点滴拾遗 - 自定义 Format 控制 String.Format 行为

点击下载示例代码

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