點擊下載示例代碼
String.Format 一重載方法的簽名如下
1 public static string Format( 2 IFormatProvider provider, 3 string format, 4 params Object[] args 5 )
可以通過自定義 IFormatProvider 接口來控制 String.Format 執行過程中的特定行為。
過程如下
1, 自定義類實現 IFormatProvider 接口（object GetFormat 方法）
1 public object GetFormat(Type formatType)
2 {
3 if (formatType == typeof(ICustomFormatter))
4 return this;
5 else
6 return null;
7 }
2, 實現 ICustomFormatter 借口（string Format 方法）
摘自 MSDN。功能為控制 Int64 type 的string輸出。
1 public string Format(string fmt, object arg, IFormatProvider formatProvider)
2 {
3 // Provide default formatting if arg is not an Int64.
4 if (arg.GetType() != typeof(Int64))
5 try
6 {
7 return HandleOtherFormats(fmt, arg);
8 }
9 catch (FormatException e)
10 {
11 throw new FormatException(String.Format("The format of '{0}' is invalid.", fmt), e);
12 }
13
14 // Provide default formatting for unsupported format strings.
15 string ufmt = fmt.ToUpper(CultureInfo.InvariantCulture);
16 if (!(ufmt == "H" || ufmt == "I"))
17 try
18 {
19 return HandleOtherFormats(fmt, arg);
20 }
21 catch (FormatException e)
22 {
23 throw new FormatException(String.Format("The format of '{0}' is invalid.", fmt), e);
24 }
25
26 // Convert argument to a string.
27 string result = arg.ToString();
28
29 // If account number is less than 12 characters, pad with leading zeroes.
30 if (result.Length < ACCT_LENGTH)
31 result = result.PadLeft(ACCT_LENGTH, '0');
32 // If account number is more than 12 characters, truncate to 12 characters.
33 if (result.Length > ACCT_LENGTH)
34 result = result.Substring(0, ACCT_LENGTH);
35
36 if (ufmt == "I") // Integer-only format.
37 return result;
38 // Add hyphens for H format specifier.
39 else // Hyphenated format.
40 return result.Substring(0, 5) + "-" + result.Substring(5, 3) + "-" + result.Substring(8);
41 }
使用實現好的 Format 類
1 long acctNumber;
2 double balance;
3 DaysOfWeek wday;
4 string output;
5
6 acctNumber = 104254567890;
7 balance = 16.34;
8 wday = DaysOfWeek.Monday;
9
10 output = String.Format((new AcctNumberFormat(),
11 "On {2}, the balance of account {0:H} was {1:C2}.",
12 acctNumber, balance, wday);
13 Console.WriteLine(output);
執行結果如下
參考資料
Culture invariant Decimal.TryParse() http://stackoverflow.com/questions/23131414/culture-invariant-decimal-tryparse
What does IFormatProvider do? http://stackoverflow.com/questions/506676/what-does-iformatprovider-do
IFormatProvider Interface https://msdn.microsoft.com/en-us/library/system.iformatprovider.aspx
