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String.Copy(String) 方法

定義

警告

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

使用與指定的 String 相同的值,建立 String 的新執行個體。

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);
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
static member Copy : string -> string
Public Shared Function Copy (str As String) As String

參數

str
String

要複製的字串。

傳回

具有與 str 相同值的新字串。

屬性

例外狀況

strnull

備註

方法 Copy 會傳 String 回與原始字串相同的值,但代表不同的物件參考。 它與指派作業不同,它會將現有的字串參考指派給其他物件變數。

重要

從 .NET Core 3.0 開始,這個方法已過時。 不過,我們不建議在任何 .NET 實作中使用。 特別是,由於 .NET Core 3.0 中的字串插入變更,在某些情況下 Copy ,方法不會建立新的字串,而是只會傳回現有內嵌字串的參考。

根據您想要呼叫 Copy 方法的原因而定,有一些替代方法:

  • 如果您想要在修改字串的作業中使用不同的字串實例,請使用原始字串實例。 因為字串不可變,所以字串作業會建立新的字串實例,而原始字串仍不會受到影響。 在此情況下,您不應該將新的字串參考指派給原始字串變數。 下列範例提供說明。

    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.
    

    在此情況下,呼叫 Copy 方法以在呼叫 方法之前 Substring 建立新的字串,不必要地建立新的字串實例。

  • 如果您想要建立具有與原始字串相同內容的可變動緩衝區,請呼叫 String.ToCharArrayStringBuilder.StringBuilder(String) 建構函式。 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.
    
  • 如果您想要建立字串的可變動複本,以便使用不安全的程式碼來修改字串內容,請使用 Marshal.StringToHGlobalUni 方法。 下列範例會 Marshal.StringToHGlobalUni 使用 方法來取得非受控記憶體中複製字串位置的指標、將字串中每個字元的 Unicode 字碼點遞增一個,並將產生的字串複製到 Managed 字串。

    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/'
    

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