private void OnButtonClick(object sender, EventArgs e)
        {
            switch (((Button)sender).Name)
            {
            case "btnTest":
                TestExpression();
                break;

            case "btnClear":
                textQuery.Clear();
                break;

            case "btnShowValues":
                break;

            case "btnCancel":
                Close();
                break;

            case "btnOk":
                if (TestExpression())
                {
                    _layersHandler.VisibilityExpression(textQuery.Text, ExpressionTarget);
                    Close();
                }
                break;
            }
        }