ReadBytes Method

BinaryReader.ReadBytes Method (Int32)


Reads the specified number of bytes from the current stream into a byte array and advances the current position by that number of bytes.

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

Public Overridable Function ReadBytes (
	count As Integer
) As Byte()


Type: System.Int32

The number of bytes to read. This value must be 0 or a non-negative number or an exception will occur.

Return Value

Type: System.Byte()

A byte array containing data read from the underlying stream. This might be less than the number of bytes requested if the end of the stream is reached.

Exception Condition

The number of decoded characters to read is greater than count. This can happen if a Unicode decoder returns fallback characters or a surrogate pair.


An I/O error occurs.


The stream is closed.


count is negative.

BinaryReader does not restore the file position after an unsuccessful read operation.

For a list of common I/O tasks, see Common I/O Tasks.

The following code example shows how to write binary data using memory as a backing store, and then verify that the data was written correctly.

Imports System
Imports System.IO

Public Class BinaryRW

    Shared Sub Main()

        Const upperBound As Integer = 1000

        ' Create random data to write to the stream.
        Dim dataArray(upperBound) As Byte
        Dim randomGenerator As New Random

        Dim binWriter As New BinaryWriter(New MemoryStream())

        ' Write the data to the stream.
        Console.WriteLine("Writing the data.")

        ' Create the reader using the stream from the writer.
        Dim binReader As New BinaryReader(binWriter.BaseStream)

        ' Set the stream position to the beginning of the stream.
        binReader.BaseStream.Position = 0

        ' Read and verify the data.
        Dim verifyArray() As Byte = _
        If verifyArray.Length <> dataArray.Length Then
            Console.WriteLine("Error writing the data.")
        End If
        For i As Integer = 0 To upperBound
            If verifyArray(i) <> dataArray(i) Then
                Console.WriteLine("Error writing the data.")
            End If
        Next i
        Console.WriteLine("The data was written and verified.")

    End Sub
End Class

This example reads the contents of a file and displays it to the console as dump text. The end of the file that is being read is detected when the length of the Byte array returned from ReadBytes is zero.

Imports System
Imports System.IO
Imports System.Text

Module Module1
    Private ReadOnly CHUNK_SIZE As Integer = 1024
    Public Sub Main(args() As String)
        If ((args.Length = 0) OrElse Not File.Exists(args(0))) Then
            Console.WriteLine("Please provide an existing file name.")
            Using fs As FileStream = New FileStream(args(0), FileMode.Open, FileAccess.Read)
                Using br As New BinaryReader(fs, New ASCIIEncoding())
                    Dim chunk(CHUNK_SIZE) As Byte
                    chunk = br.ReadBytes(CHUNK_SIZE)

                    While chunk.Length > 0
                        DumpBytes(chunk, chunk.Length)
                        chunk = br.ReadBytes(CHUNK_SIZE)
                    End While
                End Using
            End Using
        End If
    End Sub

    Public Sub DumpBytes(bdata() As Byte, len As Integer)
        Dim i As Integer
        Dim j As Integer = 0
        Dim dchar As Char
        ' 3 * 16 chars for hex display, 16 chars for text and 8 chars
        ' for the 'gutter' int the middle.
        Dim dumptext As New StringBuilder("        ", 16 * 4 + 8)
        For i = 0 To len - 1
            dumptext.Insert(j * 3, String.Format("{0:X2} ", CType(bdata(i), Integer)))
            dchar = Convert.ToChar(bdata(i))
            ' replace 'non-printable' chars with a '.'.
            If Char.IsWhiteSpace(dchar) Or Char.IsControl(dchar) Then
                dchar = "."
            End If
            j += 1
            If j = 16 Then
                dumptext.Length = 0
                dumptext.Append("        ")
                j = 0
            End If
        Next i
        ' display the remaining line
        If j > 0 Then
            ' add blank hex spots to align the 'gutter'.
            For i = j To 15
                dumptext.Insert(j * 3, "   ")
            Next i
        End If
    End Sub

End Module

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
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