StreamReader.ReadAsync Method

Definition

Namespace:

System.IO

Assembly:

System.Runtime.dll

Overloads

ReadAsync(Memory<Char>, CancellationToken)	Asynchronously reads the characters from the current stream into a memory block.
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)

Source:

StreamReader.cs

Asynchronously reads the characters from the current stream into a memory block.

public override System.Threading.Tasks.ValueTask<int> ReadAsync (Memory<char> buffer, System.Threading.CancellationToken cancellationToken = default);

override this.ReadAsync : Memory<char> * System.Threading.CancellationToken -> System.Threading.Tasks.ValueTask<int>

Public Overrides Function ReadAsync (buffer As Memory(Of Char), Optional cancellationToken As CancellationToken = Nothing) As ValueTask(Of Integer)

Parameters

buffer

Memory<Char>

When this method returns, contains the specified memory block of characters replaced by the characters read from the current source.

cancellationToken

CancellationToken

The token to monitor for cancellation requests. The default value is None.

Returns

ValueTask<Int32>

A value task that represents the asynchronous read operation. The value of the type parameter of the value task contains the number of characters that have been read, or 0 if at the end of the stream and no data was read. The number will be less than or equal to the buffer length, depending on whether the data is available within the stream.

Exceptions

OperationCanceledException

The cancellation token was canceled. This exception is stored into the returned task.

ReadAsync(Char[], Int32, Int32)

Source:

StreamReader.cs

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);

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

Task<Int32>

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.

Exceptions

ArgumentNullException

buffer is null.

ArgumentOutOfRangeException

index or count is negative.

ArgumentException

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

ObjectDisposedException

The stream has been disposed.

InvalidOperationException

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.

This method stores in the task it returns all non-usage exceptions that the method's synchronous counterpart can throw. If an exception is stored into the returned task, that exception will be thrown when the task is awaited. Usage exceptions, such as ArgumentException, are still thrown synchronously. For the stored exceptions, see the exceptions thrown by Read(Char[], Int32, Int32).