System.String.Format 方法

项目
01/30/2024

本文提供了此 API 参考文档的补充说明。

重要

可以不调用 String.Format 方法或使用复合格式字符串，而改为使用内插字符串（如果受语言支持的话）。内插字符串是包含内插表达式的字符串。每个内插表达式都使用表达式的值进行解析，并在分配字符串时包含在结果字符串中。有关详细信息，请参阅字符串内插（C# 参考）和内插字符串（Visual Basic 参考）。

示例

在本文中，调用该方法 Format 的许多示例都插在一起。还可以下载一组String.Format完整的示例，其中包括适用于 C# 的 .NET Core 项目。

以下是本文中包含的一些示例：

创建格式字符串

插入字符串
 格式项
 设置具有相同索引的项的格式

控制格式化输出

控件格式
 控制间距
 控件对齐方式
 控制整型数字的数量
 控制小数分隔符后的位数
 在结果字符串中包含文本大括号

使格式字符串区分区域性

自定义格式设置操作

自定义格式设置操作
 拦截提供程序和罗马数字格式化程序

String.Format 方法入门

如果需要将对象、变量或表达式的值插入另一个字符串，请使用 String.Format 。例如，可以将值的值 Decimal 插入字符串中，以单个字符串的形式向用户显示该值：

Decimal pricePerOunce = 17.36m;
String s = String.Format("The current price is {0} per ounce.",
                         pricePerOunce);
Console.WriteLine(s);
// Result: The current price is 17.36 per ounce.

let pricePerOunce = 17.36m
String.Format("The current price is {0} per ounce.", pricePerOunce)
|> printfn "%s"
// Result: The current price is 17.36 per ounce.

Dim pricePerOunce As Decimal = 17.36D
Dim s As String = String.Format("The current price is {0} per ounce.",
                              pricePerOunce)
' Result: The current price is 17.36 per ounce.

你可以控制该值的格式：

Decimal pricePerOunce = 17.36m;
String s = String.Format("The current price is {0:C2} per ounce.",
                         pricePerOunce);
Console.WriteLine(s);
// Result if current culture is en-US:
//      The current price is $17.36 per ounce.

let pricePerOunce = 17.36m
String.Format("The current price is {0:C2} per ounce.", pricePerOunce)
|> printfn "%s"
// Result if current culture is en-US:
//      The current price is $17.36 per ounce.

Dim pricePerOunce As Decimal = 17.36D
Dim s As String = String.Format("The current price is {0:C2} per ounce.",
                              pricePerOunce)
' Result if current culture is en-US:
'      The current price is $17.36 per ounce.

除了设置格式，还可以控制对齐和间距。

插入字符串

String.Format 以格式字符串开头，后跟一个或多个对象或表达式，这些对象或表达式将转换为字符串并在格式字符串中的指定位置插入。例如：

decimal temp = 20.4m;
string s = String.Format("The temperature is {0}°C.", temp);
Console.WriteLine(s);
// Displays 'The temperature is 20.4°C.'

let temp = 20.4m
String.Format("The temperature is {0}°C.", temp)
|> printfn "%s"
// Displays 'The temperature is 20.4°C.'

Dim temp As Decimal = 20.4D
Dim s As String = String.Format("The temperature is {0}°C.", temp)
Console.WriteLine(s)
' Displays 'The temperature is 20.4°C.'

格式 {0} 字符串是一个格式项。 0 是对象索引，其字符串值将插入到该位置。（索引从 0 开始。如果要插入的对象不是字符串，则调用其 ToString 方法将其转换为一个，然后再将其插入到结果字符串中。

下面是另一个示例，该示例使用对象列表中的两个格式项和两个对象：

string s = String.Format("At {0}, the temperature is {1}°C.",
                         DateTime.Now, 20.4);
Console.WriteLine(s);
// Output similar to: 'At 4/10/2015 9:29:41 AM, the temperature is 20.4°C.'

String.Format("At {0}, the temperature is {1}°C.", DateTime.Now, 20.4)
|> printfn "%s"
// Output similar to: 'At 4/10/2015 9:29:41 AM, the temperature is 20.4°C.'

Dim s As String = String.Format("At {0}, the temperature is {1}°C.",
                              Date.Now, 20.4)
' Output similar to: 'At 4/10/2015 9:29:41 AM, the temperature is 20.4°C.'

只要每个格式项的索引在对象列表中具有匹配的对象，就可以有任意数量的格式项和任意数量的对象。你也不必担心调用哪个重载;编译器将为你选择相应的编译器。

控件格式

可以使用格式字符串跟踪格式项中的索引，以控制对象的格式设置方式。例如， {0:d} 将“d”格式字符串应用于对象列表中的第一个对象。下面是一个包含单个对象和两个格式项的示例：

string s = String.Format("It is now {0:d} at {0:t}", DateTime.Now);
Console.WriteLine(s);
// Output similar to: 'It is now 4/10/2015 at 10:04 AM'

String.Format("It is now {0:d} at {0:t}", DateTime.Now)
|> printfn "%s"
// Output similar to: 'It is now 4/10/2015 at 10:04 AM'

Dim s As String = String.Format("It is now {0:d} at {0:t}",
                              Date.Now)
' Output similar to: 'It is now 4/10/2015 at 10:04 AM'

许多类型支持格式字符串，包括所有数值类型（标准格式字符串和自定义格式字符串）、所有日期和时间（标准格式字符串和自定义格式字符串）和时间间隔（标准和自定义格式字符串）、所有枚举类型枚举类型和 GUID。还可以向自己的类型添加对格式字符串的支持。

控制间距

可以使用插入 12 个字符的字符串等 {0,12}语法定义插入结果字符串的宽度。在这种情况下，第一个对象的字符串表示形式在 12 个字符的字段中右对齐。（如果第一个对象的字符串表示形式长度超过 12 个字符，则忽略首选字段宽度，并将整个字符串插入到结果字符串中。

下面的示例定义了一个 6 个字符的字段，用于保存字符串“Year”和某些年份字符串，以及用于保存字符串“Population”和某些总体数据的 15 个字符字段。请注意，字符在字段中右对齐。

int[] years = { 2013, 2014, 2015 };
int[] population = { 1025632, 1105967, 1148203 };
var sb = new System.Text.StringBuilder();
sb.Append(String.Format("{0,6} {1,15}\n\n", "Year", "Population"));
for (int index = 0; index < years.Length; index++)
    sb.Append(String.Format("{0,6} {1,15:N0}\n", years[index], population[index]));

Console.WriteLine(sb);

// Result:
//      Year      Population
//
//      2013       1,025,632
//      2014       1,105,967
//      2015       1,148,203

open System
open System.Text
 
let years = [| 2013; 2014; 2015 |]
let population = [| 1025632; 1105967; 1148203 |]
let sb = StringBuilder()
sb.Append(String.Format("{0,6} {1,15}\n\n", "Year", "Population")) |> ignore
for i = 0 to years.Length - 1 do
   sb.Append(String.Format("{0,6} {1,15:N0}\n", years[i], population[i])) |> ignore

printfn $"{sb}"

// Result:
//      Year      Population
//
//      2013       1,025,632
//      2014       1,105,967
//      2015       1,148,203

Dim years() As Integer = {2013, 2014, 2015}
Dim population() As Integer = {1025632, 1105967, 1148203}
Dim sb As New StringBuilder()
sb.Append(String.Format("{0,6} {1,15}{2}{2}",
                       "Year", "Population", vbCrLf))
For index As Integer = 0 To years.Length - 1
    sb.AppendFormat("{0,6} {1,15:N0}{2}",
                  years(index), population(index), vbCrLf)
Next
' Result:
'      Year      Population
'
'      2013       1,025,632
'      2014       1,105,967
'      2015       1,148,203

控件对齐方式

默认情况下，如果指定字段宽度，字符串在其字段中右对齐。若要在字段中左对齐字符串，请为字段宽度加上负号，例如 {0,-12} 定义 12 个字符的左对齐字段。

以下示例与上一个示例类似，只不过它与标签和数据左对齐。

int[] years = { 2013, 2014, 2015 };
int[] population = { 1025632, 1105967, 1148203 };
String s = String.Format("{0,-10} {1,-10}\n\n", "Year", "Population");
for (int index = 0; index < years.Length; index++)
    s += String.Format("{0,-10} {1,-10:N0}\n",
                       years[index], population[index]);
Console.WriteLine($"\n{s}");
// Result:
//    Year       Population
//
//    2013       1,025,632
//    2014       1,105,967
//    2015       1,148,203

let years = [| 2013; 2014; 2015 |]
let population = [| 1025632; 1105967; 1148203 |]
let mutable s = String.Format("{0,-10} {1,-10}\n\n", "Year", "Population")
for i = 0 to years.Length - 1 do
    s <- s + String.Format("{0,-10} {1,-10:N0}\n", years[i], population[i])
printfn $"\n{s}"
// Result:
//    Year       Population
//
//    2013       1,025,632
//    2014       1,105,967
//    2015       1,148,203

Dim years() As Integer = {2013, 2014, 2015}
Dim population() As Integer = {1025632, 1105967, 1148203}
Dim s As String = String.Format("{0,-10} {1,-10}{2}{2}",
                               "Year", "Population", vbCrLf)
For index As Integer = 0 To years.Length - 1
    s += String.Format("{0,-10} {1,-10:N0}{2}",
                     years(index), population(index), vbCrLf)
Next
' Result:
'    Year       Population
'
'    2013       1,025,632
'    2014       1,105,967
'    2015       1,148,203

String.Format 利用复合格式设置功能。有关更多信息，请参见复合格式设置。

我调用哪种方法？

操作	调用
使用当前区域性的约定设置一个或多个对象的格式。	除包含 `provider` 参数的重载外，其余 Format 重载还包括一个后跟一 String 个或多个对象参数的参数。因此，无需确定要调用的 Format 重载。语言编译器根据参数列表从没有 `provider` 参数的重载中选择适当的重载。例如，如果参数列表有五个参数，编译器将调用该方法 Format(String, Object[]) 。
使用特定区域性的约定设置一个或多个对象的格式。	以参数开头`provider`的每个Format重载后跟一个String参数和一个或多个对象参数。因此，无需确定要调用的特定 Format 重载。语言编译器根据参数列表从具有 `provider` 参数的重载中选择适当的重载。例如，如果参数列表有五个参数，编译器将调用该方法 Format(IFormatProvider, String, Object[]) 。
使用 ICustomFormatter 实现或 IFormattable 实现执行自定义格式设置操作。	具有参数的四个重载中的任何一个 `provider` 。编译器根据参数列表从具有 `provider` 参数的重载中选择适当的重载。

简短的 Format 方法

该方法的每个重载 Format 都使用复合格式功能在复合格式字符串中包含从零开始的索引占位符（称为 格式项）。在运行时，每个格式项都将替换为参数列表中相应参数的字符串表示形式。如果参数 null的值为，则格式项将 String.Empty替换为。例如，对方法的以下调用Format(String, Object, Object, Object)包括一个格式字符串，其中包含三个格式项，{1}{0}以及{2}具有三个项的参数列表。

DateTime dat = new DateTime(2012, 1, 17, 9, 30, 0); 
string city = "Chicago";
int temp = -16;
string output = String.Format("At {0} in {1}, the temperature was {2} degrees.",
                              dat, city, temp);
Console.WriteLine(output);
// The example displays output like the following:
//    At 1/17/2012 9:30:00 AM in Chicago, the temperature was -16 degrees.

open System

let dat = DateTime(2012, 1, 17, 9, 30, 0) 
let city = "Chicago"
let temp = -16
String.Format("At {0} in {1}, the temperature was {2} degrees.", dat, city, temp)
|> printfn "%s"
// The example displays output like the following:
//    At 1/17/2012 9:30:00 AM in Chicago, the temperature was -16 degrees.

Dim dat As Date = #1/17/2012 9:30AM#
Dim city As String = "Chicago"
Dim temp As Integer = -16
Dim output As String = String.Format("At {0} in {1}, the temperature was {2} degrees.",
                                   dat, city, temp)
Console.WriteLine(output)
' The example displays the following output:
'    At 1/17/2012 9:30:00 AM in Chicago, the temperature was -16 degrees.

格式项

格式项具有以下语法：

{index[,alignment][:formatString]}

括号表示可选元素。需要左大括号和右大括号。（若要在格式字符串中包含文本左大括号或右大括号，请参阅复合格式设置文章中的转义大括号部分。

例如，设置货币值格式的格式项可能如下所示：

var value = String.Format("{0,-10:C}", 126347.89m);
Console.WriteLine(value);

open System

String.Format("{0,-10:C}", 126347.89m)         
|> printfn "%s"

String.Format("{0,-10:C}", 126347.89D)

格式项具有以下元素：

index
参数的从零开始的索引，其字符串表示形式将包含在字符串中的此位置。如果为此参数 null，字符串中将包含空字符串。

alignment
可选。一个带符号整数，指示插入自变量的字段的总长度，以及它是右对齐（正整数）还是左对齐（负整数）。如果省略对齐方式，则相应的参数的字符串表示形式将插入到没有前导或尾随空格的字段中。

如果对齐值小于要插入的参数的长度，则忽略对齐方式，并将参数的字符串表示形式的长度用作字段宽度。

formatString
可选。一个字符串，指定相应参数的结果字符串的格式。如果省略 formatString，将调用相应的参数的无 ToString 参数方法以生成其字符串表示形式。如果指定 formatString，格式项引用的参数必须实现 IFormattable 接口。支持格式字符串的类型包括：

所有整型和浮点类型。（请参阅标准数值格式字符串和自定义数字格式字符串。）
DateTime 和 DateTimeOffset。（请参阅标准日期和时间格式字符串以及自定义日期和时间格式字符串。）
所有枚举类型。（请参阅枚举格式字符串。）
TimeSpan 值。（请参阅标准 TimeSpan 格式字符串和自定义 TimeSpan 格式字符串。）
GUID。（请参阅方法 Guid.ToString(String) 。

但是，请注意，任何自定义类型都可以实现 IFormattable 或扩展现有类型的 IFormattable 实现。

以下示例使用 alignment 和 formatString 参数生成格式化输出。

// Create array of 5-tuples with population data for three U.S. cities, 1940-1950.
Tuple<string, DateTime, int, DateTime, int>[] cities = 
    { Tuple.Create("Los Angeles", new DateTime(1940, 1, 1), 1504277, 
                   new DateTime(1950, 1, 1), 1970358),
      Tuple.Create("New York", new DateTime(1940, 1, 1), 7454995, 
                   new DateTime(1950, 1, 1), 7891957),  
      Tuple.Create("Chicago", new DateTime(1940, 1, 1), 3396808, 
                   new DateTime(1950, 1, 1), 3620962),  
      Tuple.Create("Detroit", new DateTime(1940, 1, 1), 1623452, 
                   new DateTime(1950, 1, 1), 1849568) };

// Display header
var header = String.Format("{0,-12}{1,8}{2,12}{1,8}{2,12}{3,14}\n",
                              "City", "Year", "Population", "Change (%)");
Console.WriteLine(header);
foreach (var city in cities) {
   var output = String.Format("{0,-12}{1,8:yyyy}{2,12:N0}{3,8:yyyy}{4,12:N0}{5,14:P1}",
                          city.Item1, city.Item2, city.Item3, city.Item4, city.Item5,
                          (city.Item5 - city.Item3)/ (double)city.Item3);
   Console.WriteLine(output);
}
// The example displays the following output:
//    City            Year  Population    Year  Population    Change (%)
//  
//    Los Angeles     1940   1,504,277    1950   1,970,358        31.0 %
//    New York        1940   7,454,995    1950   7,891,957         5.9 %
//    Chicago         1940   3,396,808    1950   3,620,962         6.6 %
//    Detroit         1940   1,623,452    1950   1,849,568        13.9 %

// Create a list of 5-tuples with population data for three U.S. cities, 1940-1950.
let cities = 
    [ "Los Angeles", DateTime(1940, 1, 1), 1504277, DateTime(1950, 1, 1), 1970358
      "New York", DateTime(1940, 1, 1), 7454995, DateTime(1950, 1, 1), 7891957
      "Chicago", DateTime(1940, 1, 1), 3396808, DateTime(1950, 1, 1), 3620962
      "Detroit", DateTime(1940, 1, 1), 1623452, DateTime(1950, 1, 1), 1849568 ]

// Display header
String.Format("{0,-12}{1,8}{2,12}{1,8}{2,12}{3,14}\n", "City", "Year", "Population", "Change (%)")
|> printfn "%s"

for name, year1, pop1, year2, pop2 in cities do
    String.Format("{0,-12}{1,8:yyyy}{2,12:N0}{3,8:yyyy}{4,12:N0}{5,14:P1}",
                  name, year1, pop1, year2, pop2,
                  double (pop2 - pop1) / double pop1)
    |> printfn "%s"
// The example displays the following output:
//    City            Year  Population    Year  Population    Change (%)
//  
//    Los Angeles     1940   1,504,277    1950   1,970,358        31.0 %
//    New York        1940   7,454,995    1950   7,891,957         5.9 %
//    Chicago         1940   3,396,808    1950   3,620,962         6.6 %
//    Detroit         1940   1,623,452    1950   1,849,568        13.9 %

Module Example3
    Public Sub Main()
        ' Create array of 5-tuples with population data for three U.S. cities, 1940-1950.
        Dim cities() =
          {Tuple.Create("Los Angeles", #1/1/1940#, 1504277, #1/1/1950#, 1970358),
            Tuple.Create("New York", #1/1/1940#, 7454995, #1/1/1950#, 7891957),
            Tuple.Create("Chicago", #1/1/1940#, 3396808, #1/1/1950#, 3620962),
            Tuple.Create("Detroit", #1/1/1940#, 1623452, #1/1/1950#, 1849568)}

        ' Display header
        Dim header As String = String.Format("{0,-12}{1,8}{2,12}{1,8}{2,12}{3,14}",
                                           "City", "Year", "Population", "Change (%)")
        Console.WriteLine(header)
        Console.WriteLine()
        For Each city In cities
            Dim output = String.Format("{0,-12}{1,8:yyyy}{2,12:N0}{3,8:yyyy}{4,12:N0}{5,14:P1}",
                                city.Item1, city.Item2, city.Item3, city.Item4, city.Item5,
                                (city.Item5 - city.Item3) / city.Item3)
            Console.WriteLine(output)
        Next
    End Sub
End Module
' The example displays the following output:
'    City            Year  Population    Year  Population    Change (%)
'    
'    Los Angeles     1940   1,504,277    1950   1,970,358        31.0 %
'    New York        1940   7,454,995    1950   7,891,957         5.9 %
'    Chicago         1940   3,396,808    1950   3,620,962         6.6 %
'    Detroit         1940   1,623,452    1950   1,849,568        13.9 %

如何设置参数的格式

格式项从字符串的开头按顺序处理。每个格式项都有一个索引，该索引对应于方法的参数列表中的对象。该方法 Format 检索参数并派生其字符串表示形式，如下所示：

如果参数为 null，该方法将插入 String.Empty 结果字符串。无需考虑处理 NullReferenceException null 参数。
如果调用 Format(IFormatProvider, String, Object[]) 重载并且 provider 对象的 IFormatProvider.GetFormat 实现返回非 null ICustomFormatter 实现，则参数将传递给其 ICustomFormatter.Format(String, Object, IFormatProvider) 方法。如果格式项包含 formatString 参数，则会将其作为第一个参数传递给该方法。 ICustomFormatter如果实现可用并生成非 null 字符串，则将该字符串作为参数的字符串表示形式返回;否则，下一步将执行。
如果参数实现 IFormattable 接口，则调用其 IFormattable.ToString 实现。
调用自变量的无 ToString 参数方法（替代或继承基类实现）。

有关截获对 ICustomFormatter.Format 方法的调用并允许您查看该方法传递给复合格式字符串中每个格式项的格式方法的信息 Format ，请参阅示例：拦截提供程序和罗马数字格式化程序。

有关详细信息，请参阅处理订单。

设置具有相同索引的项的格式

如果索引项的索引大于或等于参数列表中的参数数，该方法 Format 将 FormatException 引发异常。但是， format 只要多个格式项具有相同的索引，就可以包含比有参数更多的格式项。在对以下示例中方法的调用 Format(String, Object) 中，参数列表有一个参数，但格式字符串包含两个格式项：一个显示数字的小数值，另一个显示其十六进制值。

short[] values= { Int16.MinValue, -27, 0, 1042, Int16.MaxValue };
Console.WriteLine("{0,10}  {1,10}\n", "Decimal", "Hex");
foreach (short value in values)
{
   string formatString = String.Format("{0,10:G}: {0,10:X}", value);
   Console.WriteLine(formatString);
}   
// The example displays the following output:
//       Decimal         Hex
//    
//        -32768:       8000
//           -27:       FFE5
//             0:          0
//          1042:        412
//         32767:       7FFF

open System

let values= [| Int16.MinValue; -27s; 0s; 1042s; Int16.MaxValue |]
printfn "%10s  %10s\n" "Decimal" "Hex"
for value in values do
    String.Format("{0,10:G}: {0,10:X}", value)
    |> printfn "%s"
// The example displays the following output:
//       Decimal         Hex
//    
//        -32768:       8000
//           -27:       FFE5
//             0:          0
//          1042:        412
//         32767:       7FFF

Module Example1
    Public Sub Main()
        Dim values() As Short = {Int16.MinValue, -27, 0, 1042, Int16.MaxValue}
        Console.WriteLine("{0,10}  {1,10}", "Decimal", "Hex")
        Console.WriteLine()
        For Each value As Short In values
            Dim formatString As String = String.Format("{0,10:G}: {0,10:X}", value)
            Console.WriteLine(formatString)
        Next
    End Sub
End Module
' The example displays the following output:
'       Decimal         Hex
'    
'        -32768:       8000
'           -27:       FFE5
'             0:          0
'          1042:        412
'         32767:       7FFF

格式和区域性

通常，参数列表中的对象通过使用当前区域性的约定（由属性返回 CultureInfo.CurrentCulture ）转换为其字符串表示形式。可以通过调用其中一个包含provider参数的Format重载来控制此行为。该 provider 参数是一个 IFormatProvider 实现，它提供自定义和区域性特定的格式设置信息，用于审查格式设置过程。

该 IFormatProvider 接口具有单个成员， GetFormat该成员负责返回提供格式信息的对象。 .NET 有三 IFormatProvider 个实现，提供区域性特定的格式设置：

CultureInfo。其 GetFormat 方法返回一个区域性特定的 NumberFormatInfo 对象，用于设置数值的格式设置，并返回一个区域性特定的 DateTimeFormatInfo 对象，用于设置日期和时间值的格式。
DateTimeFormatInfo，用于日期和时间值区域性特定的格式设置。其 GetFormat 方法返回自身。
NumberFormatInfo，用于特定于区域性的数值格式设置。其 GetFormat(Type) 方法返回自身。

自定义格式设置操作

还可以调用具有类型IFormatProvider参数的方法provider的任何重载Format来执行自定义格式设置操作。例如，可以将整数的格式格式化为标识号或电话号码。若要执行自定义格式设置，参数 provider 必须同时实现 IFormatProvider 接口和 ICustomFormatter 接口。 Format当方法作为参数传递ICustomFormatter实现provider时，该方法Format将调用其IFormatProvider.GetFormat实现并请求类型的ICustomFormatter对象。然后，它调用返回 ICustomFormatter 的对象 Format 的方法来设置传递给它的复合字符串中的每个格式项的格式。

有关提供自定义格式解决方案的详细信息，请参阅如何：定义和使用自定义数字格式提供程序和 ICustomFormatter。有关将整数转换为格式化自定义数字的示例，请参阅示例：自定义格式设置操作。有关将无符号字节转换为罗马数字的示例，请参阅示例：拦截提供程序和罗马数字格式化程序。

示例：自定义格式设置操作

本示例定义格式提供程序，该格式将整数值的格式设置为 x-xxxxx-xx 形式的客户帐户号。

using System;

public class TestFormatter
{
   public static void Main()
   {
      int acctNumber = 79203159;
      Console.WriteLine(String.Format(new CustomerFormatter(), "{0}", acctNumber));
      Console.WriteLine(String.Format(new CustomerFormatter(), "{0:G}", acctNumber));
      Console.WriteLine(String.Format(new CustomerFormatter(), "{0:S}", acctNumber));
      Console.WriteLine(String.Format(new CustomerFormatter(), "{0:P}", acctNumber));
      try {
         Console.WriteLine(String.Format(new CustomerFormatter(), "{0:X}", acctNumber));
      }
      catch (FormatException e) {
         Console.WriteLine(e.Message);
      }
   }
}

public class CustomerFormatter : IFormatProvider, ICustomFormatter
{
   public object GetFormat(Type formatType) 
   {
      if (formatType == typeof(ICustomFormatter))        
         return this; 
      else
         return null;
   }
   
   public string Format(string format, 
                         object arg, 
                         IFormatProvider formatProvider) 
   {                       
      if (! this.Equals(formatProvider))
      {
         return null;
      }
      else
      {
         if (String.IsNullOrEmpty(format)) 
            format = "G";
         
         string customerString = arg.ToString();
         if (customerString.Length < 8)
            customerString = customerString.PadLeft(8, '0');
         
         format = format.ToUpper();
         switch (format)
         {
            case "G":
               return customerString.Substring(0, 1) + "-" +
                                     customerString.Substring(1, 5) + "-" +
                                     customerString.Substring(6);
            case "S":                          
               return customerString.Substring(0, 1) + "/" +
                                     customerString.Substring(1, 5) + "/" +
                                     customerString.Substring(6);
            case "P":                          
               return customerString.Substring(0, 1) + "." +
                                     customerString.Substring(1, 5) + "." +
                                     customerString.Substring(6);
            default:
               throw new FormatException( 
                         String.Format("The '{0}' format specifier is not supported.", format));
         }
      }   
   }
}
// The example displays the following output:
//       7-92031-59
//       7-92031-59
//       7/92031/59
//       7.92031.59
//       The 'X' format specifier is not supported.

open System

type CustomerFormatter() = 
    interface IFormatProvider with
        member this.GetFormat(formatType) =
            if formatType = typeof<ICustomFormatter> then
                this
            else
                null

    interface ICustomFormatter with
        member this.Format(format, arg, formatProvider: IFormatProvider) = 
            if this.Equals formatProvider |> not then
                null
            else
                let format = 
                    if String.IsNullOrEmpty format then "G"
                    else format.ToUpper()
                
                let customerString = 
                    let s = string arg
                    if s.Length < 8 then
                        s.PadLeft(8, '0')
                    else s
                
                match format with
                | "G" ->
                    customerString.Substring(0, 1) + "-" +
                        customerString.Substring(1, 5) + "-" +
                        customerString.Substring 6
                | "S" ->                          
                    customerString.Substring(0, 1) + "/" +
                        customerString.Substring(1, 5) + "/" +
                        customerString.Substring 6
                | "P" ->                          
                    customerString.Substring(0, 1) + "." +
                        customerString.Substring(1, 5) + "." +
                        customerString.Substring 6
                | _ ->
                    raise (FormatException $"The '{format}' format specifier is not supported.")

let acctNumber = 79203159
String.Format(CustomerFormatter(), "{0}", acctNumber)
|> printfn "%s"
String.Format(CustomerFormatter(), "{0:G}", acctNumber)
|> printfn "%s"
String.Format(CustomerFormatter(), "{0:S}", acctNumber)
|> printfn "%s"
String.Format(CustomerFormatter(), "{0:P}", acctNumber)
|> printfn "%s"
try
    String.Format(CustomerFormatter(), "{0:X}", acctNumber)
    |> printfn "%s"
with :? FormatException as e ->
    printfn $"{e.Message}"

// The example displays the following output:
//       7-92031-59
//       7-92031-59
//       7/92031/59
//       7.92031.59
//       The 'X' format specifier is not supported.

Module TestFormatter
   Public Sub Main()
      Dim acctNumber As Integer = 79203159
      Console.WriteLine(String.Format(New CustomerFormatter, "{0}", acctNumber))
      Console.WriteLine(String.Format(New CustomerFormatter, "{0:G}", acctNumber))
      Console.WriteLine(String.Format(New CustomerFormatter, "{0:S}", acctNumber))
      Console.WriteLine(String.Format(New CustomerFormatter, "{0:P}", acctNumber))
      Try
         Console.WriteLine(String.Format(New CustomerFormatter, "{0:X}", acctNumber))
      Catch e As FormatException
         Console.WriteLine(e.Message)
      End Try   
   End Sub
End Module

Public Class CustomerFormatter : Implements IFormatProvider, ICustomFormatter
   Public Function GetFormat(type As Type) As Object  _
                   Implements IFormatProvider.GetFormat
      If type Is GetType(ICustomFormatter) Then
         Return Me
      Else
         Return Nothing
      End If
   End Function
   
   Public Function Format(fmt As String, _
                           arg As Object, _
                           formatProvider As IFormatProvider) As String _
                    Implements ICustomFormatter.Format
      If Not Me.Equals(formatProvider) Then
         Return Nothing
      Else
         If String.IsNullOrEmpty(fmt) Then fmt = "G"
         
         Dim customerString As String = arg.ToString()
         if customerString.Length < 8 Then _
            customerString = customerString.PadLeft(8, "0"c)
         
         Select Case fmt
            Case "G"
               Return customerString.Substring(0, 1) & "-" & _
                                     customerString.Substring(1, 5) & "-" & _
                                     customerString.Substring(6)
            Case "S"                         
               Return customerString.Substring(0, 1) & "/" & _
                                     customerString.Substring(1, 5) & "/" & _
                                     customerString.Substring(6)
            Case "P"
               Return customerString.Substring(0, 1) & "." & _
                                     customerString.Substring(1, 5) & "." & _
                                     customerString.Substring(6)
            Case Else
               Throw New FormatException( _
                         String.Format("The '{0}' format specifier is not supported.", fmt))
         End Select                                                     
      End If   
   End Function
End Class
' The example displays the following output:
'       7-92031-59
'       7-92031-59
'       7/92031/59
'       7.92031.59
'       The 'X' format specifier is not supported.

示例：拦截提供程序和罗马数字格式化程序

此示例定义一个自定义格式提供程序，该提供程序实现 ICustomFormatter 和 IFormatProvider 接口以执行两项操作：

它显示传递给其 ICustomFormatter.Format 实现的参数。这样，我们便可以查看该方法传递给尝试设置格式的每个对象的自定义格式实现的参数 Format(IFormatProvider, String, Object[]) 。在调试应用程序时，这非常有用。
如果要格式化的对象是使用“R”标准格式字符串格式化的无符号字节值，则自定义格式化程序将数值格式化为罗马数字。

using System;
using System.Globalization;

public class InterceptProvider : IFormatProvider, ICustomFormatter
{
   public object GetFormat(Type formatType)
   {
      if (formatType == typeof(ICustomFormatter))
         return this;
      else
         return null;
   }
   
   public string Format(String format, Object obj, IFormatProvider provider) 
   {
      // Display information about method call.
      string formatString = format ?? "<null>";
      Console.WriteLine("Provider: {0}, Object: {1}, Format String: {2}",
                        provider.GetType().Name, obj ?? "<null>", formatString);
                        
      if (obj == null) return String.Empty;
            
      // If this is a byte and the "R" format string, format it with Roman numerals.
      if (obj is Byte && formatString.ToUpper().Equals("R")) {
         Byte value = (Byte) obj;
         int remainder;
         int result;
         String returnString = String.Empty;

         // Get the hundreds digit(s)
         result = Math.DivRem(value, 100, out remainder);
         if (result > 0)  
            returnString = new String('C', result);
         value = (Byte) remainder;
         // Get the 50s digit
         result = Math.DivRem(value, 50, out remainder);
         if (result == 1)
            returnString += "L";
         value = (Byte) remainder;
         // Get the tens digit.
         result = Math.DivRem(value, 10, out remainder);
         if (result > 0)
            returnString += new String('X', result);
         value = (Byte) remainder; 
         // Get the fives digit.
         result = Math.DivRem(value, 5, out remainder);
         if (result > 0)
            returnString += "V";
         value = (Byte) remainder;
         // Add the ones digit.
         if (remainder > 0) 
            returnString += new String('I', remainder);
         
         // Check whether we have too many X characters.
         int pos = returnString.IndexOf("XXXX");
         if (pos >= 0) {
            int xPos = returnString.IndexOf("L"); 
            if (xPos >= 0 & xPos == pos - 1)
               returnString = returnString.Replace("LXXXX", "XC");
            else
               returnString = returnString.Replace("XXXX", "XL");   
         }
         // Check whether we have too many I characters
         pos = returnString.IndexOf("IIII");
         if (pos >= 0)
            if (returnString.IndexOf("V") >= 0)
               returnString = returnString.Replace("VIIII", "IX");
            else
               returnString = returnString.Replace("IIII", "IV");    

         return returnString; 
      }   

      // Use default for all other formatting.
      if (obj is IFormattable)
         return ((IFormattable) obj).ToString(format, CultureInfo.CurrentCulture);
      else
         return obj.ToString();
   }
}

public class Example
{
   public static void Main()
   {
      int n = 10;
      double value = 16.935;
      DateTime day = DateTime.Now;
      InterceptProvider provider = new InterceptProvider();
      Console.WriteLine(String.Format(provider, "{0:N0}: {1:C2} on {2:d}\n", n, value, day));
      Console.WriteLine(String.Format(provider, "{0}: {1:F}\n", "Today: ", 
                                      (DayOfWeek) DateTime.Now.DayOfWeek));
      Console.WriteLine(String.Format(provider, "{0:X}, {1}, {2}\n", 
                                      (Byte) 2, (Byte) 12, (Byte) 199));
      Console.WriteLine(String.Format(provider, "{0:R}, {1:R}, {2:R}\n", 
                                      (Byte) 2, (Byte) 12, (Byte) 199));
   }
}
// The example displays the following output:
//    Provider: InterceptProvider, Object: 10, Format String: N0
//    Provider: InterceptProvider, Object: 16.935, Format String: C2
//    Provider: InterceptProvider, Object: 1/31/2013 6:10:28 PM, Format String: d
//    10: $16.94 on 1/31/2013
//    
//    Provider: InterceptProvider, Object: Today: , Format String: <null>
//    Provider: InterceptProvider, Object: Thursday, Format String: F
//    Today: : Thursday
//    
//    Provider: InterceptProvider, Object: 2, Format String: X
//    Provider: InterceptProvider, Object: 12, Format String: <null>
//    Provider: InterceptProvider, Object: 199, Format String: <null>
//    2, 12, 199
//    
//    Provider: InterceptProvider, Object: 2, Format String: R
//    Provider: InterceptProvider, Object: 12, Format String: R
//    Provider: InterceptProvider, Object: 199, Format String: R
//    II, XII, CXCIX

open System
open System.Globalization

type InterceptProvider() =
    interface IFormatProvider with
        member this.GetFormat(formatType) =
            if formatType = typeof<ICustomFormatter> then
                this
            else
                null
    interface ICustomFormatter with
        member _.Format(format, obj, provider: IFormatProvider) = 
            // Display information about method call.
            let formatString =
                if format = null then "<null>" else format
            printfn $"Provider: {provider.GetType().Name}, Object: %A{obj}, Format String: %s{formatString}"
                                
            if obj = null then
                String.Empty
            else
                // If this is a byte and the "R" format string, format it with Roman numerals.
                match obj with
                | :? byte as value when formatString.ToUpper().Equals "R" -> 
                    let mutable returnString = String.Empty

                    // Get the hundreds digit(s)
                    let struct (result, remainder) = Math.DivRem(value, 100uy)
                    if result > 0uy then
                        returnString <- String('C', int result)
                    let value = byte remainder
                    // Get the 50s digit
                    let struct (result, remainder) = Math.DivRem(value, 50uy)
                    if result = 1uy then
                        returnString <- returnString + "L"
                    let value = byte remainder
                    // Get the tens digit.
                    let struct (result, remainder) = Math.DivRem(value, 10uy)
                    if result > 0uy then
                        returnString <- returnString + String('X', int result)
                    let value = byte remainder 
                    // Get the fives digit.
                    let struct (result, remainder) = Math.DivRem(value, 5uy)
                    if result > 0uy then
                        returnString <- returnString + "V"
                    let value = byte remainder
                    // Add the ones digit.
                    if remainder > 0uy then 
                        returnString <- returnString + String('I', int remainder)
                    
                    // Check whether we have too many X characters.
                    let pos = returnString.IndexOf "XXXX"
                    if pos >= 0 then
                        let xPos = returnString.IndexOf "L" 
                        returnString <-
                            if xPos >= 0 && xPos = pos - 1 then
                                returnString.Replace("LXXXX", "XC")
                            else
                                returnString.Replace("XXXX", "XL")   
                    // Check whether we have too many I characters
                    let pos = returnString.IndexOf "IIII"
                    if pos >= 0 then
                        returnString <-
                            if returnString.IndexOf "V" >= 0 then
                                returnString.Replace("VIIII", "IX")
                            else
                                returnString.Replace("IIII", "IV")    
                    returnString 

                // Use default for all other formatting.
                | :? IFormattable as x ->
                    x.ToString(format, CultureInfo.CurrentCulture)
                | _ ->
                    string obj

let n = 10
let value = 16.935
let day = DateTime.Now
let provider = InterceptProvider()
String.Format(provider, "{0:N0}: {1:C2} on {2:d}\n", n, value, day)
|> printfn "%s"
String.Format(provider, "{0}: {1:F}\n", "Today: ", DateTime.Now.DayOfWeek)
|> printfn "%s"
String.Format(provider, "{0:X}, {1}, {2}\n", 2uy, 12uy, 199uy)
|> printfn "%s"
String.Format(provider, "{0:R}, {1:R}, {2:R}\n", 2uy, 12uy, 199uy)
|> printfn "%s"
// The example displays the following output:
//    Provider: InterceptProvider, Object: 10, Format String: N0
//    Provider: InterceptProvider, Object: 16.935, Format String: C2
//    Provider: InterceptProvider, Object: 1/31/2013 6:10:28 PM, Format String: d
//    10: $16.94 on 1/31/2013
//    
//    Provider: InterceptProvider, Object: Today: , Format String: <null>
//    Provider: InterceptProvider, Object: Thursday, Format String: F
//    Today: : Thursday
//    
//    Provider: InterceptProvider, Object: 2, Format String: X
//    Provider: InterceptProvider, Object: 12, Format String: <null>
//    Provider: InterceptProvider, Object: 199, Format String: <null>
//    2, 12, 199
//    
//    Provider: InterceptProvider, Object: 2, Format String: R
//    Provider: InterceptProvider, Object: 12, Format String: R
//    Provider: InterceptProvider, Object: 199, Format String: R
//    II, XII, CXCIX

Imports System.Globalization

Public Class InterceptProvider : Implements IFormatProvider, ICustomFormatter
   Public Function GetFormat(formatType As Type) As Object _
         Implements IFormatProvider.GetFormat
      If formatType Is GetType(ICustomFormatter) Then
         Return Me
      Else
         Return Nothing
      End If
   End Function
   
   Public Function Format(fmt As String, obj As Object, provider As IFormatProvider) As String _
         Implements ICustomFormatter.Format

      Dim formatString As String = If(fmt IsNot Nothing, fmt, "<null>")
      Console.WriteLine("Provider: {0}, Object: {1}, Format String: {2}",
                        provider, If(obj IsNot Nothing, obj, "<null>"), formatString)

      If obj Is Nothing Then Return String.Empty
            
      ' If this is a byte and the "R" format string, format it with Roman numerals.
      If TypeOf(obj) Is Byte AndAlso formatString.ToUpper.Equals("R") Then
         Dim value As Byte = CByte(obj)
         Dim remainder As Integer
         Dim result As Integer
         Dim returnString As String = String.Empty

         ' Get the hundreds digit(s)
         result = Math.DivRem(value, 100, remainder)
         If result > 0 Then returnString = New String("C"c, result)
         value = CByte(remainder)
         ' Get the 50s digit
         result = Math.DivRem(value, 50, remainder)
         If result = 1 Then returnString += "L"
         value = CByte(remainder)
         ' Get the tens digit.
         result = Math.DivRem(value, 10, remainder)
         If result > 0 Then returnString += New String("X"c, result)
         value = CByte(remainder) 
         ' Get the fives digit.
         result = Math.DivRem(value, 5, remainder)
         If result > 0 Then returnString += "V"
         value = CByte(remainder)
         ' Add the ones digit.
         If remainder > 0 Then returnString += New String("I"c, remainder)
         
         ' Check whether we have too many X characters.
         Dim pos As Integer = returnString.IndexOf("XXXX")
         If pos >= 0 Then
            Dim xPos As Integer = returnString.IndexOf("L") 
            If xPos >= 0 And xPos = pos - 1 Then
               returnString = returnString.Replace("LXXXX", "XC")
            Else
               returnString = returnString.Replace("XXXX", "XL")   
            End If         
         End If
         ' Check whether we have too many I characters
         pos = returnString.IndexOf("IIII")
         If pos >= 0 Then
            If returnString.IndexOf("V") >= 0 Then
               returnString = returnString.Replace("VIIII", "IX")
            Else
               returnString = returnString.Replace("IIII", "IV")    
            End If
         End If
         Return returnString 
      End If   

      ' Use default for all other formatting.
      If obj Is GetType(IFormattable)
         Return CType(obj, IFormattable).ToString(fmt, CultureInfo.CurrentCulture)
      Else
         Return obj.ToString()
      End If
   End Function
End Class

Module Example
   Public Sub Main()
      Dim n As Integer = 10
      Dim value As Double = 16.935
      Dim day As DateTime = Date.Now
      Dim provider As New InterceptProvider()
      Console.WriteLine(String.Format(provider, "{0:N0}: {1:C2} on {2:d}", n, value, day))
      Console.WriteLine()
      Console.WriteLine(String.Format(provider, "{0}: {1:F}", "Today", 
                                      CType(Date.Now.DayOfWeek, DayOfWeek)))
      Console.WriteLine()
      Console.WriteLine(String.Format(provider, "{0:X}, {1}, {2}\n", 
                                      CByte(2), CByte(12), CByte(199)))
      Console.WriteLine()
      Console.WriteLine(String.Format(provider, "{0:R}, {1:R}, {2:R}", 
                                      CByte(2), CByte(12), CByte(199)))
   End Sub
End Module
' The example displays the following output:
'    Provider: InterceptProvider, Object: 10, Format String: N0
'    Provider: InterceptProvider, Object: 16.935, Format String: C2
'    Provider: InterceptProvider, Object: 1/31/2013 6:10:28 PM, Format String: d
'    10: $16.94 on 1/31/2013
'    
'    Provider: InterceptProvider, Object: Today: , Format String: <null>
'    Provider: InterceptProvider, Object: Thursday, Format String: F
'    Today: : Thursday
'    
'    Provider: InterceptProvider, Object: 2, Format String: X
'    Provider: InterceptProvider, Object: 12, Format String: <null>
'    Provider: InterceptProvider, Object: 199, Format String: <null>
'    2, 12, 199
'    
'    Provider: InterceptProvider, Object: 2, Format String: R
'    Provider: InterceptProvider, Object: 12, Format String: R
'    Provider: InterceptProvider, Object: 199, Format String: R
'    II, XII, CXCIX

常见问题解答

为什么建议对方法的调用 `String.Format` 进行字符串内插？

字符串内插为：

更灵活。它可以在任何字符串中使用，而无需调用支持复合格式的方法。否则，必须调用 Format 支持复合格式的方法或其他方法，例如 Console.WriteLine 或 StringBuilder.AppendFormat。
更易于阅读。由于要插入字符串的表达式出现在内插表达式而不是参数列表中，因此内插字符串更易于编码和读取。由于它们的可读性更高，内插字符串不仅可以替换对复合格式方法的调用，还可以在字符串串联操作中使用它们来生成更简洁、更清晰的代码。

以下两个代码示例的比较说明了内插字符串相对于字符串串联的优越性，以及对复合格式设置方法的调用。以下示例中使用多个字符串串联操作将生成详细且难以读取的代码。

string[] names = { "Balto", "Vanya", "Dakota", "Samuel", "Koani", "Yiska", "Yuma" };
string output = names[0] + ", " + names[1] + ", " + names[2] + ", " +
                names[3] + ", " + names[4] + ", " + names[5] + ", " +
                names[6];

output += "\n";
var date = DateTime.Now;
output += String.Format("It is {0:t} on {0:d}. The day of the week is {1}.",
                        date, date.DayOfWeek);
Console.WriteLine(output);
// The example displays the following output:
//     Balto, Vanya, Dakota, Samuel, Koani, Yiska, Yuma
//     It is 10:29 AM on 1/8/2018. The day of the week is Monday.

open System

let names = [| "Balto"; "Vanya"; "Dakota"; "Samuel"; "Koani"; "Yiska"; "Yuma" |]
let output = 
    names[0] + ", " + names[1] + ", " + names[2] + ", " + 
    names[3] + ", " + names[4] + ", " + names[5] + ", " + 
    names[6] + "\n"

let date = DateTime.Now
output + String.Format("It is {0:t} on {0:d}. The day of the week is {1}.", date, date.DayOfWeek)
|> printfn "%s"
// The example displays the following output:
//     Balto, Vanya, Dakota, Samuel, Koani, Yiska, Yuma
//     It is 10:29 AM on 1/8/2018. The day of the week is Monday.


Module Example12
    Public Sub Main()
        Dim names = {"Balto", "Vanya", "Dakota", "Samuel", "Koani", "Yiska", "Yuma"}
        Dim output = names(0) + ", " + names(1) + ", " + names(2) + ", " +
                   names(3) + ", " + names(4) + ", " + names(5) + ", " +
                   names(6)

        output += vbCrLf
        Dim dat = DateTime.Now
        output += String.Format("It is {0:t} on {0:d}. The day of the week is {1}.",
                              dat, dat.DayOfWeek)
        Console.WriteLine(output)
    End Sub
End Module
' The example displays the following output:
'     Balto, Vanya, Dakota, Samuel, Koani, Yiska, Yuma
'     It is 10:29 AM on 1/8/2018. The day of the week is Monday.

相比之下，以下示例中的内插字符串的使用比字符串串联语句和对上一示例中方法的调用 Format 更清晰、更简洁。

string[] names = { "Balto", "Vanya", "Dakota", "Samuel", "Koani", "Yiska", "Yuma" };
string output = $"{names[0]}, {names[1]}, {names[2]}, {names[3]}, {names[4]}, " +
                $"{names[5]}, {names[6]}";

var date = DateTime.Now;
output += $"\nIt is {date:t} on {date:d}. The day of the week is {date.DayOfWeek}.";
Console.WriteLine(output);
// The example displays the following output:
//     Balto, Vanya, Dakota, Samuel, Koani, Yiska, Yuma
//     It is 10:29 AM on 1/8/2018. The day of the week is Monday.

open System

let names = [| "Balto"; "Vanya"; "Dakota"; "Samuel"; "Koani"; "Yiska"; "Yuma" |]
let output = $"{names[0]}, {names[1]}, {names[2]}, {names[3]}, {names[4]}, {names[5]}, {names[6]}"  

let date = DateTime.Now
output + $"\nIt is {date:t} on {date:d}. The day of the week is {date.DayOfWeek}."
|> printfn "%s" 
// The example displays the following output:
//     Balto, Vanya, Dakota, Samuel, Koani, Yiska, Yuma
//     It is 10:29 AM on 1/8/2018. The day of the week is Monday.


Module Example13
    Public Sub Main()
        Dim names = {"Balto", "Vanya", "Dakota", "Samuel", "Koani", "Yiska", "Yuma"}
        Dim output = $"{names(0)}, {names(1)}, {names(2)}, {names(3)}, {names(4)}, " +
                   $"{names(5)}, {names(6)}"

        Dim dat = DateTime.Now
        output += $"{vbCrLf}It is {dat:t} on {dat:d}. The day of the week is {dat.DayOfWeek}."
        Console.WriteLine(output)
    End Sub
End Module
' The example displays the following output:
'     Balto, Vanya, Dakota, Samuel, Koani, Yiska, Yuma
'     It is 10:29 AM on 1/8/2018. The day of the week is Monday.

在哪里可以找到预定义的格式字符串？

有关所有整型和浮点类型，请参阅标准数值格式字符串和自定义数值格式字符串。
有关日期和时间值，请参阅标准日期和时间格式字符串以及自定义日期和时间格式字符串。
有关枚举值，请参阅枚举格式字符串。
有关 TimeSpan 值，请参阅标准 TimeSpan 格式字符串和自定义 TimeSpan 格式字符串。
有关 Guid 值，请参阅参考页的 Guid.ToString(String) “备注”部分。

如何实现控制替换格式项的结果字符串的对齐方式？

格式项的一般语法为：

{index[,alignment][: formatString]}

其中对齐方式是定义字段宽度的带符号整数。如果此值为负值，则字段中的文本左对齐。如果它是正的，则文本右对齐。

如何实现控制小数点分隔符后的位数？

除“D”（仅用于整数）、“G”、“R”和“X”之外的所有标准数值格式字符串都允许定义结果字符串中小数位数的精度说明符。以下示例使用标准数值格式字符串来控制结果字符串中的十进制数字数。

object[] values = { 1603, 1794.68235, 15436.14 };
string result;
foreach (var value in values)
{
    result = String.Format("{0,12:C2}   {0,12:E3}   {0,12:F4}   {0,12:N3}  {1,12:P2}\n",
                           Convert.ToDouble(value), Convert.ToDouble(value) / 10000);
    Console.WriteLine(result);
}
// The example displays output like the following:
//       $1,603.00     1.603E+003      1603.0000      1,603.000       16.03 %
//    
//       $1,794.68     1.795E+003      1794.6824      1,794.682       17.95 %
//    
//      $15,436.14     1.544E+004     15436.1400     15,436.140      154.36 %

open System

let values: obj list = [ 1603, 1794.68235, 15436.14 ]
for value in values do
   String.Format("{0,12:C2}   {0,12:E3}   {0,12:F4}   {0,12:N3}  {1,12:P2}\n", Convert.ToDouble(value), Convert.ToDouble(value) / 10000.)
   |> printfn "%s"
// The example displays output like the following:
//       $1,603.00     1.603E+003      1603.0000      1,603.000       16.03 %
//    
//       $1,794.68     1.795E+003      1794.6824      1,794.682       17.95 %
//    
//      $15,436.14     1.544E+004     15436.1400     15,436.140      154.36 %

Module Example7
    Public Sub Main()
        Dim values() As Object = {1603, 1794.68235, 15436.14}
        Dim result As String
        For Each value In values
            result = String.Format("{0,12:C2}   {0,12:E3}   {0,12:F4}   {0,12:N3}  {1,12:P2}",
                                value, CDbl(value) / 10000)
            Console.WriteLine(result)
            Console.WriteLine()
        Next
    End Sub
End Module
' The example displays the following output:
'       $1,603.00     1.603E+003      1603.0000      1,603.000       16.03 %
'    
'       $1,794.68     1.795E+003      1794.6824      1,794.682       17.95 %
'    
'      $15,436.14     1.544E+004     15436.1400     15,436.140      154.36 %

如果使用自定义数字格式字符串，请使用“0”格式说明符来控制结果字符串中的十进制数字数，如以下示例所示。

decimal value = 16309.5436m;
string result = String.Format("{0,12:#.00000} {0,12:0,000.00} {0,12:000.00#}",
                              value);
Console.WriteLine(result);
// The example displays the following output:
//        16309.54360    16,309.54    16309.544

let value = 16309.5436m
String.Format("{0,12:#.00000} {0,12:0,000.00} {0,12:000.00#}", value)
|> printfn "%s"
// The example displays the following output:
//        16309.54360    16,309.54    16309.544

Module Example8
    Public Sub Main()
        Dim value As Decimal = 16309.5436D
        Dim result As String = String.Format("{0,12:#.00000} {0,12:0,000.00} {0,12:000.00#}",
                                           value)
        Console.WriteLine(result)
    End Sub
End Module
' The example displays the following output:
'    16309.54360    16,309.54    16309.544

如何实现控制整型数字的数量？

默认情况下，格式设置操作仅显示非零整型数字。如果要设置整数格式，则可以使用精度说明符和“D”和“X”标准格式字符串来控制数字数。

int value = 1326;
string result = String.Format("{0,10:D6} {0,10:X8}", value);
Console.WriteLine(result);
// The example displays the following output:
//     001326   0000052E

open System

let value = 1326
String.Format("{0,10:D6} {0,10:X8}", value)
|> printfn "%s"
// The example displays the following output:
//     001326   0000052E

Module Example10
    Public Sub Main()
        Dim value As Integer = 1326
        Dim result As String = String.Format("{0,10:D6} {0,10:X8}", value)
        Console.WriteLine(result)
    End Sub
End Module
' The example displays the following output:
'       001326   0000052E

可以使用“0” 自定义数字格式说明符填充整数或浮点数，以生成具有指定数字整数数的结果字符串，如以下示例所示。

int value = 16342;
string result = String.Format("{0,18:00000000} {0,18:00000000.000} {0,18:000,0000,000.0}",
                              value);
Console.WriteLine(result);
// The example displays the following output:
//           00016342       00016342.000    0,000,016,342.0

open System

let value = 16342
String.Format("{0,18:00000000} {0,18:00000000.000} {0,18:000,0000,000.0}", value)
|> printfn "%s"
// The example displays the following output:
//           00016342       00016342.000    0,000,016,342.0

Module Example9
    Public Sub Main()
        Dim value As Integer = 16342
        Dim result As String = String.Format("{0,18:00000000} {0,18:00000000.000} {0,18:000,0000,000.0}",
                                           value)
        Console.WriteLine(result)
    End Sub
End Module
' The example displays the following output:
'           00016342       00016342.000    0,000,016,342.0

格式列表中可以包含多少项？

没有实际限制。方法的第 Format(IFormatProvider, String, Object[]) 二个参数使用 ParamArrayAttribute 属性进行标记，该属性允许将分隔列表或对象数组作为格式列表包含。

如何实现结果字符串中包含文本大括号（“{”和“}”）？

例如，如何阻止以下方法调用引发 FormatException 异常？

result = String.Format("The text has {0} '{' characters and {1} '}' characters.",
                       nOpen, nClose);

let result = 
    String.Format("The text has {0} '{' characters and {1} '}' characters.", nOpen, nClose)

result = String.Format("The text has {0} '{' characters and {1} '}' characters.",
                 nOpen, nClose)

单个左大括号或右大括号始终解释为格式项的开头或结尾。若要按字面解释，必须对其进行转义。通过添加另一个大括号（“{{”和“}}”而不是“{”和“}”）来转义大括号，如以下方法调用所示：

string result;
int nOpen = 1;
int nClose = 2;
result = String.Format("The text has {0} '{{' characters and {1} '}}' characters.",
                       nOpen, nClose);
Console.WriteLine(result);

let result =
    String.Format("The text has {0} '{{' characters and {1} '}}' characters.", nOpen, nClose)

result = String.Format("The text has {0} '{{' characters and {1} '}}' characters.",
                 nOpen, nClose)

然而，即使是转义的大括号也很容易被误解。建议在格式列表中包括大括号，并使用格式项在结果字符串中插入它们，如以下示例所示。

string result;
int nOpen = 1;
int nClose = 2;
result = String.Format("The text has {0} '{1}' characters and {2} '{3}' characters.",
                       nOpen, "{", nClose, "}");
Console.WriteLine(result);

let result =
    String.Format("The text has {0} '{1}' characters and {2} '{3}' characters.", nOpen, "{", nClose, "}")

result = String.Format("The text has {0} '{1}' characters and {2} '{3}' characters.",
                 nOpen, "{", nClose, "}")

为什么调用 String.Format 方法会引发 FormatException？

异常的最常见原因是格式项的索引与格式列表中的对象不对应。通常，这表示你错误地对格式项的索引进行了编号，或者忘记了在格式列表中包括对象。尝试包含未转义的左大括号或右大括号字符也会引发一个 FormatException。有时，异常是拼写错误的结果;例如，典型的错误是错误键入“[”（左括号），而不是“{”（左大括号）。

如果 Format（System.IFormatProvider，System.String，System.Object[]）方法支持参数数组，为什么在使用数组时我的代码会引发异常？

例如，以下代码引发异常 FormatException ：

Random rnd = new Random();
int[] numbers = new int[4];
int total = 0;
for (int ctr = 0; ctr <= 2; ctr++)
{
    int number = rnd.Next(1001);
    numbers[ctr] = number;
    total += number;
}
numbers[3] = total;
Console.WriteLine("{0} + {1} + {2} = {3}", numbers);

open System

let rnd = Random()
let mutable total = 0
let numbers = Array.zeroCreate<int> 4
for i = 0 to 2 do
   let number = rnd.Next 1001
   numbers[i] <- number
   total <- total + number
numbers[3] <- total
Console.WriteLine("{0} + {1} + {2} = {3}", numbers)

Imports System.Collections.Generic

Module Example5
    Public Sub Main()
        Dim rnd As New Random()
        Dim numbers(3) As Integer
        Dim total As Integer = 0
        For ctr = 0 To 2
            Dim number As Integer = rnd.Next(1001)
            numbers(ctr) = number
            total += number
        Next
        numbers(3) = total
        Console.WriteLine("{0} + {1} + {2} = {3}", numbers)
    End Sub
End Module

这是编译器重载解析的问题。由于编译器无法将整数数组转换为对象数组，因此它将整数数组视为单个参数，因此它会调用 Format(String, Object) 该方法。引发异常的原因是有四个格式项，但格式列表中只有一个项。

由于 Visual Basic 和 C# 都不能将整数数组转换为对象数组，因此必须在调用 Format(String, Object[]) 该方法之前自行执行转换。以下示例提供了一个实现。

Random rnd = new Random();
int[] numbers = new int[4];
int total = 0;
for (int ctr = 0; ctr <= 2; ctr++)
{
    int number = rnd.Next(1001);
    numbers[ctr] = number;
    total += number;
}
numbers[3] = total;
object[] values = new object[numbers.Length];
numbers.CopyTo(values, 0);
Console.WriteLine("{0} + {1} + {2} = {3}", values);

open System

let rnd = Random()
let numbers = Array.zeroCreate<int> 4
let mutable total = 0
for i = 0 to 2 do
   let number = rnd.Next 1001
   numbers[i] <- number
   total <- total + number
numbers[3] <- total
let values = Array.zeroCreate<obj> numbers.Length
numbers.CopyTo(values, 0)
Console.WriteLine("{0} + {1} + {2} = {3}", values)

Imports System.Collections.Generic

Module Example6
    Public Sub Main()
        Dim rnd As New Random()
        Dim numbers(3) As Integer
        Dim total As Integer = 0
        For ctr = 0 To 2
            Dim number As Integer = rnd.Next(1001)
            numbers(ctr) = number
            total += number
        Next
        numbers(3) = total
        Dim values(numbers.Length - 1) As Object
        numbers.CopyTo(values, 0)
        Console.WriteLine("{0} + {1} + {2} = {3}", values)
    End Sub
End Module

通过

System.String.Format 方法

示例

创建格式字符串

控制格式化输出

使格式字符串区分区域性

自定义格式设置操作

String.Format 方法入门

插入字符串

控件格式

控制间距

控件对齐方式

我调用哪种方法？

简短的 Format 方法

格式项

如何设置参数的格式

设置具有相同索引的项的格式

格式和区域性

自定义格式设置操作

示例：自定义格式设置操作

示例：拦截提供程序和罗马数字格式化程序

常见问题解答

为什么建议对方法的调用 String.Format 进行字符串内插？

在哪里可以找到预定义的格式字符串？

如何实现控制替换格式项的结果字符串的对齐方式？

如何实现控制小数点分隔符后的位数？

如何实现控制整型数字的数量？

格式列表中可以包含多少项？

如何实现结果字符串中包含文本大括号（“{”和“}”） ？

为什么调用 String.Format 方法会引发 FormatException？

如果 Format（System.IFormatProvider，System.String，System.Object[]） 方法支持参数数组，为什么在使用数组时我的代码会引发异常？

其他资源

为什么建议对方法的调用 `String.Format` 进行字符串内插？

如何实现结果字符串中包含文本大括号（“{”和“}”）？

如果 Format（System.IFormatProvider，System.String，System.Object[]）方法支持参数数组，为什么在使用数组时我的代码会引发异常？