Effect C#的第5条讲到了IFormatProvider接口，不太明白于是找到了这个比较具有说明性的例子。

[C#] 
// Sample for the IFormatProvider interface and
// the IFormatProvider.GetFormat( Type ) method.
using System;

// This class implements the "Ra" formatting code. An instance of this 
// class should be passed to methods requiring an IFormatProvider.
public class AnyRadix : ICustomFormatter, IFormatProvider
{
    // The value to be formatted is returned as a signed string 
    // of digits from the rDigits array. 
    const string radixCode = "Ra";
    private static char[] rDigits = {
        '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 
        'A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 
        'K', 'L', 'M', 'N', 'O', 'P', 'Q', 'R', 'S', 'T', 
        'U', 'V', 'W', 'X', 'Y', 'Z' };
        
    // This method returns an object that implements ICustomFormatter 
    // to do the formatting. 
    public object GetFormat( Type argType )
    {
        // Here, the same object (this) is returned, but it would 
        // be possible to return an object of a different type.
        if ( argType == typeof( ICustomFormatter ) )
            return this;
        else
            return null;
    } 
        
    // This method does the formatting only if it recognizes the 
    // format codes. 
    public string Format( string formatString, 
        object argToBeFormatted, IFormatProvider provider )
    {
        // If no format string is provided or the format string cannot 
        // be handled, use IFormattable or standard string processing.
        if( formatString == null || 
            ! formatString.Trim( ).StartsWith( radixCode ) )
        {
            if( argToBeFormatted is IFormattable )
                return ( (IFormattable)argToBeFormatted ).
                    ToString( formatString, provider );
            else
                return argToBeFormatted.ToString( );
        }

        // The formatting is handled here.
        int     digitIndex = 0;
        long    radix;
        long    longToBeFormatted;
        long    longPositive;
        char[ ] outDigits = new char[ 63 ];
            
        // Extract the radix from the format string.
        formatString = formatString.Replace( radixCode, "" );
        try
        {
            radix = Convert.ToInt64( formatString );
        }
        catch( Exception ex )
        {
            throw new ArgumentException( String.Format( 
                "The radix \"{0}\" is invalid.", 
                formatString ), ex );
        }

        // Verify that the radix is in the proper range.
        if( radix <2 || radix > 36 )
            throw new ArgumentException( String.Format( 
                "The radix \"{0}\" is not in the range 2..36.", 
                formatString ) );
            
        // Verify that the argument can be converted to a long integer.
        try
        {
            longToBeFormatted = (long)argToBeFormatted;
        }
        catch( Exception ex )
        {
            throw new ArgumentException( String.Format(
                "The argument \"{0}\" cannot be " +
                "converted to an integer value.", 
                argToBeFormatted ), ex );
        }
            
        // Extract the magnitude for conversion.
        longPositive = Math.Abs( longToBeFormatted );

        // Convert the magnitude to a digit string.
        for( digitIndex = 0; digitIndex <= 64; digitIndex++ )
        {
            if( longPositive == 0 ) break;

            outDigits[ outDigits.Length - digitIndex - 1 ] = 
                rDigits[ longPositive % radix ];
            longPositive /= radix;
        }
            
        // Add a minus sign if the argument is negative.
        if( longToBeFormatted < 0 )
            outDigits[ outDigits.Length - digitIndex++ - 1 ] = 
                '-';

        return new string( outDigits, 
            outDigits.Length - digitIndex, digitIndex );
    } 
} 

class IFormatProviderDemo
{
    static void ConvertToAnyRadix( object argToConvert, 
        string formatStr )
    {
        AnyRadix    provider = new AnyRadix( );
        string      messageStr = 
            String.Format( "{{0:{0}}}", formatStr );

        // Write the first part of the output line.
        Console.Write( "{0,18}  {1,-6}", argToConvert, formatStr );

        // Convert the specified argument using the specified format.
        try
        {
            Console.WriteLine( String.Format( 
                provider, messageStr, argToConvert ) );
        }
        catch( Exception ex )
        {
            // Display the exception without the stack trace.
            int lineEnd = ex.ToString( ).IndexOf( '\n' );
            Console.WriteLine( "{0}\n",
                ex.ToString( ).Substring( 0, lineEnd ) );
        }
    } 
        
    static void Main( )
    {
        long twoToThe32 = 4294967296;
        long fifteenNines = 999999999999999;
            
        Console.WriteLine(
            "This example of the IFormatProvider interface \n" +
            "and the IFormatProvider.GetFormat( Type ) method " +
            "\ngenerates the following output.\n" );
        Console.WriteLine( "{0,18} Format Result", "Number" );
        Console.WriteLine( "{0,18} ------ ------", "------" );
            
        // These are valid conversions.
        ConvertToAnyRadix( twoToThe32, "Ra2" );
        ConvertToAnyRadix( twoToThe32, "Ra5" );
        ConvertToAnyRadix( twoToThe32, "Ra16" );
        ConvertToAnyRadix( twoToThe32, "Ra23" );
        ConvertToAnyRadix( twoToThe32, "Ra36" );
        ConvertToAnyRadix( fifteenNines, "Ra2" );
        ConvertToAnyRadix( fifteenNines, "Ra3" );
        ConvertToAnyRadix( fifteenNines, "Ra8" );
        ConvertToAnyRadix( fifteenNines, "Ra11" );
        ConvertToAnyRadix( fifteenNines, "Ra16" );
        ConvertToAnyRadix( fifteenNines, "Ra23" );
        ConvertToAnyRadix( fifteenNines, "Ra36" );
        ConvertToAnyRadix( fifteenNines, "E16" );
        ConvertToAnyRadix( fifteenNines, "" );
            
        // These are error conditions.
        ConvertToAnyRadix( fifteenNines, "Ra37" );
        ConvertToAnyRadix( "ABCDEFGHIJKLM", "Ra16" );
    } 
} 

/*
This example of the IFormatProvider interface
and the IFormatProvider.GetFormat( Type ) method
generates the following output.

            Number Format Result
            ------ ------ ------
        4294967296  Ra2   100000000000000000000000000000000
        4294967296  Ra5   32244002423141
        4294967296  Ra16  100000000
        4294967296  Ra23  1606K7IC
        4294967296  Ra36  1Z141Z4
   999999999999999  Ra2   11100011010111111010100100110001100111111111111111
   999999999999999  Ra3   11212010201001210101011021212000
   999999999999999  Ra8   34327724461477777
   999999999999999  Ra11  26A6A3689065639
   999999999999999  Ra16  38D7EA4C67FFF
   999999999999999  Ra23  1134DIFHLMM4
   999999999999999  Ra36  9UGXNORJLR
   999999999999999  E16   9.9999999999999900E+014
   999999999999999        999999999999999
   999999999999999  Ra37  System.ArgumentException: The radix "37" is not in th
e range 2..36.

     ABCDEFGHIJKLM  Ra16  System.ArgumentException: The argument "ABCDEFGHIJKLM
" cannot be converted to an integer value. ---> System.InvalidCastException: Sp
ecified cast is not valid.
*/