点滴拾遗 - 自定义 Format 控制 String.Format 行为

String.Format 一重载方法的签名如下

 public static string Format(

     IFormatProvider provider,

     string format,

     params Object[] args

 )

可以通过自定义 IFormatProvider 接口来控制 String.Format 执行过程中的特定行为。

过程如下

1, 自定义类实现 IFormatProvider 接口（object GetFormat 方法）

 public object GetFormat(Type formatType)

 {

     if (formatType == typeof(ICustomFormatter))

         return this;

     else

         return null;

 }

2, 实现 ICustomFormatter 借口（string Format 方法）

摘自 MSDN。功能为控制 Int64 type 的string输出。

 public string Format(string fmt, object arg, IFormatProvider formatProvider)

 {

     // Provide default formatting if arg is not an Int64.

     if (arg.GetType() != typeof(Int64))

         try

         {

             return HandleOtherFormats(fmt, arg);

         }

         catch (FormatException e)

         {

             throw new FormatException(String.Format("The format of '{0}' is invalid.", fmt), e);

         }

     // Provide default formatting for unsupported format strings.

     string ufmt = fmt.ToUpper(CultureInfo.InvariantCulture);

     if (!(ufmt == "H" || ufmt == "I"))

         try

         {

             return HandleOtherFormats(fmt, arg);

         }

         catch (FormatException e)

         {

             throw new FormatException(String.Format("The format of '{0}' is invalid.", fmt), e);

         }

     // Convert argument to a string.

     string result = arg.ToString();

     // If account number is less than 12 characters, pad with leading zeroes.

     if (result.Length < ACCT_LENGTH)

         result = result.PadLeft(ACCT_LENGTH, '');

     // If account number is more than 12 characters, truncate to 12 characters.

     if (result.Length > ACCT_LENGTH)

         result = result.Substring(, ACCT_LENGTH);

     if (ufmt == "I")                    // Integer-only format.

         return result;

     // Add hyphens for H format specifier.

     else                                          // Hyphenated format.

         return result.Substring(, ) + "-" + result.Substring(, ) + "-" + result.Substring();

 }

使用实现好的 Format 类

 long acctNumber;

 double balance;

 DaysOfWeek wday;

 string output;

 acctNumber = ;

 balance = 16.34;

 wday = DaysOfWeek.Monday;

 output = String.Format((new AcctNumberFormat(),

                        "On {2}, the balance of account {0:H} was {1:C2}.",

                        acctNumber, balance, wday);

 Console.WriteLine(output);

执行结果如下

参考资料

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

巴特西

点滴拾遗 - 自定义 Format 控制 String.Format 行为

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