MessageBox.Show called from backgound worker with the handle of the main UI thread

Question

I have this code:

public void Blah(IWin32Window _this)
{
    for (int i = 0; i < item_quantity; i++)
    {
        try { File.Delete(item[0, i]); }
        catch (Exception ex)
        {
            if (MessageBox.Show(_this, String.Format("Error while accessing {0}\n{1}"
                , item[0, i], ex.Message), "Error", MessageBoxButtons.RetryCancel
                , MessageBoxIcon.Error) == DialogResult.Retry)
            { i--; }
        }
    }
}

...and this code in the main UI thread:

private void backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
{
    AnotherClass.Blah(this);
}

When I execute this code, I get the unsafe cross-thread exception. What's the safe way to do this operation?

Answer 1

I'm not condoning the design, but you can pass in the Form to Blah() and then Invoke() against the referenced form:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private void button1_Click(object sender, EventArgs e)
    {
        if (!backgroundWorker.IsBusy)
        {
            button1.Enabled = false;
            backgroundWorker.RunWorkerAsync();
        }
    }

    private void backgroundWorker_DoWork(object sender, DoWorkEventArgs e)
    {
        SomeClass AnotherClass = new SomeClass();
        AnotherClass.Blah(this);
    }

    private void backgroundWorker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
    {
        button1.Enabled = true;
        MessageBox.Show("Done!");
    }

}

public class SomeClass
{
    public void Blah(Form frm)
    {
        int item_quantity = 5;

        for (int i = 0; i < item_quantity; i++)
        {
            try
            {
                //File.Delete(item[0, i]);

                Console.WriteLine("i = " + i.ToString());
                throw new Exception("duh");
            }
            catch (Exception ex)
            {
                frm.Invoke(new Action(() =>
                    {
                        DialogResult result = MessageBox.Show(frm, String.Format("Error while accessing {0}\n{1}", "something", ex.Message), "Error", MessageBoxButtons.RetryCancel, MessageBoxIcon.Error);
                        if (result == DialogResult.Retry)
                        {
                            i--;
                        }
                    }));
            }
        }
    }
}