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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, '0');
    // If account number is more than 12 characters, truncate to 12 characters. 
    if (result.Length > ACCT_LENGTH)
        result = result.Substring(0, ACCT_LENGTH);

    if (ufmt == "I")                    // Integer-only format.  
        return result;
    // Add hyphens for H format specifier. 
    else                                          // Hyphenated format. 
        return result.Substring(0, 5) + "-" + result.Substring(5, 3) + "-" + result.Substring(8);
}

使用实现好的 Format 类

long acctNumber;
double balance;
DaysOfWeek wday;
string output;

acctNumber = 104254567890;
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