Question

我正在使用System.Security.Cryptography.SHA512Cng.ComputeHash计算一个非常大的文件的SHA-512哈希值，我想为它显示一个进度条。如何在不重新实现算法的情况下检查方法的完成情况？

Answer 1

你可以继承Stream！这是从文件中读取的ProgressStream：

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

只需处理ProgressChanged事件。

同样的方法适用于所有其他类型的流，您也可以将它用于某些其他长时间运行的任务，这些任务根据需要而不是一次性读取文件。

Answer 2

一种常见的方法是使用TransformBlock和TransformFinalBlock并以块的形式计算哈希值。这使您可以不关心正在使用的流的类型，或者您可能想要散列多个流（比如目录中的所有文件）。

这是一个小例子：

Using stream As New FileStream("C:\somefile.dat", FileMode.Open),
      hash As New SHA512Cng()
    Const BUFFER_SIZE As Integer = 2048
    Dim buffer(BufferSize - 1) As Byte
    Dim bytesRead As Integer

    Do
        bytesRead = stream.Read(buffer, 0, BUFFER_SIZE)
        hash.TransformBlock(buffer, 0, bytesRead, buffer, 0)

        Dim percentComplete = stream.Position / stream.Length * 100
        'Handle percentComplete here somehow, perhaps with an event
    Loop While stream.Length - stream.Position > BUFFER_SIZE

    bytesRead = stream.Read(buffer, 0, BUFFER_SIZE) ' read the final block
    hash.TransformFinalBlock(buffer, 0, bytesRead)

    'We're now 100% complete, raise an event with 100% completion
    Dim theHash = hash.Hash 'The final hash values
End Using

缓冲区大小越小，结果越精细，但可能会以性能为代价。

如何在文件上显示其他库中操作的进度？

2 个答案: