public void ExpressionWithAdditionAndClickClearEntryButtonTest()
 {
     calculator.AddNumber("5");
     calculator.AddComma();
     calculator.AddNumber("2");
     calculator.AddOperation("+");
     calculator.AddNumber("1");
     calculator.AddComma();
     calculator.AddNumber("1");
     calculator.Counting();
     Assert.AreEqual(6, 3, Convert.ToDouble(calculator.CurrentEntry));
     calculator.ClearEntry();
     Assert.AreEqual("", calculator.CurrentEntry);
     Assert.AreEqual("", calculator.CurrentExpression);
 }
 /// <summary>
 /// Click a comma button.
 /// </summary>
 private void buttonComma_Click(object sender, EventArgs e) => calculator.AddComma();