String.Intern(String) 方法

public:
 static System::String ^ Intern(System::String ^ str);

public static string Intern(string str);

static member Intern : string -> string

Public Shared Function Intern (str As String) As String

参数

返回

如果系统被实习生所引用，则为系统引用;否则为对值为 <a0/> 的字符串的新引用。

例外

示例

以下示例创建两个值相等的字符串，并演示它们之间的交错会产生相同的引用。

// Sample for String.Intern(String)
using System;
using System.Text;

class Sample
{
    public static void Main()
    {
        string s1 = new StringBuilder().Append("My").Append("Test").ToString();
        string s2 = new StringBuilder().Append("My").Append("Test").ToString();
        Console.WriteLine($"s1 == {s1}");
        Console.WriteLine($"s2 == {s2}");
        Console.WriteLine($"Are s1 and s2 equal in value? {s1 == s2}");
        Console.WriteLine($"Are s1 and s2 the same reference? {Object.ReferenceEquals(s1, s2)}");

        string i1 = String.Intern(s1);
        string i2 = String.Intern(s2);
        Console.WriteLine($"After interning:");
        Console.WriteLine($"  Are i1 and i2 equal in value? {i1 == i2}");
        Console.WriteLine($"  Are i1 and i2 the same reference? {Object.ReferenceEquals(i1, i2)}");
    }
}
/*
This example produces the following results:
s1 == MyTest
s2 == MyTest
Are s1 and s2 equal in value? True
Are s1 and s2 the same reference? False
After interning:
  Are i1 and i2 equal in value? True
  Are i1 and i2 the same reference? True
*/

// Sample for String.Intern(String)
open System
open System.Text

let s1 = StringBuilder().Append("My").Append("Test").ToString()
let s2 = StringBuilder().Append("My").Append("Test").ToString()
printfn $"s1 = {s1}"
printfn $"s2 = {s2}"
printfn $"Are s1 and s2 equal in value? {s1 = s2}"
printfn $"Are s1 and s2 the same reference? {Object.ReferenceEquals(s1, s2)}"

let i1 = String.Intern s1
let i2 = String.Intern s2
printfn "After interning:"
printfn $"  Are i1 and i2 equal in value? {i1 = i2}"
printfn $"  Are i1 and i2 the same reference? {Object.ReferenceEquals(i1, i2)}"
(*
This example produces the following results:
s1 = MyTest
s2 = MyTest
Are s1 and s2 equal in value? True
Are s1 and s2 the same reference? False
After interning:
  Are i1 and i2 equal in value? True
  Are i1 and i2 the same reference? True
*)

Imports System.Text

Class Sample

    Public Shared Sub Run()
        Dim s1 As String = New StringBuilder().Append("My").Append("Test").ToString()
        Dim s2 As String = New StringBuilder().Append("My").Append("Test").ToString()
        Console.WriteLine($"s1 = {s1}")
        Console.WriteLine($"s2 = {s2}")
        Console.WriteLine($"Are s1 and s2 equal in value? {s1 = s2}")
        Console.WriteLine($"Are s1 and s2 the same reference? {s1 Is s2}")

        Dim i1 As String = String.Intern(s1)
        Dim i2 As String = String.Intern(s2)
        Console.WriteLine("After interning:")
        Console.WriteLine($"  Are i1 and i2 equal in value? {i1 = i2}")
        Console.WriteLine($"  Are i1 and i2 the same reference? {i1 Is i2}")
    End Sub
End Class
'
's1 = MyTest
's2 = MyTest
'Are s1 and s2 equal in value? True
'Are s1 and s2 the same reference? False
'After interning:
'  Are i1 and i2 equal in value? True
'  Are i1 and i2 the same reference? True
'

注解

![注意] > 虽然 String.Intern 保证两个值相等的字符串返回相同的实习生引用，但它不保证返回的引用与字符串文本相同。

        The common language runtime maintains a table, called the *intern pool*, that holds a single reference for each unique string value. The <xref:System.String.Intern*> method uses the intern pool to search for a string equal to the value of `str`. If no such string exists, a reference to `str` is added to the pool, and that reference is returned. (In contrast, the <xref:System.String.IsInterned(System.String)> method returns a null reference if the requested string doesn't exist in the intern pool.)

运行时可以使用实习生池来节省字符串存储。 但是，无法保证字符串文本的自动插入 -- 根据程序集的编译和执行方式，某些文本可能不会添加到池中。

在以下示例中，字符串 s1 的值为“MyTest”。 该 System.Text.StringBuilder 类生成一个新的字符串对象，其值与 s1 相同。 将该字符串的引用分配给s2。 该方法Intern搜索与s2值相同的字符串。 如果s1已驻留（例如，因为程序集需要字符串字面量驻留），该方法将返回与s1相同的引用，然后将该引用分配给s3，然后s1和s3进行比较以验证相等性。 否则，将为s2创建一个新的驻留条目并将其分配给s3，而s1与s3比较不相等。 在任一情况下， s1 并 s2 比较不相等，因为它们引用不同的对象。

string s1 = "MyTest"; 
string s2 = new StringBuilder().Append("My").Append("Test").ToString(); 
string s3 = String.Intern(s2); 
Console.WriteLine((Object)s2==(Object)s1); // Different references.
Console.WriteLine((Object)s3==(Object)s1); // The same reference.

let s1 = "MyTest"
let s2 = StringBuilder().Append("My").Append("Test").ToString()
let s3 = String.Intern s2
printfn $"{s2 :> obj = s1 :> obj}" // Different references.
printfn $"{s3 :> obj = s1 :> obj}" // The same reference.

Dim s1 As String = "MyTest" 
Dim s2 As String = New StringBuilder().Append("My").Append("Test").ToString() 
Dim s3 As String = String.Intern(s2) 
Console.WriteLine(CObj(s2) Is CObj(s1))      ' Different references.
Console.WriteLine(CObj(s3) Is CObj(s1))      ' The same reference.

性能注意事项

如果您尝试减少应用程序分配的内存总量，请记住，驻留字符串会产生两个不良的副作用。 首先，在公共语言运行库（CLR）终止之前，为驻留 String 对象分配的内存通常不会被释放。 原因是 CLR 对驻留对象 String 的引用可能在您的应用程序甚至应用程序域终止后仍然存在。 其次，要在内存中驻留字符串，必须先创建字符串。 使用String对象的内存仍必须进行分配，即使该内存最终会被垃圾回收。

CompilationRelaxations.NoStringInterning 枚举成员将程序集标记为不需要字符串文字驻留。 默认情况下，C# 编译器在每个程序集上发出一个 CompilationRelaxationsAttribute 带有 NoStringInterning 标志的标志，以提高性能，这意味着不能保证将字符串文本添加到实习生池。 可以使用NoStringInterning特性对程序集CompilationRelaxationsAttribute进行自定义。

使用 本机 AOT 发布应用时，不支持关闭 NoStringInterning 。 使用本机 AOT 时，无法保证将字符串文本添加到实习生池，因此 Intern 可能无法找到与源代码中似乎是文本的字符串匹配项。

另请参阅

• IsInterned(String)