ファイルの読み込み書き込みを非同期処理で実行する方法

ファイルの読み込み書き込みを非同期処理で実行する方法
VB.NET

Private readBytes As Byte() = Nothing
Private allBytes As System.IO.MemoryStream = Nothing

Private Sub btn1_Click(ByVal sender As Object, ByVal e As EventArgs) _
        Handles btn1.Click
    readBytes = New Byte(4095) {}
    If allBytes IsNot Nothing Then
        allBytes.Close()
    End If
    allBytes = New System.IO.MemoryStream()
    Dim fileName As String = "C:\test.txt"

    Dim fs As New System.IO.FileStream(fileName, _
        System.IO.FileMode.Open, _
        System.IO.FileAccess.Read, _
        System.IO.FileShare.Read, _
        &H1000, _
        True)
    fs.BeginRead(readBytes, _
                 0, _
                 readBytes.Length, _
                 New AsyncCallback(AddressOf ReadBytesCallback), _
                 fs)
End Sub

Private Sub ReadBytesCallback(ByVal ar As IAsyncResult)
    Dim fs As System.IO.FileStream = _
        DirectCast(ar.AsyncState, System.IO.FileStream)
    Dim readSize As Integer = fs.EndRead(ar)

    If readSize > 0 Then
        allBytes.Write(readBytes, 0, readSize)

        fs.BeginRead(readBytes, _
                     0, _
                     readBytes.Length, _
                     New AsyncCallback(AddressOf ReadBytesCallback), _
                     fs)
    Else
        fs.Close()
    End If
End Sub

C#

private byte[] readBytes = null;
System.IO.MemoryStream allBytes = null;

private void btn1_Click(object sender, System.EventArgs e)
{
    readBytes = new byte[0x1000];
    if (allBytes != null)
    {
        allBytes.Close();
    }
    allBytes = new System.IO.MemoryStream();
    string fileName = @"C:\test.txt";

    System.IO.FileStream fs = new System.IO.FileStream(
        fileName,
        System.IO.FileMode.Open,
        System.IO.FileAccess.Read,
        System.IO.FileShare.Read,
        0x1000,
        true);
    fs.BeginRead(readBytes, 0, readBytes.Length,
        new AsyncCallback(ReadBytesCallback), fs);
}

private void ReadBytesCallback(IAsyncResult ar)
{
    System.IO.FileStream fs =
        (System.IO.FileStream)ar.AsyncState;
    int readSize = fs.EndRead(ar);

    if (readSize > 0)
    {

        allBytes.Write(readBytes, 0, readSize);

        fs.BeginRead(readBytes, 0, readBytes.Length,
            new AsyncCallback(ReadBytesCallback), fs);
    }
    else
    {
        fs.Close();
    }
}