Generate MD5 and SHA1 - vb.net

Question

I'm using the below function to generate an MD5\SH1 hash for SQL backup files.
This works well, has progress report etc but is slow if using large files.
Could I generate the MD5 at the same time as SH1 rather than having to process the file twice, doubling the time taken? What about converting an MD5 result to SHA1?
Imports System
Imports System.IO
Imports System.Security.Cryptography
Imports System.Text
Public Class ASyncFileHashAlgorithm
Protected hashAlgorithm As HashAlgorithm
Protected m_hash As Byte()
Protected cancel As Boolean = False
Protected m_bufferSize As Integer = 4096
Public Delegate Sub FileHashingProgressHandler(ByVal sender As Object, _
ByVal e As FileHashingProgressArgs)
Public Event FileHashingProgress As FileHashingProgressHandler
Public Sub New(ByVal hashAlgorithm As HashAlgorithm)
Me.hashAlgorithm = hashAlgorithm
End Sub
Public Function ComputeHash(ByVal stream As Stream) As Byte()
cancel = False
m_hash = Nothing
Dim _bufferSize As Integer = m_bufferSize
' this makes it impossible to change the buffer size while computing
Dim readAheadBuffer As Byte(), buffer As Byte()
Dim readAheadBytesRead As Integer, bytesRead As Integer
Dim size As Long, totalBytesRead As Long = 0
size = stream.Length
readAheadBuffer = New Byte(_bufferSize - 1) {}
readAheadBytesRead = stream.Read(readAheadBuffer, 0, _
readAheadBuffer.Length)
totalBytesRead += readAheadBytesRead
Do
bytesRead = readAheadBytesRead
buffer = readAheadBuffer
readAheadBuffer = New Byte(_bufferSize - 1) {}
readAheadBytesRead = stream.Read(readAheadBuffer, 0, _
readAheadBuffer.Length)
totalBytesRead += readAheadBytesRead
If readAheadBytesRead = 0 Then
hashAlgorithm.TransformFinalBlock(buffer, 0, bytesRead)
Else
hashAlgorithm.TransformBlock(buffer, 0, bytesRead, buffer, 0)
End If
RaiseEvent FileHashingProgress(Me, New _
FileHashingProgressArgs(totalBytesRead, size))
Loop While readAheadBytesRead <> 0 AndAlso Not cancel
If cancel Then
Return InlineAssignHelper(m_hash, Nothing)
End If
Return InlineAssignHelper(m_hash, hashAlgorithm.Hash)
End Function
Public Property BufferSize() As Integer
Get
Return m_bufferSize
End Get
Set(ByVal value As Integer)
m_bufferSize = value
End Set
End Property
Public ReadOnly Property Hash() As Byte()
Get
Return m_hash
End Get
End Property
'Public Sub Cancel()
' cancel = True
'End Sub
Public Overrides Function ToString() As String
Dim hex As String = ""
For Each b As Byte In Hash
hex += b.ToString("x2")
Next
Return hex
End Function
Private Shared Function InlineAssignHelper(Of T)(ByRef target As T, _
ByVal value As T) As T
target = value
Return value
End Function
End Class
Public Class FileHashingProgressArgs
Inherits EventArgs
Public Property TotalBytesRead() As Long
Get
Return m_TotalBytesRead
End Get
Set(ByVal value As Long)
m_TotalBytesRead = Value
End Set
End Property
Private m_TotalBytesRead As Long
Public Property Size() As Long
Get
Return m_Size
End Get
Set(ByVal value As Long)
m_Size = Value
End Set
End Property
Private m_Size As Long
Public Sub New(ByVal totalBytesRead__1 As Long, ByVal size__2 As Long)
TotalBytesRead = totalBytesRead__1
Size = size__2
End Sub
End Class
The following is how I'm generating a hash using the above:
Shared hasher As New ASyncFileHashAlgorithm(SHA1.Create())
Private Function Test(Byval (strFilePathAndName, as String)
Dim stream As IO.Stream = DirectCast(File.Open(strFilePathAndName, _
FileMode.Open), Stream)
AddHandler hasher.FileHashingProgress, _
AddressOf OnFileHashingProgress
Dim t = New Thread(AddressOf hasher.ComputeHash)
t.Start(stream)
While t.IsAlive
Application.DoEvents()
End While
'LblMD5.Text = hasher.ToString???
LblSHA1.Text = hasher.ToString
stream.Dispose()
End Sub
Public Sub OnFileHashingProgress(ByVal sender As Object, _
ByVal e As FileHashingProgressArgs)
SetControlPropertyValue(uxChildForm.ProgressBar, "Position", _
CInt(e.TotalBytesRead / e.Size * 100))
End Sub

Answer 1

You ToString method may create tremendous overhead for large strings since concatenation is expensive (it creates large temporary buffer) and you do it often. Use a StringBuilder (initialised with the correct size) instead.