StreamReader.ReadAsync StreamReader.ReadAsync StreamReader.ReadAsync StreamReader.ReadAsync Method

定义

重载

ReadAsync(Memory<Char>, CancellationToken) ReadAsync(Memory<Char>, CancellationToken)
ReadAsync(Char[], Int32, Int32) ReadAsync(Char[], Int32, Int32) ReadAsync(Char[], Int32, Int32) ReadAsync(Char[], Int32, Int32)

从当前流中异步读取指定的最大字符,并且从指定的索引位置开始将该数据写入缓冲区。Reads a specified maximum number of characters from the current stream asynchronously and writes the data to a buffer, beginning at the specified index.

ReadAsync(Memory<Char>, CancellationToken) ReadAsync(Memory<Char>, CancellationToken)

public override System.Threading.Tasks.ValueTask<int> ReadAsync (Memory<char> buffer, System.Threading.CancellationToken cancellationToken = null);
override this.ReadAsync : Memory<char> * System.Threading.CancellationToken -> System.Threading.Tasks.ValueTask<int>

参数

buffer
Memory<Char>

返回

ReadAsync(Char[], Int32, Int32) ReadAsync(Char[], Int32, Int32) ReadAsync(Char[], Int32, Int32) ReadAsync(Char[], Int32, Int32)

从当前流中异步读取指定的最大字符,并且从指定的索引位置开始将该数据写入缓冲区。Reads a specified maximum number of characters from the current stream asynchronously and writes the data to a buffer, beginning at the specified index.

public:
 override System::Threading::Tasks::Task<int> ^ ReadAsync(cli::array <char> ^ buffer, int index, int count);
[System.Runtime.InteropServices.ComVisible(false)]
public override System.Threading.Tasks.Task<int> ReadAsync (char[] buffer, int index, int count);
override this.ReadAsync : char[] * int * int -> System.Threading.Tasks.Task<int>
Public Overrides Function ReadAsync (buffer As Char(), index As Integer, count As Integer) As Task(Of Integer)

参数

buffer
Char[]

当此方法返回时,包含指定的字符数组,此数组中 index 和 (index + count - 1) 之间的值被从当前源中读取的字符所替换。When this method returns, contains the specified character array with the values between index and (index + count - 1) replaced by the characters read from the current source.

index
Int32 Int32 Int32 Int32

buffer 中开始写入的位置。The position in buffer at which to begin writing.

count
Int32 Int32 Int32 Int32

要读取的最大字符数。The maximum number of characters to read. 如果在将指定的字符数写入缓冲区之前已到达流结尾,则将返回当前方法。If the end of the stream is reached before the specified number of characters is written into the buffer, the current method returns.

返回

表示异步读取操作的任务。A task that represents the asynchronous read operation. TResult 参数的值包含读入缓冲区的总字符数。The value of the TResult parameter contains the total number of characters read into the buffer. 如果当前可用字符数少于所请求的字符数,则该结果值可小于所请求的字符数;如果已到达流结尾时,则为 0(零)。The result value can be less than the number of characters requested if the number of characters currently available is less than the requested number, or it can be 0 (zero) if the end of the stream has been reached.

异常

indexcount 的总和大于缓冲区长度。The sum of index and count is larger than the buffer length.

以前的读取操作当前正在使用读取器。The reader is currently in use by a previous read operation.

示例

下面的示例演示如何使用ReadAsync(Char[], Int32, Int32)方法读取文件中的所有字符。The following example shows how to read all the characters in a file by using the ReadAsync(Char[], Int32, Int32) method. 它在将字符添加到StringBuilder类的实例之前, 检查每个字符是否为字母、数字或空格。It checks whether each character is a letter, digit, or white space before adding the character to an instance of the StringBuilder class.

using System;
using System.Windows;
using System.IO;
using System.Text;

namespace WpfApplication
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private async void Button_Click_1(object sender, RoutedEventArgs e)
        {
            string filename = @"C:\Example\existingfile.txt";
            char[] result;
            StringBuilder builder = new StringBuilder();

            using (StreamReader reader = File.OpenText(filename))
            {
                result = new char[reader.BaseStream.Length];
                await reader.ReadAsync(result, 0, (int)reader.BaseStream.Length);
            }
           
            foreach (char c in result)
            {
                if (char.IsLetterOrDigit(c) || char.IsWhiteSpace(c))
                {
                    builder.Append(c);
                }
            }
            FileOutput.Text = builder.ToString();
        }
    }
}
Imports System.Text
Imports System.IO

Class MainWindow

    Private Async Sub Button_Click_1(sender As Object, e As RoutedEventArgs)
        Dim filename As String = "C:\Example\existingfile.txt"
        Dim result() As Char
        Dim builder As StringBuilder = New StringBuilder()

        Using reader As StreamReader = File.OpenText(filename)
            ReDim result(reader.BaseStream.Length)
            Await reader.ReadAsync(result, 0, reader.BaseStream.Length)
        End Using

        For Each c As Char In result
            If (Char.IsLetterOrDigit(c) Or Char.IsWhiteSpace(c)) Then
                builder.Append(c)
            End If
            Next
        FileOutput.Text = builder.ToString()
    End Sub
End Class

注解

在读取count参数指定的字符数或到达流的末尾后, 任务完成。The task completes after either the number of characters specified by the count parameter are read or the end of the stream is reached.

适用于