StreamReader.ReadAsync Method (Char(), Int32, Int32)

.NET Framework (current version)

Reads a specified maximum number of characters from the current stream asynchronously and writes the data to a buffer, beginning at the specified index.

Namespace:   System.IO
Assembly:  mscorlib (in mscorlib.dll)

<HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading := True)>
Public Overrides Function ReadAsync (
	buffer As Char(),
	index As Integer,
	count As Integer
) As Task(Of Integer)


Type: System.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.

Type: System.Int32

The position in buffer at which to begin writing.

Type: System.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.

Return Value

Type: System.Threading.Tasks.Task(Of Int32)

A task that represents the asynchronous read operation. The value of the TResult parameter contains the total number of bytes read into the buffer. The result value can be less than the number of bytes requested if the number of bytes currently available is less than the requested number, or it can be 0 (zero) if the end of the stream has been reached.

Exception Condition

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.

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

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.

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
            End If
        FileOutput.Text = builder.ToString()
    End Sub
End Class

Universal Windows Platform
Available since 8
.NET Framework
Available since 4.5
Portable Class Library
Supported in: portable .NET platforms
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1
Return to top