How to handle a blocked clipboard and other oddities

Another workaround would be to use Clipboard.SetDataObject instead of Clipboard.SetText.

According to this MSDN article this method has two parameters - retryTimes and retryDelay - that you can use like this:

System.Windows.Forms.Clipboard.SetDataObject(
    "some text", // Text to store in clipboard
    false,       // Do not keep after our application exits
    5,           // Retry 5 times
    200);        // 200 ms delay between retries

As the clipboard is shared by all UI applications, you will run into this from time to time. Obviously, you don't want your application to crash if it failed to write to the clipboard, so gracefully handling ExternalException is reasonable. I would suggest presenting an error to the user if the SetObjectData call to write to the clipboard fails.

A suggestion would be to use (via P/Invoke) user32!GetOpenClipboardWindow to see if another application has the clipboard open. It will return the HWND of the window which has the clipboard open, or IntPtr.Zero if no application had it open. You could spin on the value until its IntPtr.Zero for a specified amount of time.

I ran into this error today. I decided to handle it by telling the user about the potentially misbehaving application. To do so, you can do something like this:

[System.Runtime.InteropServices.DllImport("user32.dll")]
static extern IntPtr GetOpenClipboardWindow();

[System.Runtime.InteropServices.DllImport("user32.dll")]
static extern int GetWindowText(int hwnd, StringBuilder text, int count);

private void btnCopy_Click(object sender, EventArgs e)
{
    try
    {
        Clipboard.Clear();
        Clipboard.SetText(textBox1.Text);
    }
    catch (Exception ex)
    {
        string msg = ex.Message;
        msg += Environment.NewLine;
        msg += Environment.NewLine;
        msg += "The problem:";
        msg += Environment.NewLine;
        msg += getOpenClipboardWindowText();
        MessageBox.Show(msg);
    }
}

private string getOpenClipboardWindowText()
{
    IntPtr hwnd = GetOpenClipboardWindow();
    StringBuilder sb = new StringBuilder(501);
    GetWindowText(hwnd.ToInt32(), sb, 500);
    return sb.ToString();
    // example:
    // skype_plugin_core_proxy_window: 02490E80
}

For me, the problem window title was "skype_plugin_core_proxy_window". I searched for info on that, and was surprised that it yielded only one hit, and that was in Russian. So I'm adding this answer, both to give another hit for that string, and to provide further help to bring potentially-misbehaving apps to light.