public string Evaluate(string expression) { IOperand result; try { var infixExpression = parser.Parse(expression); var postfixExpression = postfixConverter.ConvertInfixToPostfix(infixExpression); result = postfixCalculator.EvaluateExpression(postfixExpression); } catch (Exception ex) { return(ex.Message); //throw new ArgumentNullException(); } return(result.Value.ToString()); }