ASP.NET | Calcolare il CRC32 di un file

Protected Function Crc32(ByVal FileName As String) As String
            Dim _fileStream As New IO.FileStream(FileName, IO.FileMode.Open, IO.FileAccess.Read)
            Dim _sha1 As New System.Security.Cryptography.SHA1CryptoServiceProvider
            _sha1.ComputeHash(_fileStream)
            Dim _hash() As Byte = _sha1.Hash
            Dim _buffer As New StringBuilder
            For Each _hashByte As Byte In _hash
                _buffer.Append(String.Format("{0:X1}", _hashByte))
            Next
            Return _buffer.ToString.ToLower
End Function