Background worker - report progress with string array

Question

I need to return multiple STRING values from my backgroundworker in each loop, so I tried to use ReportProgress second parameter as string array. Example of code:

private void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
    string[] workerResult = new string[2];
    for (int i=0; i<someNumber; i++)
    {
        //do some heavy calculating
        workerResult[0] = "this string";
        workerResult[1] = "some other string";
        backgroundWorker1.ReportProgress(i, workerResult) // also tried workerResult[] and [2]
    }
}

private void backgroundWorker1_ProgressChanged(object sender, ProgressChangedEventArgs e)
{
    string[] results = (string[])e.UserState;

    MessageBox.Show(results[0]); // line of error
    MessageBox.Show(results[1]); // line of error
}

It compiles, but on runtime in the moment I try to access Userstate returned string, I get an error: "Object reference not set to an instance of an object."

For me it seems lik something is wrong when passing array parameter to ProgressChanged delegate, or in ProgressChanged method when trying to set results array values.

Answer 1

Your code snippet is incapable of reproducing the problem. A standard mistake is to call ReportProgress() and then to continue modifying the object. It takes a while for the event handler to run, it will see the modified object, not the original. You avoid this by simply creating a new object so that the event handler always works with the original. Like this:

        //do some heavy calculating
        for (int i = 0; i < 2; ++i) {
            string[] workerResult = new string[2];
            workerResult[0] = "this string";
            workerResult[1] = "some other string";
            backgroundWorker1.ReportProgress(i, workerResult);
        }

Note how the array creation statement is moved inside the loop.