Question

I’m changing the Text of a button from a BackgroundWorker and it works. I thought that was supposed to throw an exception. Why doesn’t it?

Why don’t I get a Cross-thread operation not valid: ... accessed from a thread other than the thread it was created on.?

EDIT: Thanks everyone.

Perhaps the reason was that there was a: Thread.Sleep(1000); on the UI.

public Form1()
{
    InitializeComponent();

    backgroundWorker1.DoWork += new DoWorkEventHandler(backgroundWorker1_DoWork);
    backgroundWorker1.RunWorkerAsync();
    Thread.Sleep(1000);
}

void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
{
    button1.Text = "a";
}

However, I noticed that this following code runs fine as well, despite affecting the UI (indirectly).

public partial class Form1 : Form
{
    int i;

    public Form1()
    {
        InitializeComponent();

        i = 1;
        backgroundWorker1.DoWork += new DoWorkEventHandler(backgroundWorker1_DoWork);
        backgroundWorker1.RunWorkerAsync();
        for (int j = 0; j < 100000000; j++) ;
        button1.Text = i.ToString();
    }

    void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
    {
        i = 2;
    }
}

Why?

Answer 1

The answer is probably that cross thread operations can be executed. They’re just prone to trouble. And a Control (which is supposed to throw an exception) probably doesn’t ‘mind’ cross threading when it’s aSleep.