formatCode 是可选的格式化代码字符串。（详细内容请搜索“格式化字符串”查看）

必须用“{”和“}”将格式与其他字符分开。如果恰好在格式中也要使用大括号，可以用连续的两个大括号表示一个大括号，即： “{{”或者“}}”。

常用格式举例：

（1） int i=12345;

this.textBox1.Text=i.ToString();

//结果 12345（this指当前对象，或叫当前类的实例）

this.textBox2.Text=i.ToString("d8");

//结果 00012345

（2） int i=123;

double j=123.45;

string s1=string.Format("the value is {0,7:d}",i);

string s2=string.Format("the value is {0,7:f3}",j);

this.textBox1.Text=s1 ;

//结果 the value is 123

this.textBox2.Text=s2;

//结果 the value is 123.450

（3）double i=12345.6789;

this.textBox1.Text=i.ToString("f2"); //结果 12345.68

this.textBox2.Text=i.ToString("f6");

//结果 12345.678900

（4）double i=12345.6789;

this.textBox1.Text=i.ToString("n"); //结果 12,345.68

this.textBox2.Text=i.ToString(“n4”); //结果 12,345.6789

（5）double i=0.126;

string s=string.Format("the value is {0:p}",i);

this.textBox1.Text=i.ToString("p"); //结果 12.6%

this.textBox2.Text=s; //结果 the value is 12.6%

（6） DateTime dt =new DateTime(2003,5,25);

this.textBox1.Text=dt.ToString("yy.M.d");

//结果 03.5.25

this.textBox2.Text=dt.ToString(“yyyy年M月”);

//结果 2003年5月

Convert.ToDateTime("2005/12/22 22:22:22").ToString("yyyy/MM/dd HH:mm:ss")
"2005/12/22 22:22:22"

（7） int i=123;

double j=123.45;

string s=string.Format("i:{0,-7},j:{1,7}",i,j);

//-7表示左对齐，占7位

this.textBox1.Text=s ;

//结果i:123 ,j: 123.45 

Byte.ToString 方法 (String, IFormatProvider)的C#例子

下面的代码示例使用 ToString 方法的几个重载格式化 Byte 值。

// Example for the Byte.ToString( ) methods.
using System;
using System.Globalization;

class ByteToStringDemo
{
    static void RunToStringDemo( )
    {
        byte smallValue = http://www.mamicode.com/13;
        byte largeValue = http://www.mamicode.com/234;
            
        // Format the Byte values without and with format strings.
        Console.WriteLine( "/nIFormatProvider is not used:" );
        Console.WriteLine( "   {0,-20}{1,10}{2,10}", 
            "No format string:", smallValue.ToString( ), 
            largeValue.ToString( ) );
        Console.WriteLine( "   {0,-20}{1,10}{2,10}", 
            "‘X2‘ format string:", smallValue.ToString( "X2" ), 
            largeValue.ToString( "X2" ) );
            
        // Get the NumberFormatInfo object from the 
        // invariant culture.
        CultureInfo         culture = new CultureInfo( "" );
        NumberFormatInfo    numInfo = culture.NumberFormat;
            
        // Set the digit grouping to 1, set the digit separator
        // to underscore, and set decimal digits to 0.
        numInfo.NumberGroupSizes = new int[ ] { 1 };
        numInfo.NumberGroupSeparator = "_";
        numInfo.NumberDecimalDigits = 0;
            
        // Use the NumberFormatInfo object for an IFormatProvider.
        Console.WriteLine( 
            "/nA NumberFormatInfo object with digit group " +
            "size = 1 and /ndigit separator " +
            "= ‘_‘ is used for the IFormatProvider:" );
        Console.WriteLine( "   {0,-20}{1,10}{2,10}", 
            "No format string:", smallValue.ToString( numInfo ), 
            largeValue.ToString( numInfo ) );
        Console.WriteLine( "   {0,-20}{1,10}{2,10}", 
            "‘N‘ format string:", 
            smallValue.ToString( "N", numInfo ), 
            largeValue.ToString( "N", numInfo ) );
    } 
        
    static void Main( )
    {
        Console.WriteLine( "This example of/n" +
            "   Byte.ToString( ),/n" +
            "   Byte.ToString( String ),/n" + 
            "   Byte.ToString( IFormatProvider ), and/n" +
            "   Byte.ToString( String, IFormatProvider )/n" +
            "generates the following output when formatting " +
            "Byte values /nwith combinations of format " +
            "strings and IFormatProvider." );
            
        RunToStringDemo( );
    }
}

/*
This example of
   Byte.ToString( ),
   Byte.ToString( String ),
   Byte.ToString( IFormatProvider ), and
   Byte.ToString( String, IFormatProvider )
generates the following output when formatting Byte values
with combinations of format strings and IFormatProvider.

IFormatProvider is not used:
   No format string:           13       234
   ‘X2‘ format string:         0D        EA

A NumberFormatInfo object with digit group size = 1 and
digit separator = ‘_‘ is used for the IFormatProvider:
   No format string:           13       234
   ‘N‘ format string:         1_3     2_3_4
*/