Question

I have a try-catch statement in reference to downloading a file. When the file is downloaded, I then enable one of my menu items then turn off a timer set to retry the download in one minute.

My problem is that for some reason my trafficManagementToolStripMenuItem.Enabled = true; line is activating the catch statement even though the menu item is being enabled. The file correctly downloads and when I comment out that line, it works perfectly. But every time I run it, I get the “error” message box and the timer is not disabled even though the menu item correctly enables after the download is complete.

Any ideas?

try
{
    ////downloads Data

    string address = "http://website.file.txt";
    string filename = "vsd.txt";
    WebClient client = new WebClient();
    client.DownloadFile(address, filename);
    trafficManagementToolStripMenuItem.Enabled = true;
    timer1.Enabled = false;
}
catch
{
    timer1.Enabled = true;
    MessageBox.Show("error", "test");
}

Answer 1

You don’t say what the exception is, but I’d guess that you’re running your download routine in a background thread, is that right? You can only access controls from the thread that created them: the main UI thread. Accessing a control such as a ToolStripMenuItem from a background thread will cause an exception.

If this is the case, use the Control.Invoke or Control.BeginInvoke method to run the .Enabled = true call on the ToolStripMenuItem’s thread. To do this, you’ll need a Control (unfortunately ToolStripMenuItem is only a Component). I’ll assume you can get a reference to the containing Form from somewhere. Now you can write this:

Action enableAction = delegate() { trafficManagementToolStripMenuItem.Enabled = true; }
form.BeginInvoke(enableAction);

This causes the enableAction to run on the correct UI thread for form.