asked on

backgroundWorker.. DoWork and ProgressChanged

Hello,

I using a backgroundWorker to execute a time consuming operation and print the result as they come an a listView. I'm basically doing something similar to the following:

private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
while (condition)
{
if (backgroundWorker1.CancellationPending)
{
e.Cancel = true;
}
else
{
// code... bla bla bla

globalItemName = someVar;

Console.WriteLine("In DoWork");

backgroundWorker1.ReportProgress(1);
}
}
}

private void backgroundWorker1_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
Console.WriteLine("In ProgressChanged");

listView1.Items.Add(globalItemName);

progressBar1.Value = e.ProgressPercentage;
}

It's not working, all the 'In DoWork' is printed in the output window first then 'In ProgressChanged' is printed after.

What's wrong?

Thanks.

AlexFM

>> 'In DoWork' is printed in the output window first then 'In ProgressChanged' is printed after.

This is exactly what is written in your code:

Console.WriteLine("In DoWork");
backgroundWorker1.ReportProgress(1); // In ProgressChanged

What is the problem exactly?

i950

ASKER

Hi Alex,

I'm getting:

Console.WriteLine("In DoWork");
Console.WriteLine("In DoWork");
Console.WriteLine("In DoWork");
Console.WriteLine("In DoWork");
Console.WriteLine("In DoWork");
Console.WriteLine("In DoWork");
Console.WriteLine("In DoWork");
Console.WriteLine("In ProgressChanged");
Console.WriteLine("In ProgressChanged");
Console.WriteLine("In ProgressChanged");
Console.WriteLine("In ProgressChanged");
Console.WriteLine("In ProgressChanged");
Console.WriteLine("In ProgressChanged");
Console.WriteLine("In ProgressChanged");
Console.WriteLine("In ProgressChanged");

Not:

Console.WriteLine("In DoWork");
Console.WriteLine("In ProgressChanged");
Console.WriteLine("In DoWork");
Console.WriteLine("In ProgressChanged");
Console.WriteLine("In DoWork");
Console.WriteLine("In ProgressChanged");
Console.WriteLine("In DoWork");
Console.WriteLine("In ProgressChanged");

AlexFM

I made small test and reproduced this behavour. This means, BackgroundWorker.ReportProgress invokes ProgressChanged event asynchronously. Line
backgroundWorker1.ReportProgress(1);
is executed, and worker thread code continues immediately, without waiting for ProgressChanged function. Computer makes thread switch with some time interval. In your case, backgroundWorker1_DoWork function is executed in one time interval scheduled to this thread. When thread exits, main thread is activated and executes all ProgressChanged events from event queue.
Make small change:
backgroundWorker1.ReportProgress(1);
System.Threading.Thread.Sleep(0);

Now you can see expected result. Sleep(0) causes thread context switch, and ProgressChanged is called immediately.
However, Sleep(0) is added only for test, remove it. Asynchronous behavior of BackgroundWorker is absolutely OK, this is the way multithreading must work.

AlexFM

BTW, you can add more code to backgroundWorker1_DoWork to make it more time consuming. This this case output will look like this (without Sleep):

Console.WriteLine("In DoWork");
Console.WriteLine("In DoWork");
Console.WriteLine("In ProgressChanged");
Console.WriteLine("In ProgressChanged");
Console.WriteLine("In DoWork");
Console.WriteLine("In DoWork");
Console.WriteLine("In ProgressChanged");
Console.WriteLine("In DoWork");
Console.WriteLine("In DoWork");
Console.WriteLine("In DoWork");
Console.WriteLine("In ProgressChanged");
Console.WriteLine("In ProgressChanged");
Console.WriteLine("In ProgressChanged");
Console.WriteLine("In ProgressChanged");

Every thread executes its work in time interval given to this thread by operating system. This is exactly what we expect from parellel asynchronous execution.

i950

ASKER

Ahaaaa... it's clear now.

So my whole idea to update the UI in ProgressChanged not going to work as expected. What I'm doing is declaring a global variable for the listView item text and I assigning this variable in DoWork then I call ReportProgress and in ReportProgress I use the global variable to get the list view item name, which may or may not be changed!!!!!

What's the correct way to update the UI in this case?

AlexFM

Correct way is using BackgroundWorker.ReportProgress Method (Int32, Object) overload. It allows to add instance of any class or primitive type (like string) as parameter.

backgroundWorker1.ReportProgress(1, someVar);

private void backgroundWorker1_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
string s = (string)e.UserState;
...
}

i950

ASKER

It worked! Thanks!

Just one more thing...

How can I pass several variables (integers and strings) to ReportProgress?

AlexFM

You need to define your own class which contains all these variables, and pass instance of this class as parameter.

BackgroundWorkerData d = new BackgroundWorkerData(); // your class
d.name = somevar;
d.number1 = ...; // fill with any data

backgroundWorker1.ReportProgress(1, d);

private void backgroundWorker1_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
BackgroundWorkerData d = (BackgroundWorkerData)e.UserState;
// read data here
...
}

i950

ASKER

Is there any performance overhead when initiating many BackgroundWorkerData here:

BackgroundWorkerData d = new BackgroundWorkerData(); // your class
d.name = somevar;
d.number1 = ...; // fill with any data

I'll be using it in a loop that may be repeated several thousand times.

ASKER CERTIFIED SOLUTION

AlexFM

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