write(data.timeIndex.ToString() + ", ");
    foreach (UInt16 dd in data.intData)
        sw.Write(dd.ToString() + ", ");
    sw.WriteLine();
    if (icnt > 10)
    {
        sw.Close();
        return;
    }
    this.label1.Text = dataQ.Count.ToString();
}

That's all of them.

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Answer 1

The problem is due to no synchronisation on the StreamWriter you are writing to. The order is not sequential.

Answer 2

Is it because you are writing to the same array UInt16[] intDist over and over again? Shouldn't you be using separate arrays for each sensorData object? (Btw, is sensorData.intData suppose to be sensorData.data in your sample code?)

CLARIFICATION:

Only one intDist array is created in calculateAndEnqueueData() , so different sensorData instances are all sharing the same array --- this is ok if the adding+writing and removal+writing occur in lock-step; otherwise, some data points may be missing/repeated.

SUGGESTION:

Populate sensorData instances directly, without using the intDist array, in calculateAndEnqueueData() :

    // create new sensorData instance
    sensorData dat = new sensorData();
    dat.TimeStamp = time;
    dat.timeIndex = j;

    // Calculate four Sine curves
    for (int i = 0; i < collectedData.numberOfChannels; i++)
    {
        dat.data[i] = (UInt16) Math.Abs(Math.Sin(2.0 * Math.PI * myFrequency[i] * time);
    }

    // enqueue
    lock (locker) dataQ.Enqueue(dat);