public void Test()
        {
            var textBox = new FormsTextBox(_form._textBox);
            var button  = new FormsButton(_form._button);

            textBox.EmulateChangeText("abc");
            button.SetFocus();

            string err = _form._errorProvider.GetError(_form._textBox);

            Assert.AreEqual("数字じゃないよ。", err);
        }