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 ability to cancel the copy? But I'm getting a 0 byte file What I'm doing wrong? ``` Public Class Form1 Dim cancelled As Boolean = Nothing Dim input = New System.IO.FileStream("C:\1.txt", System.IO.FileMode.Open, System.IO.FileAccess.Read, System.IO.FileShare.Read) Dim output = New System.IO.FileStream("C:\Nueva carpeta\1.txt", System.IO.FileMode.CreateNew, System.IO.FileAccess.Write, System.IO.FileShare.Write) Public Sub CopyStream(ByVal inputStream As System.IO.Stream, ByVal outputStream As System.IO.Stream) 'Dim buffer = System.IO.File.ReadAllBytes("C:\1.txt") Dim buffer = New Byte((1024) - 1) {} Dim bytesRead As Integer = 1 While (inputStream.Read(buffer, 0, buffer.Length) > 0) outputStream.Write(buffer, 0, bytesRead) 'bytesRead += 1 If cancelled Then MsgBox("operacion cancelada") Return End If End While inputStream.Close() ou…

Accepted Answer

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.

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.

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
```

Copy a file in little chunks

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