Copy a file in little chunks

Question

I want to copy a file in little chunks (to cancel the copy operation if needed).

I\'m trying to follow the unmarked solution here: How to copy a file with the abilit

Answer 1

You have to understand what you're doing and how it works.

First, you allocate a buffer.

Dim buffer = New Byte(1024) {}

Then you go and read some data from the input stream. The data is put in buffer, starting at 0, at most buffer.Length bytes. The method returns how many bytes it has actually read and put in the buffer.

bytesRead = inputStream.Read(buffer, 0, buffer.Length)

If the number of bytes you've read is bigger than 0, then you haven't reached the end of the file yet.

While (bytesRead > 0)

Then you write exactly those bytes that were read (bytesRead bytes) to the output stream. Write the bytes from buffer, start at index 0 and write bytesRead number of bytes.

outputStream.Write(buffer, 0, bytesRead)

Since the stream will buffer what you wrote for efficiency reasons, you'll have to flush and close the output stream.

outputStream.Flush()
outputStream.Close()

Putting it together:

Dim buffer = New Byte(1024) {}
Dim bytesRead As Integer

' Read some bytes
While ((bytesRead = inputStream.Read(buffer, 0, buffer.Length)) > 0)
    ' Write them to the output
    outputStream.Write(buffer, 0, bytesRead)

    If cancelled Then
        MsgBox("operacion cancelada")
        Return
    End If

    ' Repeat
End

outputStream.Flush()
outputStream.Close()

_{The last time I wrote VB is more than a decade ago. You'll have to ensure the syntax is correct.}

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Notice how the original code contains this line:

While ((bytesRead = inputStream.Read(buffer, 0, buffer.Length)) > 0)

You see the bytesRead = part? You haven't copied that into your code. It is essential to store the number of bytes read.

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Final code: this works on my computer:

Imports System
Imports System.IO

Namespace ConsoleApplication1
    Friend Class Program
        Private Shared Sub Main(args As String())
            Program.CopyMyFiles()
            Console.WriteLine("DONE!")
            Console.ReadLine()
        End Sub

        Private Shared Sub CopyMyFiles()
            Dim input_filepath As String = "Test.txt"
            Dim output_filepath As String = "Test_New.txt"
            Dim input As FileStream = New FileStream(input_filepath, FileMode.Open, FileAccess.ReadWrite)
            Dim output As FileStream = New FileStream(output_filepath, FileMode.Create, FileAccess.ReadWrite)
            Program.CopyStream(input, output)
        End Sub

        Public Shared Sub CopyStream(inputStream As Stream, outputStream As Stream)
            Dim buffer As Byte() = New Byte(1025)
            Dim bytesRead As Integer
            bytesRead = inputStream.Read(buffer, 0, buffer.Length)
            While bytesRead > 0
                outputStream.Write(buffer, 0, bytesRead)
                bytesRead = inputStream.Read(buffer, 0, buffer.Length)
            End While
            outputStream.Flush()
            inputStream.Close()
            outputStream.Close()
        End Sub
    End Class
End Namespace