I have a Clipboard GetText Problem about MessageBox

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C#

clipboard

forms

As you see my timer check if user click ctrl+ mouse middle, after timer1() send ctrl+c keys to active program and get selected text to textbox1 but sometimes get some my MessageBoxs text content why? How can I solve this problem

Such as Textbox1 text content

---------------------------
Message Title
---------------------------
Message
---------------------------
OK
---------------------------

private void timer1_Tick(object sender, EventArgs e){
            if (comboBox1.SelectedIndex == 0 && hotkey.Ctrl_Middle()) //ctrl+mouse middle
            {
                timer_1();
            }
}

void timer_1(){
            string clp = "";
            if (!Clipboard.GetText().Contains(" -MessageBox text content -")) 
            {
                clp = Clipboard.GetText(); //Old clipboardtext backup
            }
            SendKeys.Send("^c");
            if (!string.IsNullOrWhiteSpace(Clipboard.GetText()) && Clipboard.GetText() != clp)
            {
                textBox1.Text = Clipboard.GetText();  //***************
            }
            
            if (!string.IsNullOrWhiteSpace(clp))
            {
                try
                {
                    Clipboard.SetText(clp);
                }
                catch (Exception)
                {
                    MessageBox.Show("Error : " + clp);
                }
            }
}