Parallel function call and thread issue

Question

Sorry to post one issue here which is related to code. i was debugging a code which was wrote long time back by one developer. the issue is two function is called parallel using thread and one function set a variable true at end and another function just
looping until the variable is set to true. the variable value is not getting set to true. here i am posting a code snippet and just tell me what is mistake in the code for which variable is not getting set to true by first function.
when user click button then two function is invoked
private void UPDATE_Click(object sender, System.EventArgs e)
SavedFirst();
AddCheckThread();
public void SavedFirst()
IsOpen = true;
System.Threading.Thread loadT = new System.Threading.Thread(new System.Threading.ThreadStart(SaveAll));
loadT.Start();
IsOpen = false;
private void AddCheckThread()
if (!ALLFlag)
loadingThread = new System.Threading.Thread(new System.Threading.ThreadStart(addCheck));
loadingThread.Priority = System.Threading.ThreadPriority.Lowest;
loadingThread.Start();
private void SaveAll()
if (this.InvokeRequired)
this.Invoke(new MethodInvoker(delegate
ALLFlag = false;
if (!ALLFlag)
loadingThread = new System.Threading.Thread(new System.Threading.ThreadStart(AddProducts));
loadingThread.Priority = System.Threading.ThreadPriority.Lowest;
loadingThread.Start();
return;
private void AddProducts()
if (this.InvokeRequired)
this.Invoke(new MethodInvoker(delegate
ALLFlag = false;
if (System.Windows.Forms.MessageBox.Show(this, "Would you like to add the details into the Database?", "Add?", System.Windows.Forms.MessageBoxButtons.YesNo) == System.Windows.Forms.DialogResult.Yes)
if (comboOUR.SelectedItem == null || comboCountry.SelectedItem == null || comboSelleble.SelectedItem == null || txtOereff.Text == "" || txtUKPrice.Text == "" || txtUSPrice.Text == "" || txtSUrCharge.Text == "" || txtOURUS.Text == "" || txtOURUK.Text == "")
FormValidation();
else
Gather_Data();
bool isInserted = false;
if (System.Windows.Forms.MessageBox.Show(this, "Would you like to add the details into \"Detailed-Product\" Too?", "Add?", System.Windows.Forms.MessageBoxButtons.YesNo) == System.Windows.Forms.DialogResult.Yes)
isInserted = bbaProduct.BBASaveProduct();
if (isInserted == true)
isInserted = false;
isInserted = bbaProduct.InsertInProductTable(BBAProduct.DetailProductID);
if (isInserted == true)
System.Windows.Forms.MessageBox.Show(this, "Product Successfully Added ", "Add");
else
System.Windows.Forms.MessageBox.Show(this, "Error Occurred !! Not Added Into the Database ", "Add");
else
System.Windows.Forms.MessageBox.Show(this, "Error Occurred !! Not Added Into the Database ", "Add");
else
isInserted = bbaProduct.InsertInProductTable(0);
if (isInserted == true)
System.Windows.Forms.MessageBox.Show(this, "Successfully Inserted Into the database", "Add");
else
System.Windows.Forms.MessageBox.Show(this, "Error Occurred !! Not Added Into the Database", "Add");
else
System.Windows.Forms.MessageBox.Show(this, "Process Cancelled By The User", "Add");
ALLFlag = true;
return;
#region Add Check
private void addCheck()
if (this.InvokeRequired)
this.Invoke(new MethodInvoker(delegate
this.Opacity = 0.8;
axShockwaveFlash1.Visible = true;
axShockwaveFlash1.Play();
while (!ALLFlag)
int x = 0;
axShockwaveFlash1.Visible = false;
axShockwaveFlash1.Visible = false;
axShockwaveFlash1.StopPlay();
this.Opacity = 1;
ALLFlag = false;
loadingThread.Abort();
return;
help me to locate where is the mistake for which ALLFlag value is not getting set to true. thanks

Answer 1

Your reliance on a field value across threads isn't going to work reliably.  In a multi-core/multi-threaded world the memory model allows for reading data out of order for optimization reasons.  Additionally the CPU can and will cache recently
used data in memory that isn't shared across processors.  This can get you into trouble as you will not be reading the same value across processors.
The only correct way to ensure that multiple threads access the same data correctly is to use sync objects that are designed for that such as a Semaphore or Mutex.  Sync objects allow you to safely share data across threads.
Overall your code appears to be trying to update the UI using a thread but since it is calling Invoke each time you are basically spawning a thread that then blocks waiting for the UI thread.  You could pretty much eliminate your issues by getting rid
of the threading calls altogether along with any sort of state management variables that were arbitrarily created (ALLflags) and simply use BeginInvoke.  You didn't post who was calling your higher level save methods but if it is a UI element then BeginInvoke
and InvokeRequired are completely redundant as is all the threading.
You can cleanly update this code to eliminate all the extra variables and threading and simply use the newer async/await functionality to help resolve the issues your code is having.  But you still have problems with the invoked code.  In one of
the methods you are looping while waiting for a variable to be set.  But since you invoked it then it is occurring on the UI thread.  Because it is on the UI thread and that can only do 1 thing at a time, if ALLflag is false your code will deadlock. 
The only other code that would set it to true cannot run because it is also invoked on the UI thread and only one of them can run at any one time.  Basically each of your threads are going to try to run on the same thread and only 1 of them will be able
to.  Until the first thread method finishes the second one will never run.
If you need Boolean flags then consider using a ManualResetEvent instead.  It is thread safe, works across threads and doesn't have any of the problems of a Boolean field when talking about threading. But you still have a deadlock between your invoked
code that nothing will solve except rewriting your code.
Michael Taylor
http://blogs.msmvps.com/p3net