OpenTween.FilterDialog.FilterTextBox_KeyDown C# (CSharp) Method

        private void FilterTextBox_KeyDown(object sender, KeyEventArgs e)
        {
            if (e.KeyCode == Keys.Space && e.Modifiers == (Keys.Shift | Keys.Control))
            {
                TweenMain main = (TweenMain)this.Owner;
                TextBox tbox = (TextBox)sender;
                if (tbox.SelectionStart > 0)
                {
                    int endidx = tbox.SelectionStart - 1;
                    string startstr = "";
                    for (int i = tbox.SelectionStart - 1; i >= 0; i--)
                    {
                        char c = tbox.Text[i];
                        if (Char.IsLetterOrDigit(c) || c == '_')
                        {
                            continue;
                        }
                        if (c == '@')
                        {
                            startstr = tbox.Text.Substring(i + 1, endidx - i);
                            main.ShowSuplDialog(tbox, main.AtIdSupl, startstr.Length + 1, startstr);
                        }
                        else if (c == '#')
                        {
                            startstr = tbox.Text.Substring(i + 1, endidx - i);
                            main.ShowSuplDialog(tbox, main.HashSupl, startstr.Length + 1, startstr);
                        }
                        else
                        {
                            break;
                        }
                    }
                    e.Handled = true;
                }
            }
        }