StreamReader.ReadAsync Method

Definition

Overloads

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

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)

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>

Parameters

buffer
Memory<Char>
cancellationToken
CancellationToken

Returns

ReadAsync(Char[], Int32, Int32)

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)

Parameters

buffer
Char[]

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

The position in buffer at which to begin writing.

count
Int32

The maximum number of characters to read. If the end of the stream is reached before the specified number of characters is written into the buffer, the current method returns.

Returns

A task that represents the asynchronous read operation. The value of the TResult parameter contains the total number of characters read into the buffer. 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.

Attributes

Exceptions

buffer is null.

index or count is negative.

The sum of index and count is larger than the buffer length.

The stream has been disposed.

The reader is currently in use by a previous read operation.

Examples

The following example shows how to read all the characters in a file by using the ReadAsync(Char[], Int32, Int32) method. 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

Remarks

The task completes after either the number of characters specified by the count parameter are read or the end of the stream is reached.

Applies to