DateTime中Parse(string s)和TryParse(string s, out datetime)都是用來將字元型的日期時間轉換為等效的System.DateTime。那麼,他們之間有沒有差別呢,除了函數的參數不同。先看下程式碼:
string dateTimeStr = ""; DateTime dt = DateTime.Parse(dateTimeStr);
運行空字串,將其轉換為日期時間型,顯然不能轉化,並且Parse()會拋出一個例外: System. FormatException: s 中不包含日期和時間的有效字串表示形式。但是,運行TryParse這個轉換方法:
string dateTimeStr = ""; DateTime dt2; //dt2未经初始化,就被传递给函数TryParse() bool sucflag = DateTime.TryParse(dateTimeStr, out dt2);
轉換首先是不拋出異常的,dt2被賦值為日期時間的最小值,sucflag為false。看下對函數的註解:
當此方法傳回時,如果轉換成功,則包含與s 中包含的日期和時間等效的System.DateTime 值;如果轉換失敗,則為System.DateTime.MinValue。如果s 參數為 null,是空字串 (“”) 或不包含日期和時間的有效字串表示形式,則轉換失敗。 *該參數未經初始化即傳遞。這個函數是不會拋出任何異常的。
看到他們的不同後,進一步來講,parse()拋出異常必然影響性能,TryParse()未拋出任何異常,這是一種優化異常效能的設計模式,稱為Try-Parse Pattern。以下是微軟的官方解釋:
For extremely performance-sensitive APIs, an even faster pattern than the Tester-Doer Pattern described in the previous section should be used. The pattern calls for adjusting the member name to make a well-defined test case a part of the member semantics. For example, DateTime defines a Parse method that throws an exception if parsing of a string fails. It also defines a corresponding parse, but returns false if parsing is unsuccessful and returns the result of a successful parsing using an out parameter.
ry ##T#Tester-Doer
##ry-Par了另一個模式:
Tester-Doer模式
如下程式碼:
ICollection<int> numbers = 省略获取数据的逻辑 numbers.Add(1);//Add此处不做可写性检查以上缺陷:假如集合是唯讀的,方法Add會拋出例外。呼叫這個方法的地方會經常拋出異常,因此會影響系統的效能。為了避免這個設計缺陷,微軟提出: Sometimes performance of an exception-throwing member can be improved by breaking the member into two. 將Add()分解為:
, which in our example is the propertyICollection<int> numbers = 省略获取数据的逻辑if(!numbers.IsReadOnly) //Tester{ numbers.Add(1); //Doer}# 將Add()分解為:rrreee Tester-Do Testerer Testerer Testerer Testerer Testerer Testerer Testerer模式總結:The member used to
test a condition
IsReadOnly
, is referred to as the tester. The member used to perform apotentially throwing operation
, the Add method in our example, is referred to as the doer.
以上是.NET框架-Try-Parse和Tester-Doer的使用區別的詳細內容。更多資訊請關注PHP中文網其他相關文章!