IEnumerable<T>.GetEnumerator 方法

定義

命名空間:

System.Collections.Generic

組件:

System.Runtime.dll

傳回逐一查看集合的列舉值。

public:
 System::Collections::Generic::IEnumerator<T> ^ GetEnumerator();

public System.Collections.Generic.IEnumerator<out T> GetEnumerator ();

abstract member GetEnumerator : unit -> System.Collections.Generic.IEnumerator<'T>

Public Function GetEnumerator () As IEnumerator(Of Out T)

傳回

IEnumerator<T>

可用來逐一查看集合的列舉值。

範例

下列範例示範如何實作 IEnumerable<T> 介面，並使用該實作來建立 LINQ 查詢。 當您實作 IEnumerable<T> 時，也必須針對 C# 實 IEnumerator<T> 作 或 ，才能使用 yield 關鍵字。 實作 IEnumerator<T> 也需要 IDisposable 實作，您會在此範例中看到此範例。

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

備註

傳回 IEnumerator<T> 的功能可讓您藉由公開屬性 來逐一 Current 查看集合。您可以使用列舉值來讀取集合中的資料，但不能修改集合。

一開始，列舉程式位在集合中的第一個項目之前。 在這個位置上，Current 並未定義。 因此，您必須先呼叫 MoveNext 方法，將列舉值前移至集合的第一個專案，然後再讀取 的值 Current 。

Current 會傳回相同的物件，直到 MoveNext 再次呼叫 為 MoveNext 下一個專案的設定 Current 為止。

如果 MoveNext 傳遞集合結尾，列舉值會放在集合的最後一個專案後面，並 MoveNext 傳 false 回 。 當列舉值位於這個位置時，後續呼叫 MoveNext 也會傳回 false 。 如果最後一 MoveNext 次呼叫傳 false 回 ， Current 則為未定義。 您不能再次將 Current 設定為集合的第一個項目；您必須建立新的列舉值執行個體。

如果對集合進行變更，例如新增、修改或刪除專案，則列舉值的行為是未定義的。

列舉值沒有集合的獨佔存取權，因此只要集合保持不變，列舉值就會保持有效。 如果對集合進行變更，例如新增、修改或刪除專案，列舉值就會失效，而且您可能會得到非預期的結果。 此外，列舉集合不是安全線程的程式。 若要保證執行緒安全，您應該在列舉值期間鎖定集合，或在集合上實作同步處理。

命名空間中 System.Collections.Generic 集合的預設實作不會同步處理。

另請參閱

• IEnumerator<T>
• 逐步解說：在 Visual Basic 中實作 IEnumerable(Of T)
• 迭代器 (C#)
• 迭代器 (Visual Basic)