String.Copy(String) 方法

定义

命名空间:

System

程序集:

System.Runtime.dll

Source:

String.cs

注意

This API should not be used to create mutable strings. See https://go.microsoft.com/fwlink/?linkid=2084035 for alternatives.

创建一个与指定的 String 具有相同值的 String 的新实例。

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

[System.Obsolete("This API should not be used to create mutable strings. See https://go.microsoft.com/fwlink/?linkid=2084035 for alternatives.")]
public static string Copy (string str);

[<System.Obsolete("This API should not be used to create mutable strings. See https://go.microsoft.com/fwlink/?linkid=2084035 for alternatives.")>]
static member Copy : string -> string

Public Shared Function Copy (str As String) As String

参数

str

String

要复制的字符串。

返回

String

值与 str 相同的新字符串。

属性

ObsoleteAttribute

例外

ArgumentNullException

str 为 null。

注解

方法 Copy 返回一个 String 对象，该对象具有与原始字符串相同的值，但表示不同的对象引用。 它不同于赋值操作，后者将现有字符串引用分配给其他对象变量。

重要

从 .NET Core 3.0 开始，此方法已过时。 但是，我们不建议在任何 .NET 实现中使用它。 特别是，由于 .NET Core 3.0 中字符串插入的变化，在某些情况下， Copy 该方法不会创建新字符串，而只是返回对现有实习生字符串的引用。

根据要调用 Copy 方法的原因，有多种替代方法：

• 如果希望其他字符串实例在修改字符串的操作中使用，请使用原始字符串实例。 由于字符串是不可变的，因此字符串操作会创建新的字符串实例，并且原始字符串不受影响。 在这种情况下，不应将新字符串引用分配给原始字符串变量。 下面的示例进行了这方面的演示。

  var original = "This is a sentence. This is a second sentence.";
  var sentence1 = original.Substring(0, original.IndexOf(".") + 1);
  Console.WriteLine(original);
  Console.WriteLine(sentence1);
  // The example displays the following output:
  //    This is a sentence. This is a second sentence.
  //    This is a sentence.
  

  let original = "This is a sentence. This is a second sentence."
  let sentence1 = original.Substring(0, original.IndexOf "." + 1)
  printfn $"{original}"
  printfn $"{sentence1}"
  // The example displays the following output:
  //    This is a sentence. This is a second sentence.
  //    This is a sentence.
  

  Dim original = "This is a sentence. This is a second sentence."
  Dim sentence1 = original.Substring(0, original.IndexOf(".") + 1)
  Console.WriteLine(original)
  Console.WriteLine(sentence1)
  ' The example displays the following output:
  '    This is a sentence. This is a second sentence.
  '    This is a sentence.
  

  在这种情况下，在调用 Copy 方法之前 Substring 调用 方法以创建新字符串会不必要地创建新的字符串实例。

• 如果要创建内容与原始字符串相同的可变缓冲区，请调用 String.ToCharArray 或 StringBuilder.StringBuilder(String) 构造函数。 例如：

  private static void UseMutableBuffer()
  {
      var original = "This is a sentence. This is a second sentence.";
      var chars = original.ToCharArray();
      var span = new Span<char>(chars);
      var slice = span.Slice(span.IndexOf('.'), 3);
      slice = MergeSentence(slice);
      Console.WriteLine($"Original string: {original}");
      Console.WriteLine($"Modified string: {span.ToString()}");
  
      static Span<char> MergeSentence(Span<char> span)
      {
          if (span.Length == 0) return Span<char>.Empty;
  
          span[0] = ';';
          span[2] = Char.ToLower(span[2]);
          return span;
      }
  }
  // The example displays the following output:
  //    Original string: This is a sentence. This is a second sentence.
  //    Modified string: This is a sentence; this is a second sentence.
  

  let mergeSentence (span: Span<char>) =
      if span.Length = 0 then
          Span<char>.Empty
      else
          span[0] <- '\000'
          span[2] <- Char.ToLower span[2]
          span
  
  let useMutableBuffer () =
      let original = "This is a sentence. This is a second sentence."
      let chars = original.ToCharArray()
      let span = Span chars
      let slice = span.Slice(span.IndexOf '.', 3)
      let slice = mergeSentence slice
      let span = span.ToString()
      printfn $"Original string: {original}"
      printfn $"Modified string: {span}"
  // The example displays the following output:
  //    Original string: This is a sentence. This is a second sentence.
  //    Modified string: This is a sentence this is a second sentence.
  

  Private Sub UseMutableBuffer()
      Dim original = "This is a sentence. This is a second sentence."
      Dim sb = new StringBuilder(original)
      Dim index = original.IndexOf(".")
      sb(index) = ";"
      sb(index + 2) = Char.ToLower(sb(index + 2))
      Console.WriteLine($"Original string: {original}")
      Console.WriteLine($"Modified string: {sb.ToString()}")
  End Sub
  ' The example displays the following output:
  '    Original string: This is a sentence. This is a second sentence.
  '    Modified string: This is a sentence; this is a second sentence.
  

• 如果要创建字符串的可变副本，以便可以使用不安全的代码修改字符串内容，请使用 Marshal.StringToHGlobalUni 方法。 以下示例使用 Marshal.StringToHGlobalUni 方法获取指向非托管内存中复制字符串的位置的指针，将字符串中每个字符的 Unicode 码位递增一个，并将生成的字符串复制回托管字符串。

  private static void UseUnmanaged()
  {
      var original = "This is a single sentence.";
      var len = original.Length; 
      var ptr = Marshal.StringToHGlobalUni(original);
      string? result;
      unsafe 
      {
          char *ch = (char *) ptr.ToPointer();
          while (len-- > 0)
          {
              char c = Convert.ToChar(Convert.ToUInt16(*ch) + 1);
              *ch++ = c;
          } 
          result = Marshal.PtrToStringUni(ptr);
          Marshal.FreeHGlobal(ptr);
      }
      Console.WriteLine($"Original string: {original}");
      Console.WriteLine($"String from interop: '{result}'");
  }
  // The example displays the following output:
  //    Original string: This is a single sentence.
  //    String from interop: 'Uijt!jt!b!tjohmf!tfoufodf/'
  

  #nowarn "9"
  open FSharp.NativeInterop
  
  let useUnmanaged () =
      let original = "This is a single sentence."
      let mutable len = original.Length 
      let ptr = Marshal.StringToHGlobalUni original
      let mutable ch = ptr.ToPointer() |> NativePtr.ofVoidPtr<char> 
      while len > 0 do
          len <- len - 1
          Convert.ToUInt16(NativePtr.read ch) + 1us
          |> Convert.ToChar
          |> NativePtr.write (NativePtr.add ch 1)
          ch <- NativePtr.add ch 1
  
      let result = Marshal.PtrToStringUni ptr
      Marshal.FreeHGlobal ptr
      printfn $"Original string: {original}"
      printfn $"String from interop: '{result}'"
  
  // The example displays the following output:
  //    Original string: This is a single sentence.
  //    String from interop: 'Uijt!jt!b!tjohmf!tfoufodf/'