Question

I’m computing the SHA-512 hash of a very large file using System.Security.Cryptography.SHA512Cng.ComputeHash, and I’d like to show a progress bar for it. How can I check the completion of the method without reimplementing the algorithm?

Answer 1

You can subclass Stream! Here’s a ProgressStream that reads from a file:

Public Class ProgressStream
    Inherits FileStream

    Public Event ProgressChanged(sender As ProgressStream, progress As Integer)

    Public Sub New(fileName As String)
        MyBase.New(fileName, FileMode.Open, FileAccess.Read)
    End Sub

    Public ReadOnly Property Progress() As Integer
        Get
            Return CInt(Me.Position / Me.Length * 100)
        End Get
    End Property

    Public Overrides Function Read(array() As Byte, offset As Integer, count As Integer) As Integer
        Read = MyBase.Read(array, offset, count)
        RaiseEvent ProgressChanged(Me, Me.Progress)
    End Function
End Class

Just handle the ProgressChanged event.

The same method applies to all other types of streams, and you can also use it for certain other long-running tasks that read a file as required instead of all at once.