IEnumerable<T>.GetEnumerator 方法

定义

返回一个循环访问集合的枚举器。

public:
 System::Collections::Generic::IEnumerator<T> ^ GetEnumerator();
public System.Collections.Generic.IEnumerator<out T> GetEnumerator ();
public System.Collections.Generic.IEnumerator<T> GetEnumerator ();
abstract member GetEnumerator : unit -> System.Collections.Generic.IEnumerator<'T>
Public Function GetEnumerator () As IEnumerator(Of Out T)
Public Function GetEnumerator () As IEnumerator(Of T)

返回

用于循环访问集合的枚举数。

示例

以下示例演示如何实现 接口并 IEnumerable<T> 使用该实现来创建 LINQ 查询。 实现 IEnumerable<T>时,还必须实现 IEnumerator<T> ,或者,仅对于 C#,可以使用 yield 关键字 (keyword) 。 实现 IEnumerator<T> 还需要 IDisposable 实现,你将在此示例中看到。

using System;
using System.IO;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

public class App
{
    // Exercise the Iterator and show that it's more
    // performant.
    public static void Main()
    {
        TestStreamReaderEnumerable();
        Console.WriteLine("---");
        TestReadingFile();
    }

    public static void TestStreamReaderEnumerable()
    {
        // Check the memory before the iterator is used.
        long memoryBefore = GC.GetTotalMemory(true);
      IEnumerable<String> stringsFound;
        // Open a file with the StreamReaderEnumerable and check for a string.
      try {
         stringsFound =
               from line in new StreamReaderEnumerable(@"c:\temp\tempFile.txt")
               where line.Contains("string to search for")
               select line;
         Console.WriteLine("Found: " + stringsFound.Count());
      }
      catch (FileNotFoundException) {
         Console.WriteLine(@"This example requires a file named C:\temp\tempFile.txt.");
         return;
      }

        // Check the memory after the iterator and output it to the console.
        long memoryAfter = GC.GetTotalMemory(false);
        Console.WriteLine("Memory Used With Iterator = \t"
            + string.Format(((memoryAfter - memoryBefore) / 1000).ToString(), "n") + "kb");
    }

    public static void TestReadingFile()
    {
        long memoryBefore = GC.GetTotalMemory(true);
      StreamReader sr;
      try {
         sr = File.OpenText("c:\\temp\\tempFile.txt");
      }
      catch (FileNotFoundException) {
         Console.WriteLine(@"This example requires a file named C:\temp\tempFile.txt.");
         return;
      }

        // Add the file contents to a generic list of strings.
        List<string> fileContents = new List<string>();
        while (!sr.EndOfStream) {
            fileContents.Add(sr.ReadLine());
        }

        // Check for the string.
        var stringsFound =
            from line in fileContents
            where line.Contains("string to search for")
            select line;

        sr.Close();
        Console.WriteLine("Found: " + stringsFound.Count());

        // Check the memory after when the iterator is not used, and output it to the console.
        long memoryAfter = GC.GetTotalMemory(false);
        Console.WriteLine("Memory Used Without Iterator = \t" +
            string.Format(((memoryAfter - memoryBefore) / 1000).ToString(), "n") + "kb");
    }
}

// A custom class that implements IEnumerable(T). When you implement IEnumerable(T),
// you must also implement IEnumerable and IEnumerator(T).
public class StreamReaderEnumerable : IEnumerable<string>
{
    private string _filePath;
    public StreamReaderEnumerable(string filePath)
    {
        _filePath = filePath;
    }

    // Must implement GetEnumerator, which returns a new StreamReaderEnumerator.
    public IEnumerator<string> GetEnumerator()
    {
        return new StreamReaderEnumerator(_filePath);
    }

    // Must also implement IEnumerable.GetEnumerator, but implement as a private method.
    private IEnumerator GetEnumerator1()
    {
        return this.GetEnumerator();
    }
    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator1();
    }
}

// When you implement IEnumerable(T), you must also implement IEnumerator(T),
// which will walk through the contents of the file one line at a time.
// Implementing IEnumerator(T) requires that you implement IEnumerator and IDisposable.
public class StreamReaderEnumerator : IEnumerator<string>
{
    private StreamReader _sr;
    public StreamReaderEnumerator(string filePath)
    {
        _sr = new StreamReader(filePath);
    }

    private string _current;
    // Implement the IEnumerator(T).Current publicly, but implement
    // IEnumerator.Current, which is also required, privately.
    public string Current
    {

        get
        {
            if (_sr == null || _current == null)
            {
                throw new InvalidOperationException();
            }

            return _current;
        }
    }

    private object Current1
    {

        get { return this.Current; }
    }

    object IEnumerator.Current
    {
        get { return Current1; }
    }

    // Implement MoveNext and Reset, which are required by IEnumerator.
    public bool MoveNext()
    {
        _current = _sr.ReadLine();
        if (_current == null)
            return false;
        return true;
    }

    public void Reset()
    {
        _sr.DiscardBufferedData();
        _sr.BaseStream.Seek(0, SeekOrigin.Begin);
        _current = null;
    }

    // Implement IDisposable, which is also implemented by IEnumerator(T).
    private bool disposedValue = false;
    public void Dispose()
    {
        Dispose(disposing: true);
        GC.SuppressFinalize(this);
    }

    protected virtual void Dispose(bool disposing)
    {
        if (!this.disposedValue)
        {
            if (disposing)
            {
                // Dispose of managed resources.
            }
            _current = null;
            if (_sr != null) {
               _sr.Close();
               _sr.Dispose();
            }
        }

        this.disposedValue = true;
    }

     ~StreamReaderEnumerator()
    {
        Dispose(disposing: false);
    }
}
// This example displays output similar to the following:
//       Found: 2
//       Memory Used With Iterator =     33kb
//       ---
//       Found: 2
//       Memory Used Without Iterator =  206kb
Imports System.IO
Imports System.Collections
Imports System.Collections.Generic
Imports System.Linq

Public Module App
   ' Excercise the Iterator and show that it's more performant.
   Public Sub Main()
      TestStreamReaderEnumerable()
      Console.WriteLine("---")
      TestReadingFile()
   End Sub

   Public Sub TestStreamReaderEnumerable()
        ' Check the memory before the iterator is used.
        Dim memoryBefore As Long = GC.GetTotalMemory(true)
      Dim stringsFound As IEnumerable(Of String)
        ' Open a file with the StreamReaderEnumerable and check for a string.
      Try
         stringsFound =
               from line in new StreamReaderEnumerable("c:\temp\tempFile.txt")
               where line.Contains("string to search for")
               select line
         Console.WriteLine("Found: {0}", stringsFound.Count())
      Catch e As FileNotFoundException
         Console.WriteLine("This example requires a file named C:\temp\tempFile.txt.")
         Return
      End Try

        ' Check the memory after the iterator and output it to the console.
        Dim memoryAfter As Long = GC.GetTotalMemory(false)
        Console.WriteLine("Memory Used with Iterator = {1}{0} kb",
                        (memoryAfter - memoryBefore)\1000, vbTab)
   End Sub

   Public Sub TestReadingFile()
        Dim memoryBefore As Long = GC.GetTotalMemory(true)
      Dim sr As StreamReader
      Try
         sr = File.OpenText("c:\temp\tempFile.txt")
      Catch e As FileNotFoundException
         Console.WriteLine("This example requires a file named C:\temp\tempFile.txt.")
         Return
      End Try

        ' Add the file contents to a generic list of strings.
        Dim fileContents As New List(Of String)()
        Do While Not sr.EndOfStream
            fileContents.Add(sr.ReadLine())
      Loop

        ' Check for the string.
        Dim stringsFound =
            from line in fileContents
            where line.Contains("string to search for")
            select line

      sr.Close()
      Console.WriteLine("Found: {0}", stringsFound.Count())

        ' Check the memory after when the iterator is not used, and output it to the console.
        Dim memoryAfter As Long = GC.GetTotalMemory(False)
        Console.WriteLine("Memory Used without Iterator = {1}{0} kb",
                        (memoryAfter - memoryBefore)\1000, vbTab)
   End Sub
End Module

' A custom class that implements IEnumerable(T). When you implement IEnumerable(T),
' you must also implement IEnumerable and IEnumerator(T).
Public Class StreamReaderEnumerable : Implements IEnumerable(Of String)
    Private _filePath As String

    Public Sub New(filePath As String)
        _filePath = filePath
    End Sub

    ' Must implement GetEnumerator, which returns a new StreamReaderEnumerator.
    Public Function GetEnumerator() As IEnumerator(Of String) _
          Implements IEnumerable(Of String).GetEnumerator
        Return New StreamReaderEnumerator(_filePath)
    End Function

    ' Must also implement IEnumerable.GetEnumerator, but implement as a private method.
    Private Function GetEnumerator1() As IEnumerator _
          Implements IEnumerable.GetEnumerator
        Return Me.GetEnumerator()
    End Function
End Class

' When you implement IEnumerable(T), you must also implement IEnumerator(T),
' which will walk through the contents of the file one line at a time.
' Implementing IEnumerator(T) requires that you implement IEnumerator and IDisposable.
Public Class StreamReaderEnumerator : Implements IEnumerator(Of String)
    Private _sr As StreamReader

    Public Sub New(filePath As String)
        _sr = New StreamReader(filePath)
    End Sub

    Private _current As String

    ' Implement the IEnumerator(T).Current Publicly, but implement
    ' IEnumerator.Current, which is also required, privately.
    Public ReadOnly Property Current As String _
          Implements IEnumerator(Of String).Current
        Get
            If _sr Is Nothing OrElse _current Is Nothing
                Throw New InvalidOperationException()
            End If

            Return _current
        End Get
    End Property

    Private ReadOnly Property Current1 As Object _
          Implements IEnumerator.Current
        Get
           Return Me.Current
        End Get
    End Property

    ' Implement MoveNext and Reset, which are required by IEnumerator.
    Public Function MoveNext() As Boolean _
          Implements IEnumerator.MoveNext
        _current = _sr.ReadLine()
        if _current Is Nothing Then Return False

        Return True
    End Function

    Public Sub Reset() _
          Implements IEnumerator.Reset
        _sr.DiscardBufferedData()
        _sr.BaseStream.Seek(0, SeekOrigin.Begin)
        _current = Nothing
    End Sub

    ' Implement IDisposable, which is also implemented by IEnumerator(T).
    Private disposedValue As Boolean = False
    Public Sub Dispose() _
          Implements IDisposable.Dispose
        Dispose(disposing:=True)
        GC.SuppressFinalize(Me)
    End Sub

    Protected Overridable Sub Dispose(disposing As Boolean)
        If Not Me.disposedValue Then
            If disposing Then
                ' Dispose of managed resources.
            End If

            _current = Nothing
            If _sr IsNot Nothing Then
               _sr.Close()
               _sr.Dispose()
            End If
        End If

        Me.disposedValue = True
    End Sub

    Protected Overrides Sub Finalize()
        Dispose(disposing:=False)
    End Sub
End Class
' This example displays output similar to the following:
'       Found: 2
'       Memory Used With Iterator =     33kb
'       ---
'       Found: 2
'       Memory Used Without Iterator =  206kb

注解

返回的 IEnumerator<T> 提供通过公开 Current 属性 来循环访问集合的功能。可以使用枚举器读取集合中的数据,但不能修改集合。

最初,枚举数定位在集合中第一个元素的前面。 在此位置上,未定义 Current。 因此,在读取 的值Current之前,必须调用 MoveNext 方法来将枚举器推进到集合的第一个元素。

Current 返回相同的 对象,直到 MoveNext 再次调用 作为 MoveNext 下一个元素的集 Current

如果 MoveNext 传递集合的末尾,则枚举器位于集合中最后一个元素之后,并 MoveNext 返回 false。 当枚举器位于此位置时,对 MoveNext 的后续调用也会返回 false。 如果最后一次MoveNext调用返回 ,falseCurrent则为未定义。 无法再次将 Current 设置为集合的第一个元素;必须改为创建新的枚举器实例。

如果对集合进行了更改,例如添加、修改或删除元素,则枚举器的行为是不确定的。

枚举器对集合没有独占访问权限,因此只要集合保持不变,枚举器就保持有效。 如果对集合进行了更改(例如添加、修改或删除元素),枚举器将失效,并且可能会得到意外结果。 此外,枚举集合不是线程安全的过程。 若要保证线程安全,应在枚举器期间锁定集合,或在集合上实现同步。

命名空间中 System.Collections.Generic 集合的默认实现不会同步。

适用于

另请参阅