System.Text.StringBuilder 类
本文提供了此 API 参考文档的补充说明。
该 StringBuilder 类表示一个类似于字符串的对象，其值是可变字符序列。
StringBuilder 与字符串类型
尽管 StringBuilder 这 String 两者都表示字符序列，但它们的实现方式不同。 String 是不可变类型。 也就是说，似乎修改 String 对象的每个操作实际上都会创建新字符串。
例如，以下 C# 示例中对 String.Concat 方法的调用似乎更改了名为 value 的字符串变量的值。 事实上，该方法 Concat 返回一个 value 对象，该对象与传递给该方法的对象具有不同的值和地址 value 。 请注意，必须使用编译器选项编译 /unsafe 该示例。
using System;
public class Example7
{
public unsafe static void Main()
{
string value = "This is the first sentence" + ".";
fixed (char* start = value)
{
value = String.Concat(value, "This is the second sentence. ");
fixed (char* current = value)
{
Console.WriteLine(start == current);
}
}
}
}
// The example displays the following output:
// False
let mutable value = "This is the first sentence" + "."
use start = fixed value
value <- System.String.Concat(value, "This is the second sentence. ")
use current = fixed value
printfn $"{start = current}"
// The example displays the following output:
// False
对于执行大量字符串操作的例程（例如，在循环中多次修改字符串的应用），反复修改字符串可能会造成重大性能损失。 替代方法是使用 StringBuilder，这是可变字符串类。 可变性意味着，在创建类的实例后，可以通过追加、删除、替换或插入字符来修改该类。 对象 StringBuilder 维护一个缓冲区，以适应字符串的扩展。 如果可用空间，新数据将追加到缓冲区;否则，将分配新的较大缓冲区，将原始缓冲区中的数据复制到新缓冲区，然后将新数据追加到新缓冲区。
重要
StringBuilder尽管该类通常提供比String类更好的性能，但每当你想要操作字符串时，不应自动替换StringStringBuilder。 性能取决于字符串的大小、要为新字符串分配的内存量、要对其执行代码的系统以及操作类型。 应准备好测试代码，以确定是否 StringBuilder 确实提供了显著的性能改进。
请考虑在这些条件下使用该 String 类：
当代码对字符串所做的更改数较小时。 在这些情况下， StringBuilder 可能会提供微不足道或没有性能改进 String。
执行固定数量的串联操作时，尤其是字符串文本。 在这种情况下，编译器可以将串联操作合并为单个操作。
在生成字符串时必须执行广泛的搜索操作。 类 StringBuilder 缺少搜索方法，例如
IndexOf或StartsWith。 你必须将 StringBuilder 对象转换为这些操作的对象 String ，这可以抵消使用 StringBuilder的性能优势。 有关详细信息，请参阅 “在 StringBuilder 对象 部分中搜索文本”。
请考虑在这些条件下使用该 StringBuilder 类：
- 当你期望代码在设计时对字符串进行未知数量的更改（例如，使用循环连接包含用户输入的随机字符串时）。
- 当你期望代码对字符串进行大量更改时。
StringBuilder 的工作原理
该 StringBuilder.Length 属性指示对象当前包含的字符 StringBuilder 数。 如果将字符添加到 StringBuilder 对象，则其长度将增加，直到它等于属性的大小 StringBuilder.Capacity ，该属性定义对象可以包含的字符数。 如果添加的字符数导致对象的长度 StringBuilder 超过其当前容量，则会分配新的内存，将属性的值 Capacity 加倍，新字符将添加到 StringBuilder 对象中，并调整其 Length 属性。 将 StringBuilder 动态分配对象的其他内存，直到它达到属性定义的 StringBuilder.MaxCapacity 值。 达到最大容量后，无法为StringBuilder对象分配更多内存，并尝试添加字符或将其扩展为超出其最大容量引发异常ArgumentOutOfRangeExceptionOutOfMemoryException。
以下示例演示对象 StringBuilder 如何分配新内存，并在分配给对象的字符串扩展时动态增加其容量。 该代码通过调用其默认（无参数）构造函数来创建 StringBuilder 对象。 此对象的默认容量为 16 个字符，其最大容量超过 20 亿个字符。 追加字符串“这是一个句子”。会导致新的内存分配，因为字符串长度（19 个字符）超出了对象的默认容量 StringBuilder 。 对象的容量翻倍为 32 个字符，添加新字符串，对象长度现在等于 19 个字符。 然后，该代码将字符串“这是一个额外的句子”。追加到对象 11 次的值 StringBuilder 。 每当追加操作导致对象的长度 StringBuilder 超过其容量时，其现有容量将翻倍， Append 并且操作成功。
using System;
using System.Reflection;
using System.Text;
public class Example4
{
public static void Main()
{
StringBuilder sb = new StringBuilder();
ShowSBInfo(sb);
sb.Append("This is a sentence.");
ShowSBInfo(sb);
for (int ctr = 0; ctr <= 10; ctr++)
{
sb.Append("This is an additional sentence.");
ShowSBInfo(sb);
}
}
private static void ShowSBInfo(StringBuilder sb)
{
foreach (var prop in sb.GetType().GetProperties())
{
if (prop.GetIndexParameters().Length == 0)
Console.Write("{0}: {1:N0} ", prop.Name, prop.GetValue(sb));
}
Console.WriteLine();
}
}
// The example displays the following output:
// Capacity: 16 MaxCapacity: 2,147,483,647 Length: 0
// Capacity: 32 MaxCapacity: 2,147,483,647 Length: 19
// Capacity: 64 MaxCapacity: 2,147,483,647 Length: 50
// Capacity: 128 MaxCapacity: 2,147,483,647 Length: 81
// Capacity: 128 MaxCapacity: 2,147,483,647 Length: 112
// Capacity: 256 MaxCapacity: 2,147,483,647 Length: 143
// Capacity: 256 MaxCapacity: 2,147,483,647 Length: 174
// Capacity: 256 MaxCapacity: 2,147,483,647 Length: 205
// Capacity: 256 MaxCapacity: 2,147,483,647 Length: 236
// Capacity: 512 MaxCapacity: 2,147,483,647 Length: 267
// Capacity: 512 MaxCapacity: 2,147,483,647 Length: 298
// Capacity: 512 MaxCapacity: 2,147,483,647 Length: 329
// Capacity: 512 MaxCapacity: 2,147,483,647 Length: 360
open System.Text
let showSBInfo (sb: StringBuilder) =
for prop in sb.GetType().GetProperties() do
if prop.GetIndexParameters().Length = 0 then
printf $"{prop.Name}: {prop.GetValue sb:N0} "
printfn ""
let sb = StringBuilder()
showSBInfo sb
sb.Append "This is a sentence." |> ignore
showSBInfo sb
for i = 0 to 10 do
sb.Append "This is an additional sentence." |> ignore
showSBInfo sb
// The example displays the following output:
// Capacity: 16 MaxCapacity: 2,147,483,647 Length: 0
// Capacity: 32 MaxCapacity: 2,147,483,647 Length: 19
// Capacity: 64 MaxCapacity: 2,147,483,647 Length: 50
// Capacity: 128 MaxCapacity: 2,147,483,647 Length: 81
// Capacity: 128 MaxCapacity: 2,147,483,647 Length: 112
// Capacity: 256 MaxCapacity: 2,147,483,647 Length: 143
// Capacity: 256 MaxCapacity: 2,147,483,647 Length: 174
// Capacity: 256 MaxCapacity: 2,147,483,647 Length: 205
// Capacity: 256 MaxCapacity: 2,147,483,647 Length: 236
// Capacity: 512 MaxCapacity: 2,147,483,647 Length: 267
// Capacity: 512 MaxCapacity: 2,147,483,647 Length: 298
// Capacity: 512 MaxCapacity: 2,147,483,647 Length: 329
// Capacity: 512 MaxCapacity: 2,147,483,647 Length: 360
Imports System.Reflection
Imports System.Text
Module Example5
Public Sub Main()
Dim sb As New StringBuilder()
ShowSBInfo(sb)
sb.Append("This is a sentence.")
ShowSBInfo(sb)
For ctr As Integer = 0 To 10
sb.Append("This is an additional sentence.")
ShowSBInfo(sb)
Next
End Sub
Public Sub ShowSBInfo(sb As StringBuilder)
For Each prop In sb.GetType().GetProperties
If prop.GetIndexParameters().Length = 0 Then
Console.Write("{0}: {1:N0} ", prop.Name, prop.GetValue(sb))
End If
Next
Console.WriteLine()
End Sub
End Module
' The example displays the following output:
' Capacity: 16 MaxCapacity: 2,147,483,647 Length: 0
' Capacity: 32 MaxCapacity: 2,147,483,647 Length: 19
' Capacity: 64 MaxCapacity: 2,147,483,647 Length: 50
' Capacity: 128 MaxCapacity: 2,147,483,647 Length: 81
' Capacity: 128 MaxCapacity: 2,147,483,647 Length: 112
' Capacity: 256 MaxCapacity: 2,147,483,647 Length: 143
' Capacity: 256 MaxCapacity: 2,147,483,647 Length: 174
' Capacity: 256 MaxCapacity: 2,147,483,647 Length: 205
' Capacity: 256 MaxCapacity: 2,147,483,647 Length: 236
' Capacity: 512 MaxCapacity: 2,147,483,647 Length: 267
' Capacity: 512 MaxCapacity: 2,147,483,647 Length: 298
' Capacity: 512 MaxCapacity: 2,147,483,647 Length: 329
' Capacity: 512 MaxCapacity: 2,147,483,647 Length: 360
内存分配
对象的默认容量 StringBuilder 为 16 个字符，其默认最大容量为 Int32.MaxValue。 如果调用和StringBuilder(String)构造函数，StringBuilder()则使用这些默认值。
可以通过以下方式显式定义对象的初始容量 StringBuilder ：
通过调用在创建对象时包括
capacity参数的任何StringBuilder构造函数。通过将新值 StringBuilder.Capacity 显式分配给属性以展开现有 StringBuilder 对象。 请注意，如果新容量小于现有容量或大于 StringBuilder 对象的最大容量，则属性将引发异常。
通过调用 StringBuilder.EnsureCapacity 具有新容量的方法。 新容量不能大于 StringBuilder 对象的最大容量。 但是，与对属性的赋值 Capacity 不同，如果所需的新容量小于现有容量， EnsureCapacity 则不会引发异常;在这种情况下，方法调用不起作用。
如果分配给 StringBuilder 构造函数调用中对象的字符串长度超过默认容量或指定容量，则 Capacity 属性将设置为使用 value 参数指定的字符串的长度。
可以通过调用StringBuilder(Int32, Int32)构造函数显式定义对象的最大容量StringBuilder。 无法通过向属性分配新值 MaxCapacity 来更改最大容量，因为它是只读的。
如上一部分所示，每当现有容量不足时，将分配额外的内存，对象容量 StringBuilder 将增加一倍，达到属性定义的 MaxCapacity 值。
一般情况下，默认容量和最大容量足以满足大多数应用的需求。 可以考虑在以下条件下设置这些值：
如果对象的最终大小 StringBuilder 可能非常大，通常超过几兆字节。 在这种情况下，可能具有一些性能优势，即将初始 Capacity 属性设置为高值，以消除对过多内存重新分配的需求。
如果代码在内存有限的系统上运行。 在这种情况下，可能需要考虑将 MaxCapacity 属性设置为小于 Int32.MaxValue 代码是否正在处理可能导致它在内存受限的环境中执行的大型字符串。
实例化 StringBuilder 对象
通过调用其六个重载类构造函数之一来实例化对象 StringBuilder ，下表中列出了该构造函数。 其中三个构造函数实例化一个 StringBuilder 对象，该对象的值是空字符串，但设置其 Capacity 值和 MaxCapacity 值的方式不同。 其余三个构造函数定义具有 StringBuilder 特定字符串值和容量的对象。 三个构造函数中的两个使用默认最大容量 Int32.MaxValue，而第三个构造函数允许设置最大容量。
| 构造函数 | 字符串值 | 容量 | 最大容量 |
|---|---|---|---|
| StringBuilder() | String.Empty | 16 | Int32.MaxValue |
| StringBuilder(Int32) | String.Empty | 由 capacity 参数定义 |
Int32.MaxValue |
| StringBuilder(Int32, Int32) | String.Empty | 由 capacity 参数定义 |
由 maxCapacity 参数定义 |
| StringBuilder(String) | 由 value 参数定义 |
16 或 value. Length，无论哪一个更大 |
Int32.MaxValue |
| StringBuilder(String, Int32) | 由 value 参数定义 |
由 capacity 参数或 value. Length，无论哪一个更大。 |
Int32.MaxValue |
| StringBuilder(String, Int32, Int32, Int32) | 由 value 定义。 Substring（startIndex、 length） |
由 capacity 参数或 value. Length，无论哪一个更大。 |
Int32.MaxValue |
以下示例使用其中三个构造函数重载来实例化 StringBuilder 对象。
using System;
using System.Text;
public class Example8
{
public static void Main()
{
string value = "An ordinary string";
int index = value.IndexOf("An ") + 3;
int capacity = 0xFFFF;
// Instantiate a StringBuilder from a string.
StringBuilder sb1 = new StringBuilder(value);
ShowSBInfo(sb1);
// Instantiate a StringBuilder from string and define a capacity.
StringBuilder sb2 = new StringBuilder(value, capacity);
ShowSBInfo(sb2);
// Instantiate a StringBuilder from substring and define a capacity.
StringBuilder sb3 = new StringBuilder(value, index,
value.Length - index,
capacity);
ShowSBInfo(sb3);
}
public static void ShowSBInfo(StringBuilder sb)
{
Console.WriteLine("\nValue: {0}", sb.ToString());
foreach (var prop in sb.GetType().GetProperties())
{
if (prop.GetIndexParameters().Length == 0)
Console.Write("{0}: {1:N0} ", prop.Name, prop.GetValue(sb));
}
Console.WriteLine();
}
}
// The example displays the following output:
// Value: An ordinary string
// Capacity: 18 MaxCapacity: 2,147,483,647 Length: 18
//
// Value: An ordinary string
// Capacity: 65,535 MaxCapacity: 2,147,483,647 Length: 18
//
// Value: ordinary string
// Capacity: 65,535 MaxCapacity: 2,147,483,647 Length: 15
open System.Text
let showSBInfo (sb: StringBuilder) =
for prop in sb.GetType().GetProperties() do
if prop.GetIndexParameters().Length = 0 then
printf $"{prop.Name}: {prop.GetValue sb:N0} "
printfn ""
let value = "An ordinary string"
let index = value.IndexOf "An " + 3
let capacity = 0xFFFF
// Instantiate a StringBuilder from a string.
let sb1 = StringBuilder value
showSBInfo sb1
// Instantiate a StringBuilder from string and define a capacity.
let sb2 = StringBuilder(value, capacity)
showSBInfo sb2
// Instantiate a StringBuilder from substring and define a capacity.
let sb3 = StringBuilder(value, index, value.Length - index, capacity)
showSBInfo sb3
// The example displays the following output:
// Value: An ordinary string
// Capacity: 18 MaxCapacity: 2,147,483,647 Length: 18
//
// Value: An ordinary string
// Capacity: 65,535 MaxCapacity: 2,147,483,647 Length: 18
//
// Value: ordinary string
// Capacity: 65,535 MaxCapacity: 2,147,483,647 Length: 15
Imports System.Text
Module Example8
Public Sub Main()
Dim value As String = "An ordinary string"
Dim index As Integer = value.IndexOf("An ") + 3
Dim capacity As Integer = &HFFFF
' Instantiate a StringBuilder from a string.
Dim sb1 As New StringBuilder(value)
ShowSBInfo(sb1)
' Instantiate a StringBuilder from string and define a capacity.
Dim sb2 As New StringBuilder(value, capacity)
ShowSBInfo(sb2)
' Instantiate a StringBuilder from substring and define a capacity.
Dim sb3 As New StringBuilder(value, index,
value.Length - index,
capacity)
ShowSBInfo(sb3)
End Sub
Public Sub ShowSBInfo(sb As StringBuilder)
Console.WriteLine()
Console.WriteLine("Value: {0}", sb.ToString())
For Each prop In sb.GetType().GetProperties
If prop.GetIndexParameters().Length = 0 Then
Console.Write("{0}: {1:N0} ", prop.Name, prop.GetValue(sb))
End If
Next
Console.WriteLine()
End Sub
End Module
' The example displays the following output:
' Value: An ordinary string
' Capacity: 18 MaxCapacity: 2,147,483,647 Length: 18
'
' Value: An ordinary string
' Capacity: 65,535 MaxCapacity: 2,147,483,647 Length: 18
'
' Value: ordinary string
' Capacity: 65,535 MaxCapacity: 2,147,483,647 Length: 15
调用 StringBuilder 方法
大多数修改实例中的 StringBuilder 字符串的方法都返回对同一实例的引用。 这样，便可以通过两种方式调用 StringBuilder 方法：
可以进行单独的方法调用并忽略返回值，如以下示例所示。
using System; using System.Text; public class Example { public static void Main() { StringBuilder sb = new StringBuilder(); sb.Append("This is the beginning of a sentence, "); sb.Replace("the beginning of ", ""); sb.Insert(sb.ToString().IndexOf("a ") + 2, "complete "); sb.Replace(",", "."); Console.WriteLine(sb.ToString()); } } // The example displays the following output: // This is a complete sentence.open System.Text let sb = StringBuilder() sb.Append "This is the beginning of a sentence, " |> ignore sb.Replace("the beginning of ", "") |> ignore sb.Insert((string sb).IndexOf "a " + 2, "complete ") |> ignore sb.Replace(",", ".") |> ignore printfn $"{sb}" // The example displays the following output: // This is a complete sentence.Imports System.Text Module Example2 Public Sub Main() Dim sb As New StringBuilder() sb.Append("This is the beginning of a sentence, ") sb.Replace("the beginning of ", "") sb.Insert(sb.ToString().IndexOf("a ") + 2, "complete ") sb.Replace(",", ".") Console.WriteLine(sb.ToString()) End Sub End Module ' The example displays the following output: ' This is a complete sentence.可以在单个语句中进行一系列方法调用。 如果要编写链接连续操作的单个语句，这可以很方便。 以下示例将上一个示例中的三个方法调用合并为一行代码。
using System; using System.Text; public class Example2 { public static void Main() { StringBuilder sb = new StringBuilder("This is the beginning of a sentence, "); sb.Replace("the beginning of ", "").Insert(sb.ToString().IndexOf("a ") + 2, "complete ").Replace(",", "."); Console.WriteLine(sb.ToString()); } } // The example displays the following output: // This is a complete sentence.open System.Text let sb = StringBuilder "This is the beginning of a sentence, " sb .Replace("the beginning of ", "") .Insert((string sb).IndexOf "a " + 2, "complete ") .Replace(",", ".") |> ignore printfn $"{sb}" // The example displays the following output: // This is a complete sentence.Imports System.Text Module Example3 Public Sub Main() Dim sb As New StringBuilder("This is the beginning of a sentence, ") sb.Replace("the beginning of ", "").Insert(sb.ToString().IndexOf("a ") + 2, "complete ").Replace(", ", ".") Console.WriteLine(sb.ToString()) End Sub End Module ' The example displays the following output: ' This is a complete sentence.
执行 StringBuilder 操作
可以使用类的方法 StringBuilder 循环访问、添加、删除或修改对象中的 StringBuilder 字符。
循环访问 StringBuilder 字符
可以使用属性访问对象StringBuilder.Chars[]中的StringBuilder字符。 在 C# 中， Chars[] 是索引器;在 Visual Basic 中，它是类的默认属性 StringBuilder 。 这使你能够仅使用其索引来设置或检索单个字符，而无需显式引用 Chars[] 属性。 对象中的 StringBuilder 字符从索引 0 开始（零），并继续编制索引 Length - 1。
下面的示例演示了该 Chars[] 属性。 它将十个随机数追加到一个 StringBuilder 对象，然后循环访问每个字符。 如果该字符的 Unicode 类别为 UnicodeCategory.DecimalDigitNumber1，则它将数字减少 1（如果数字值为 0，则将其更改为 9）。 该示例在更改单个字符的值前后显示对象的内容 StringBuilder 。
using System;
using System.Globalization;
using System.Text;
public class Example3
{
public static void Main()
{
Random rnd = new Random();
StringBuilder sb = new StringBuilder();
// Generate 10 random numbers and store them in a StringBuilder.
for (int ctr = 0; ctr <= 9; ctr++)
sb.Append(rnd.Next().ToString("N5"));
Console.WriteLine("The original string:");
Console.WriteLine(sb.ToString());
// Decrease each number by one.
for (int ctr = 0; ctr < sb.Length; ctr++)
{
if (Char.GetUnicodeCategory(sb[ctr]) == UnicodeCategory.DecimalDigitNumber)
{
int number = (int)Char.GetNumericValue(sb[ctr]);
number--;
if (number < 0) number = 9;
sb[ctr] = number.ToString()[0];
}
}
Console.WriteLine("\nThe new string:");
Console.WriteLine(sb.ToString());
}
}
// The example displays the following output:
// The original string:
// 1,457,531,530.00000940,522,609.000001,668,113,564.000001,998,992,883.000001,792,660,834.00
// 000101,203,251.000002,051,183,075.000002,066,000,067.000001,643,701,043.000001,702,382,508
// .00000
//
// The new string:
// 0,346,420,429.99999839,411,598.999990,557,002,453.999990,887,881,772.999990,681,559,723.99
// 999090,192,140.999991,940,072,964.999991,955,999,956.999990,532,690,932.999990,691,271,497
// .99999
open System
open System.Globalization
open System.Text
let rnd = Random()
let sb = new StringBuilder()
// Generate 10 random numbers and store them in a StringBuilder.
for _ = 0 to 9 do
rnd.Next().ToString "N5" |> sb.Append |> ignore
printfn "The original string:"
printfn $"{sb}"
// Decrease each number by one.
for i = 0 to sb.Length - 1 do
if Char.GetUnicodeCategory(sb[i]) = UnicodeCategory.DecimalDigitNumber then
let number = Char.GetNumericValue sb.[i] |> int
let number = number - 1
let number = if number < 0 then 9 else number
sb.[i] <- number.ToString()[0]
printfn "\nThe new string:"
printfn $"{sb}"
// The example displays the following output:
// The original string:
// 1,457,531,530.00000940,522,609.000001,668,113,564.000001,998,992,883.000001,792,660,834.00
// 000101,203,251.000002,051,183,075.000002,066,000,067.000001,643,701,043.000001,702,382,508
// .00000
//
// The new string:
// 0,346,420,429.99999839,411,598.999990,557,002,453.999990,887,881,772.999990,681,559,723.99
// 999090,192,140.999991,940,072,964.999991,955,999,956.999990,532,690,932.999990,691,271,497
// .99999
Imports System.Globalization
Imports System.Text
Module Example4
Public Sub Main()
Dim rnd As New Random()
Dim sb As New StringBuilder()
' Generate 10 random numbers and store them in a StringBuilder.
For ctr As Integer = 0 To 9
sb.Append(rnd.Next().ToString("N5"))
Next
Console.WriteLine("The original string:")
Console.WriteLine(sb.ToString())
Console.WriteLine()
' Decrease each number by one.
For ctr As Integer = 0 To sb.Length - 1
If Char.GetUnicodeCategory(sb(ctr)) = UnicodeCategory.DecimalDigitNumber Then
Dim number As Integer = CType(Char.GetNumericValue(sb(ctr)), Integer)
number -= 1
If number < 0 Then number = 9
sb(ctr) = number.ToString()(0)
End If
Next
Console.WriteLine("The new string:")
Console.WriteLine(sb.ToString())
End Sub
End Module
' The example displays the following output:
' The original string:
' 1,457,531,530.00000940,522,609.000001,668,113,564.000001,998,992,883.000001,792,660,834.00
' 000101,203,251.000002,051,183,075.000002,066,000,067.000001,643,701,043.000001,702,382,508
' .00000
'
' The new string:
' 0,346,420,429.99999839,411,598.999990,557,002,453.999990,887,881,772.999990,681,559,723.99
' 999090,192,140.999991,940,072,964.999991,955,999,956.999990,532,690,932.999990,691,271,497
' .99999
在以下情况下，通过 Chars[] 属性使用基于字符的索引可能会非常缓慢：
- StringBuilder 实例很大（例如，它由数以万计的字符组成）。
- StringBuilder是“区块”。也就是说，对方法的重复调用（例如StringBuilder.Append，已自动扩展对象的StringBuilder.Capacity属性并为其分配新的内存区块）。
因为每次字符访问都会遍历区块的整个链接列表以查找要索引到的正确缓冲区，所以性能受到严重影响。
注意
即使对于大型“区块” StringBuilder 对象，使用 Chars[] 属性对一个或多个字符进行基于索引的访问也会产生微不足道的性能影响;通常，它是 O（n） 操作。 当迭代 StringBuilder 对象中的字符时，会对性能造成显著的影响，这是 O(n^2) 操作。
如果通过 StringBuilder 对象在使用基于字符的索引时遇到性能问题，可以使用以下任一解决方法：
通过调用 ToString 方法，将 StringBuilder 实例转换为 String，然后访问字符串中的字符。
将现有的 StringBuilder 对象的内容复制到新的预调整大小的 StringBuilder 对象。 由于新的 StringBuilder 对象并不笨重，因此可以提升性能。 例如：
// sbOriginal is the existing StringBuilder object var sbNew = new StringBuilder(sbOriginal.ToString(), sbOriginal.Length);' sbOriginal is the existing StringBuilder object Dim sbNew = New StringBuilder(sbOriginal.ToString(), sbOriginal.Length)通过调用 StringBuilder(Int32) 构造函数，将 StringBuilder 对象的初始容量设置为约等于其最大预期大小的值。 请注意，即使 StringBuilder 很少达到其最大容量，它仍会分配整个内存块。
向 StringBuilder 对象添加文本
该 StringBuilder 类包括用于扩展对象内容的 StringBuilder 以下方法：
该方法 Append 将字符串、子字符串、字符数组、字符数组的一部分、重复多次的单个字符或基元数据类型的字符串表示形式追加到 StringBuilder 对象。
该方法 AppendLine 将行终止符或字符串与行终止符一起追加到 StringBuilder 对象。
该方法 AppendFormat 将 复合格式字符串 追加到 StringBuilder 对象。 结果字符串中包含的对象的字符串表示形式可以反映当前系统区域性或指定区域性的格式约定。
该方法 Insert 在对象中的 StringBuilder 指定位置插入字符串、子字符串、字符串的多个重复、字符数组、字符数组的一部分或基元数据类型的字符串表示形式。 位置由从零开始的索引定义。
以下示例使用
using System;
using System.Text;
public class Example6
{
public static void Main()
{
// Create a StringBuilder object with no text.
StringBuilder sb = new StringBuilder();
// Append some text.
sb.Append('*', 10).Append(" Adding Text to a StringBuilder Object ").Append('*', 10);
sb.AppendLine("\n");
sb.AppendLine("Some code points and their corresponding characters:");
// Append some formatted text.
for (int ctr = 50; ctr <= 60; ctr++)
{
sb.AppendFormat("{0,12:X4} {1,12}", ctr, Convert.ToChar(ctr));
sb.AppendLine();
}
// Find the end of the introduction to the column.
int pos = sb.ToString().IndexOf("characters:") + 11 +
Environment.NewLine.Length;
// Insert a column header.
sb.Insert(pos, String.Format("{2}{0,12:X4} {1,12}{2}", "Code Unit",
"Character", "\n"));
// Convert the StringBuilder to a string and display it.
Console.WriteLine(sb.ToString());
}
}
// The example displays the following output:
// ********** Adding Text to a StringBuilder Object **********
//
// Some code points and their corresponding characters:
//
// Code Unit Character
// 0032 2
// 0033 3
// 0034 4
// 0035 5
// 0036 6
// 0037 7
// 0038 8
// 0039 9
// 003A :
// 003B ;
// 003C <
open System
open System.Text
// Create a StringBuilder object with no text.
let sb = StringBuilder()
// Append some text.
sb
.Append('*', 10)
.Append(" Adding Text to a StringBuilder Object ")
.Append('*', 10)
|> ignore
sb.AppendLine "\n" |> ignore
sb.AppendLine "Some code points and their corresponding characters:" |> ignore
// Append some formatted text.
for i = 50 to 60 do
sb.AppendFormat("{0,12:X4} {1,12}", i, Convert.ToChar i) |> ignore
sb.AppendLine() |> ignore
// Find the end of the introduction to the column.
let pos = (string sb).IndexOf("characters:") + 11 + Environment.NewLine.Length
// Insert a column header.
sb.Insert(pos, String.Format("{2}{0,12:X4} {1,12}{2}", "Code Unit", "Character", "\n"))
|> ignore
// Convert the StringBuilder to a string and display it.
printfn $"{sb}"
// The example displays the following output:
// ********** Adding Text to a StringBuilder Object **********
//
// Some code points and their corresponding characters:
//
// Code Unit Character
// 0032 2
// 0033 3
// 0034 4
// 0035 5
// 0036 6
// 0037 7
// 0038 8
// 0039 9
// 003A :
// 003B ;
// 003C <
Imports System.Text
Module Example7
Public Sub Main()
' Create a StringBuilder object with no text.
Dim sb As New StringBuilder()
' Append some text.
sb.Append("*"c, 10).Append(" Adding Text to a StringBuilder Object ").Append("*"c, 10)
sb.AppendLine()
sb.AppendLine()
sb.AppendLine("Some code points and their corresponding characters:")
' Append some formatted text.
For ctr = 50 To 60
sb.AppendFormat("{0,12:X4} {1,12}", ctr, Convert.ToChar(ctr))
sb.AppendLine()
Next
' Find the end of the introduction to the column.
Dim pos As Integer = sb.ToString().IndexOf("characters:") + 11 +
Environment.NewLine.Length
' Insert a column header.
sb.Insert(pos, String.Format("{2}{0,12:X4} {1,12}{2}", "Code Unit",
"Character", vbCrLf))
' Convert the StringBuilder to a string and display it.
Console.WriteLine(sb.ToString())
End Sub
End Module
' The example displays the following output:
' ********** Adding Text to a StringBuilder Object **********
'
' Some code points and their corresponding characters:
'
' Code Unit Character
' 0032 2
' 0033 3
' 0034 4
' 0035 5
' 0036 6
' 0037 7
' 0038 8
' 0039 9
' 003A :
' 003B ;
' 003C <
从 StringBuilder 对象中删除文本
StringBuilder类包括可以减少的当前大小的方法StringBuilder实例。 Clear方法中删除所有字符，并设置Length属性设置为零。 Remove方法中删除指定的数量的特定索引位置开始的字符。 此外，从末尾删除字符StringBuilder对象通过设置其Length属性的值小于当前实例的长度。
以下示例从对象中删除某些文本 StringBuilder ，显示其生成的容量、最大容量和长度属性值，然后调用 Clear 该方法以删除对象中的所有字符 StringBuilder 。
using System;
using System.Text;
public class Example5
{
public static void Main()
{
StringBuilder sb = new StringBuilder("A StringBuilder object");
ShowSBInfo(sb);
// Remove "object" from the text.
string textToRemove = "object";
int pos = sb.ToString().IndexOf(textToRemove);
if (pos >= 0)
{
sb.Remove(pos, textToRemove.Length);
ShowSBInfo(sb);
}
// Clear the StringBuilder contents.
sb.Clear();
ShowSBInfo(sb);
}
public static void ShowSBInfo(StringBuilder sb)
{
Console.WriteLine("\nValue: {0}", sb.ToString());
foreach (var prop in sb.GetType().GetProperties())
{
if (prop.GetIndexParameters().Length == 0)
Console.Write("{0}: {1:N0} ", prop.Name, prop.GetValue(sb));
}
Console.WriteLine();
}
}
// The example displays the following output:
// Value: A StringBuilder object
// Capacity: 22 MaxCapacity: 2,147,483,647 Length: 22
//
// Value: A StringBuilder
// Capacity: 22 MaxCapacity: 2,147,483,647 Length: 16
//
// Value:
// Capacity: 22 MaxCapacity: 2,147,483,647 Length: 0
open System.Text
let showSBInfo (sb: StringBuilder) =
for prop in sb.GetType().GetProperties() do
if prop.GetIndexParameters().Length = 0 then
printf $"{prop.Name}: {prop.GetValue sb:N0} "
printfn ""
let sb = StringBuilder "A StringBuilder object"
showSBInfo sb
// Remove "object" from the text.
let textToRemove = "object"
let pos = (string sb).IndexOf textToRemove
if pos >= 0 then
sb.Remove(pos, textToRemove.Length) |> ignore
showSBInfo sb
// Clear the StringBuilder contents.
sb.Clear() |> ignore
showSBInfo sb
// The example displays the following output:
// Value: A StringBuilder object
// Capacity: 22 MaxCapacity: 2,147,483,647 Length: 22
//
// Value: A StringBuilder
// Capacity: 22 MaxCapacity: 2,147,483,647 Length: 16
//
// Value:
// Capacity: 22 MaxCapacity: 2,147,483,647 Length: 0
Imports System.Text
Module Example6
Public Sub Main()
Dim sb As New StringBuilder("A StringBuilder object")
ShowSBInfo(sb)
' Remove "object" from the text.
Dim textToRemove As String = "object"
Dim pos As Integer = sb.ToString().IndexOf(textToRemove)
If pos >= 0 Then
sb.Remove(pos, textToRemove.Length)
ShowSBInfo(sb)
End If
' Clear the StringBuilder contents.
sb.Clear()
ShowSBInfo(sb)
End Sub
Public Sub ShowSBInfo(sb As StringBuilder)
Console.WriteLine()
Console.WriteLine("Value: {0}", sb.ToString())
For Each prop In sb.GetType().GetProperties
If prop.GetIndexParameters().Length = 0 Then
Console.Write("{0}: {1:N0} ", prop.Name, prop.GetValue(sb))
End If
Next
Console.WriteLine()
End Sub
End Module
' The example displays the following output:
' Value: A StringBuilder object
' Capacity: 22 MaxCapacity: 2,147,483,647 Length: 22
'
' Value: A StringBuilder
' Capacity: 22 MaxCapacity: 2,147,483,647 Length: 16
'
' Value:
' Capacity: 22 MaxCapacity: 2,147,483,647 Length: 0
修改 StringBuilder 对象中的文本
该方法 StringBuilder.Replace 替换整个 StringBuilder 对象或特定字符范围中字符或字符串的所有匹配项。 下面的示例使用 Replace 该方法将对象中的所有 StringBuilder 感叹号（！）替换为问号（？）。
using System;
using System.Text;
public class Example13
{
public static void Main()
{
StringBuilder MyStringBuilder = new StringBuilder("Hello World!");
MyStringBuilder.Replace('!', '?');
Console.WriteLine(MyStringBuilder);
}
}
// The example displays the following output:
// Hello World?
open System.Text
let myStringBuilder = StringBuilder "Hello World!"
myStringBuilder.Replace('!', '?') |> ignore
printfn $"{myStringBuilder}"
// The example displays the following output:
// Hello World?
Imports System.Text
Module Example
Public Sub Main()
Dim MyStringBuilder As New StringBuilder("Hello World!")
MyStringBuilder.Replace("!"c, "?"c)
Console.WriteLine(MyStringBuilder)
End Sub
End Module
' The example displays the following output:
' Hello World?
在 StringBuilder 对象中搜索文本
该StringBuilder类不包括与类提供的String类似String.ContainsString.IndexOf和String.StartsWith方法的方法，这允许你搜索对象中的特定字符或子字符串。 确定子字符串的存在或起始字符位置要求使用字符串搜索方法或正则表达式方法搜索 String 值。 可通过四种方法实现此类搜索，如下表所示。
| 方法 | 优点 | 缺点 |
|---|---|---|
| 在将字符串值添加到 StringBuilder 对象之前搜索这些值。 | 可用于确定子字符串是否存在。 | 当子字符串的索引位置很重要时，不能使用。 |
| 调用 ToString 并搜索返回 String 的对象。 | 如果将所有文本分配给对象 StringBuilder ，然后开始修改它，则易于使用。 | 如果在将所有文本添加到StringBuilder对象之前必须进行修改，则重复调用ToString这一点很麻烦。 如果要进行更改，则必须记住要从对象文本的 StringBuilder 末尾开始工作。 |
| 使用 Chars[] 属性按顺序搜索一系列字符。 | 如果担心单个字符或小型子字符串，则很有用。 | 如果要搜索的字符数很大，或者搜索逻辑复杂，则比较繁琐。 对于通过重复的方法调用增长非常大的对象，性能非常差。 |
| 将 StringBuilder 对象转换为 String 对象，并对该 String 对象执行修改。 | 如果修改次数较小，则很有用。 | 如果修改次数很大，则否定类的性能优势 StringBuilder 。 |
让我们更详细地研究这些技术。
如果搜索的目标是确定特定子字符串是否存在（也就是说，如果对子字符串的位置不感兴趣），则可以在对象中 StringBuilder 存储字符串之前搜索字符串。 以下示例提供了一个可能的实现。 它定义一个
StringBuilderFinder类，该类的构造函数将传递对 StringBuilder 对象的引用和要在字符串中找到的子字符串。 在这种情况下，该示例尝试确定记录的温度是华氏温度还是摄氏度，并将适当的介绍性文本添加到对象的开头 StringBuilder 。 随机数生成器用于选择一个数组，该数组包含以摄氏度或华氏度为单位的数据。using System; using System.Text; public class Example9 { public static void Main() { Random rnd = new Random(); string[] tempF = { "47.6F", "51.3F", "49.5F", "62.3F" }; string[] tempC = { "21.2C", "16.1C", "23.5C", "22.9C" }; string[][] temps = { tempF, tempC }; StringBuilder sb = new StringBuilder(); var f = new StringBuilderFinder(sb, "F"); var baseDate = new DateTime(2013, 5, 1); String[] temperatures = temps[rnd.Next(2)]; bool isFahrenheit = false; foreach (var temperature in temperatures) { if (isFahrenheit) sb.AppendFormat("{0:d}: {1}\n", baseDate, temperature); else isFahrenheit = f.SearchAndAppend(String.Format("{0:d}: {1}\n", baseDate, temperature)); baseDate = baseDate.AddDays(1); } if (isFahrenheit) { sb.Insert(0, "Average Daily Temperature in Degrees Fahrenheit"); sb.Insert(47, "\n\n"); } else { sb.Insert(0, "Average Daily Temperature in Degrees Celsius"); sb.Insert(44, "\n\n"); } Console.WriteLine(sb.ToString()); } } public class StringBuilderFinder { private StringBuilder sb; private String text; public StringBuilderFinder(StringBuilder sb, String textToFind) { this.sb = sb; this.text = textToFind; } public bool SearchAndAppend(String stringToSearch) { sb.Append(stringToSearch); return stringToSearch.Contains(text); } } // The example displays output similar to the following: // Average Daily Temperature in Degrees Celsius // // 5/1/2013: 21.2C // 5/2/2013: 16.1C // 5/3/2013: 23.5C // 5/4/2013: 22.9Copen System open System.Text type StringBuilderFinder(sb: StringBuilder, textToFind: string) = member _.SearchAndAppend(stringToSearch: string) = sb.Append stringToSearch |> ignore stringToSearch.Contains textToFind let tempF = [| "47.6F"; "51.3F"; "49.5F"; "62.3F" |] let tempC = [| "21.2C"; "16.1C"; "23.5C"; "22.9C" |] let temps = [| tempF; tempC |] let sb = StringBuilder() let f = StringBuilderFinder(sb, "F") let temperatures = temps[Random.Shared.Next(2)] let mutable baseDate = DateTime(2013, 5, 1) let mutable isFahrenheit = false for temperature in temperatures do if isFahrenheit then sb.AppendFormat("{0:d}: {1}\n", baseDate, temperature) |> ignore else isFahrenheit <- $"{baseDate:d}: {temperature}\n" |> f.SearchAndAppend baseDate <- baseDate.AddDays 1 if isFahrenheit then sb.Insert(0, "Average Daily Temperature in Degrees Fahrenheit") |> ignore sb.Insert(47, "\n\n") |> ignore else sb.Insert(0, "Average Daily Temperature in Degrees Celsius") |> ignore sb.Insert(44, "\n\n") |> ignore printfn $"{sb}" // The example displays output similar to the following: // Average Daily Temperature in Degrees Celsius // // 5/1/2013: 21.2C // 5/2/2013: 16.1C // 5/3/2013: 23.5C // 5/4/2013: 22.9CImports System.Text Module Example9 Public Sub Main() Dim rnd As New Random() Dim tempF() As String = {"47.6F", "51.3F", "49.5F", "62.3F"} Dim tempC() As String = {"21.2C", "16.1C", "23.5C", "22.9C"} Dim temps()() As String = {tempF, tempC} Dim sb As StringBuilder = New StringBuilder() Dim f As New StringBuilderFinder(sb, "F") Dim baseDate As New DateTime(2013, 5, 1) Dim temperatures() As String = temps(rnd.Next(2)) Dim isFahrenheit As Boolean = False For Each temperature In temperatures If isFahrenheit Then sb.AppendFormat("{0:d}: {1}{2}", baseDate, temperature, vbCrLf) Else isFahrenheit = f.SearchAndAppend(String.Format("{0:d}: {1}{2}", baseDate, temperature, vbCrLf)) End If baseDate = baseDate.AddDays(1) Next If isFahrenheit Then sb.Insert(0, "Average Daily Temperature in Degrees Fahrenheit") sb.Insert(47, vbCrLf + vbCrLf) Else sb.Insert(0, "Average Daily Temperature in Degrees Celsius") sb.Insert(44, vbCrLf + vbCrLf) End If Console.WriteLine(sb.ToString()) End Sub End Module Public Class StringBuilderFinder Private sb As StringBuilder Private text As String Public Sub New(sb As StringBuilder, textToFind As String) Me.sb = sb text = textToFind End Sub Public Function SearchAndAppend(stringToSearch As String) As Boolean sb.Append(stringToSearch) Return stringToSearch.Contains(text) End Function End Class ' The example displays output similar to the following: ' Average Daily Temperature in Degrees Celsius ' ' 5/1/2013: 21.2C ' 5/2/2013: 16.1C ' 5/3/2013: 23.5C ' 5/4/2013: 22.9CStringBuilder.ToString调用方法将对象转换为StringBuilder对象String。 可以使用诸如或String.StartsWith等String.LastIndexOf方法搜索字符串，也可以使用正则表达式和Regex类来搜索模式。 由于两个对象StringBuilderString都使用 UTF-16 编码来存储字符，因此两个对象中的字符、子字符串和正则表达式匹配项的索引位置相同。 这样，便可以使用 StringBuilder 方法在对象中找到 String 该文本的同一位置进行更改。
注意
如果采用此方法，则应从对象的末尾 StringBuilder 到它的开头工作，这样就不必重复将该 StringBuilder 对象转换为字符串。
下面的示例阐释了这种方法。 它存储对象中英文字母的四个 StringBuilder 匹配项。 然后，它将文本 String 转换为对象，并使用正则表达式标识每个四个字符序列的起始位置。 最后，它在除第一个序列以外的每个四个字符序列之前添加一个下划线，并将序列的第一个字符转换为大写。
using System; using System.Text; using System.Text.RegularExpressions; public class Example10 { public static void Main() { // Create a StringBuilder object with 4 successive occurrences // of each character in the English alphabet. StringBuilder sb = new StringBuilder(); for (ushort ctr = (ushort)'a'; ctr <= (ushort)'z'; ctr++) sb.Append(Convert.ToChar(ctr), 4); // Create a parallel string object. String sbString = sb.ToString(); // Determine where each new character sequence begins. String pattern = @"(\w)\1+"; MatchCollection matches = Regex.Matches(sbString, pattern); // Uppercase the first occurrence of the sequence, and separate it // from the previous sequence by an underscore character. for (int ctr = matches.Count - 1; ctr >= 0; ctr--) { Match m = matches[ctr]; sb[m.Index] = Char.ToUpper(sb[m.Index]); if (m.Index > 0) sb.Insert(m.Index, "_"); } // Display the resulting string. sbString = sb.ToString(); int line = 0; do { int nChars = line * 80 + 79 <= sbString.Length ? 80 : sbString.Length - line * 80; Console.WriteLine(sbString.Substring(line * 80, nChars)); line++; } while (line * 80 < sbString.Length); } } // The example displays the following output: // Aaaa_Bbbb_Cccc_Dddd_Eeee_Ffff_Gggg_Hhhh_Iiii_Jjjj_Kkkk_Llll_Mmmm_Nnnn_Oooo_Pppp_ // Qqqq_Rrrr_Ssss_Tttt_Uuuu_Vvvv_Wwww_Xxxx_Yyyy_Zzzzopen System open System.Text open System.Text.RegularExpressions // Create a StringBuilder object with 4 successive occurrences // of each character in the English alphabet. let sb = StringBuilder() for char in 'a' .. 'z' do sb.Append(char, 4) |> ignore // Create a parallel string object. let sbString = string sb // Determine where each new character sequence begins. let pattern = @"(\w)\1+" let matches = Regex.Matches(sbString, pattern) // Uppercase the first occurrence of the sequence, and separate it // from the previous sequence by an underscore character. for i = matches.Count - 1 downto 0 do let m = matches[i] sb[m.Index] <- Char.ToUpper sb[m.Index] if m.Index > 0 then sb.Insert(m.Index, "_") |> ignore // Display the resulting string. let sbString2 = string sb for line = 0 to (sbString2.Length - 1) / 80 do let nChars = if line * 80 + 79 <= sbString2.Length then 80 else sbString2.Length - line * 80 printfn $"{sbString2.Substring(line * 80, nChars)}" // The example displays the following output: // Aaaa_Bbbb_Cccc_Dddd_Eeee_Ffff_Gggg_Hhhh_Iiii_Jjjj_Kkkk_Llll_Mmmm_Nnnn_Oooo_Pppp_ // Qqqq_Rrrr_Ssss_Tttt_Uuuu_Vvvv_Wwww_Xxxx_Yyyy_ZzzzImports System.Text Imports System.Text.RegularExpressions Module Example10 Public Sub Main() ' Create a StringBuilder object with 4 successive occurrences ' of each character in the English alphabet. Dim sb As New StringBuilder() For ctr As UShort = AscW("a") To AscW("z") sb.Append(ChrW(ctr), 4) Next ' Create a parallel string object. Dim sbString As String = sb.ToString() ' Determine where each new character sequence begins. Dim pattern As String = "(\w)\1+" Dim matches As MatchCollection = Regex.Matches(sbString, pattern) ' Uppercase the first occurrence of the sequence, and separate it ' from the previous sequence by an underscore character. For ctr As Integer = matches.Count - 1 To 0 Step -1 Dim m As Match = matches(ctr) sb.Chars(m.Index) = Char.ToUpper(sb.Chars(m.Index)) If m.Index > 0 Then sb.Insert(m.Index, "_") Next ' Display the resulting string. sbString = sb.ToString() Dim line As Integer = 0 Do Dim nChars As Integer = If(line * 80 + 79 <= sbString.Length, 80, sbString.Length - line * 80) Console.WriteLine(sbString.Substring(line * 80, nChars)) line += 1 Loop While line * 80 < sbString.Length End Sub End Module ' The example displays the following output: ' Aaaa_Bbbb_Cccc_Dddd_Eeee_Ffff_Gggg_Hhhh_Iiii_Jjjj_Kkkk_Llll_Mmmm_Nnnn_Oooo_Pppp_ ' Qqqq_Rrrr_Ssss_Tttt_Uuuu_Vvvv_Wwww_Xxxx_Yyyy_Zzzz使用 StringBuilder.Chars[] 属性按顺序搜索对象中的 StringBuilder 字符范围。 如果要搜索的字符数很大或搜索逻辑特别复杂，则此方法可能不实用。 有关针对非常大的区块 StringBuilder 对象基于字符索引的访问的性能影响，请参阅该 StringBuilder.Chars[] 属性的文档。
以下示例的功能与上一个示例相同，但在实现中有所不同。 它使用 Chars[] 属性检测字符值何时发生更改、在该位置插入下划线，并将新序列中的第一个字符转换为大写。
using System; using System.Text; public class Example11 { public static void Main() { // Create a StringBuilder object with 4 successive occurrences // of each character in the English alphabet. StringBuilder sb = new StringBuilder(); for (ushort ctr = (ushort)'a'; ctr <= (ushort)'z'; ctr++) sb.Append(Convert.ToChar(ctr), 4); // Iterate the text to determine when a new character sequence occurs. int position = 0; Char current = '\u0000'; do { if (sb[position] != current) { current = sb[position]; sb[position] = Char.ToUpper(sb[position]); if (position > 0) sb.Insert(position, "_"); position += 2; } else { position++; } } while (position <= sb.Length - 1); // Display the resulting string. String sbString = sb.ToString(); int line = 0; do { int nChars = line * 80 + 79 <= sbString.Length ? 80 : sbString.Length - line * 80; Console.WriteLine(sbString.Substring(line * 80, nChars)); line++; } while (line * 80 < sbString.Length); } } // The example displays the following output: // Aaaa_Bbbb_Cccc_Dddd_Eeee_Ffff_Gggg_Hhhh_Iiii_Jjjj_Kkkk_Llll_Mmmm_Nnnn_Oooo_Pppp_ // Qqqq_Rrrr_Ssss_Tttt_Uuuu_Vvvv_Wwww_Xxxx_Yyyy_Zzzzopen System open System.Text // Create a StringBuilder object with 4 successive occurrences // of each character in the English alphabet. let sb = StringBuilder() for char in 'a' .. 'z' do sb.Append(char, 4) |> ignore // Iterate the text to determine when a new character sequence occurs. let mutable position = 0 let mutable current = '\u0000' while position <= sb.Length - 1 do if sb[position] <> current then current <- sb[position] sb[position] <- Char.ToUpper sb[position] if position > 0 then sb.Insert(position, "_") |> ignore position <- position + 2 else position <- position + 1 // Display the resulting string. let sbString = string sb for line = 0 to (sbString.Length - 1) / 80 do let nChars = if line * 80 + 79 <= sbString.Length then 80 else sbString.Length - line * 80 printfn $"{sbString.Substring(line * 80, nChars)}" // The example displays the following output: // Aaaa_Bbbb_Cccc_Dddd_Eeee_Ffff_Gggg_Hhhh_Iiii_Jjjj_Kkkk_Llll_Mmmm_Nnnn_Oooo_Pppp_ // Qqqq_Rrrr_Ssss_Tttt_Uuuu_Vvvv_Wwww_Xxxx_Yyyy_ZzzzImports System.Text Module Example11 Public Sub Main() ' Create a StringBuilder object with 4 successive occurrences ' of each character in the English alphabet. Dim sb As New StringBuilder() For ctr As UShort = AscW("a") To AscW("z") sb.Append(ChrW(ctr), 4) Next ' Iterate the text to determine when a new character sequence occurs. Dim position As Integer = 0 Dim current As Char = ChrW(0) Do If sb(position) <> current Then current = sb(position) sb(position) = Char.ToUpper(sb(position)) If position > 0 Then sb.Insert(position, "_") position += 2 Else position += 1 End If Loop While position <= sb.Length - 1 ' Display the resulting string. Dim sbString As String = sb.ToString() Dim line As Integer = 0 Do Dim nChars As Integer = If(line * 80 + 79 <= sbString.Length, 80, sbString.Length - line * 80) Console.WriteLine(sbString.Substring(line * 80, nChars)) line += 1 Loop While line * 80 < sbString.Length End Sub End Module ' The example displays the following output: ' Aaaa_Bbbb_Cccc_Dddd_Eeee_Ffff_Gggg_Hhhh_Iiii_Jjjj_Kkkk_Llll_Mmmm_Nnnn_Oooo_Pppp_ ' Qqqq_Rrrr_Ssss_Tttt_Uuuu_Vvvv_Wwww_Xxxx_Yyyy_Zzzz将所有未修改的文本存储在对象中StringBuilder，调用StringBuilder.ToString方法将对象String转换为StringBuilder对象，并对该String对象执行修改。 如果只有一些修改，则可以使用此方法;否则，使用不可变字符串的成本可能会否定使用 StringBuilder 对象的性能优势。
以下示例的功能与前面的两个示例相同，但在实现方面有所不同。 它创建一个 StringBuilder 对象，将其转换为对象 String ，然后使用正则表达式对字符串执行所有剩余的修改。 该方法 Regex.Replace(String, String, MatchEvaluator) 使用 lambda 表达式对每个匹配项执行替换。
using System; using System.Text; using System.Text.RegularExpressions; public class Example12 { public static void Main() { // Create a StringBuilder object with 4 successive occurrences // of each character in the English alphabet. StringBuilder sb = new StringBuilder(); for (ushort ctr = (ushort)'a'; ctr <= (ushort)'z'; ctr++) sb.Append(Convert.ToChar(ctr), 4); // Convert it to a string. String sbString = sb.ToString(); // Use a regex to uppercase the first occurrence of the sequence, // and separate it from the previous sequence by an underscore. string pattern = @"(\w)(\1+)"; sbString = Regex.Replace(sbString, pattern, m => (m.Index > 0 ? "_" : "") + m.Groups[1].Value.ToUpper() + m.Groups[2].Value); // Display the resulting string. int line = 0; do { int nChars = line * 80 + 79 <= sbString.Length ? 80 : sbString.Length - line * 80; Console.WriteLine(sbString.Substring(line * 80, nChars)); line++; } while (line * 80 < sbString.Length); } } // The example displays the following output: // Aaaa_Bbbb_Cccc_Dddd_Eeee_Ffff_Gggg_Hhhh_Iiii_Jjjj_Kkkk_Llll_Mmmm_Nnnn_Oooo_Pppp_ // Qqqq_Rrrr_Ssss_Tttt_Uuuu_Vvvv_Wwww_Xxxx_Yyyy_Zzzzopen System.Text open System.Text.RegularExpressions // Create a StringBuilder object with 4 successive occurrences // of each character in the English alphabet. let sb = StringBuilder() for char in 'a' .. 'z' do sb.Append(char, 4) |> ignore // Convert it to a string. let sbString = string sb // Use a regex to uppercase the first occurrence of the sequence, // and separate it from the previous sequence by an underscore. let pattern = @"(\w)(\1+)" let sbStringReplaced = Regex.Replace( sbString, pattern, fun m -> (if m.Index > 0 then "_" else "") + m.Groups[ 1 ].Value.ToUpper() + m.Groups[2].Value ) // Display the resulting string. for line = 0 to (sbStringReplaced.Length - 1) / 80 do let nChars = if line * 80 + 79 <= sbStringReplaced.Length then 80 else sbStringReplaced.Length - line * 80 printfn $"{sbStringReplaced.Substring(line * 80, nChars)}" // The example displays the following output: // Aaaa_Bbbb_Cccc_Dddd_Eeee_Ffff_Gggg_Hhhh_Iiii_Jjjj_Kkkk_Llll_Mmmm_Nnnn_Oooo_Pppp_ // Qqqq_Rrrr_Ssss_Tttt_Uuuu_Vvvv_Wwww_Xxxx_Yyyy_ZzzzImports System.Text Imports System.Text.RegularExpressions Module Example12 Public Sub Main() ' Create a StringBuilder object with 4 successive occurrences ' of each character in the English alphabet. Dim sb As New StringBuilder() For ctr As UShort = AscW("a") To AscW("z") sb.Append(ChrW(ctr), 4) Next ' Convert it to a string. Dim sbString As String = sb.ToString() ' Use a regex to uppercase the first occurrence of the sequence, ' and separate it from the previous sequence by an underscore. Dim pattern As String = "(\w)(\1+)" sbString = Regex.Replace(sbString, pattern, Function(m) If(m.Index > 0, "_", "") + m.Groups(1).Value.ToUpper + m.Groups(2).Value) ' Display the resulting string. Dim line As Integer = 0 Do Dim nChars As Integer = If(line * 80 + 79 <= sbString.Length, 80, sbString.Length - line * 80) Console.WriteLine(sbString.Substring(line * 80, nChars)) line += 1 Loop While line * 80 < sbString.Length End Sub End Module ' The example displays the following output: ' Aaaa_Bbbb_Cccc_Dddd_Eeee_Ffff_Gggg_Hhhh_Iiii_Jjjj_Kkkk_Llll_Mmmm_Nnnn_Oooo_Pppp_ ' Qqqq_Rrrr_Ssss_Tttt_Uuuu_Vvvv_Wwww_Xxxx_Yyyy_Zzzz
将 StringBuilder 对象转换为字符串
必须先将 StringBuilder 对象转换为 String 对象，然后才能将 StringBuilder 对象表示的字符串传递给包含 String 参数的方法，或在用户界面中显示它。 通过调用 StringBuilder.ToString 该方法来执行此转换。 有关图示，请参阅前面的示例，该示例调用 ToString 方法将对象转换为 StringBuilder 字符串，以便可以传递给正则表达式方法。
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