Q:

Why does pressing Ctrl+"+" produce a beep in a TextBox?

I am Working with C# and Windows Forms and want to use Ctrl+"Oemplus" as a function key for my application. I use a German keyboard and this key is located 2 keys right of the letter P (that ist then "+"). Whenever I press this key in combination with Ctrl and the focus is on a TextBox I get a beep.
This also happens when I switch to an US keyboard layout (still using my German keyboard). This is then the ] key.
The same happens when pressing this key while in Internet Explorers address bar. 
My question is:

Why does this key combination produce a beep in a TextBox.
How can I avoid the beep?

Thanks for any efforts you put on this.
Update:
I tried it on an US/Thai keyboard and get the beep as well. This happens no matter what logical keyboard layout I use (German, US, Thai).
The beep also happens in Windows Explorer in the address bar but not in the search box.

A:

It is very unclear what you hope to happen when you press that keystroke.  TextBox leaves no doubt about it, it BEEPs! because it can see that the user is trying to do something special but it doesn't know exactly what.  Good reason to beep you.  Solution is to implement magic, in the //.. comment in this next snippet.  With the extra code to stop the beep at the end:
    private void textBox1_KeyDown(object sender, KeyEventArgs e) {
        if (e.KeyData == (Keys.Control | Keys.Oemplus)) {
            // Invoke magic
            //...

            // Magic is done now:
            e.Handled = e.SuppressKeyPress = true;
        }
    }

A:

What I believe is happening is that the key combination is not allowed for the Textbox, therefore you are getting the error.  You can test for the Key Combination by this code( using the right bracket key in EN Windows) it is using SuppressKeyPress to prevent the Key Combination from being passed to the underlying control to prevent the beep.
private void textBox1_KeyDown(object sender, KeyEventArgs e)
{
    if (ModifierKeys == Keys.Control)
        if (e.KeyValue == 221) // You may need to determine this value for your keyboard layout.
        {
            textBox1.Text += " + "; // Handle the Key combination.
            e.SuppressKeyPress = true;  // Prevents key from being passed to underlying control
        }

}

