Most efficient way to jump through a file and read lines? - vb.net

Question

I want to use a FileStream and seek from the beginning of the file while moving forward in the file .01% of the file size at a time.
So I want to seek to a position in the file, read the entire line, if it matches my criteria I am done. If not, I seek ahead another .01.
C# is OK but VB.NET preferred.
I used to do it something like this in VB6...
FileOpen(1, CurrentFullPath, OpenMode.Input, OpenAccess.Read, OpenShare.Shared)
Dim FileLength As Long = LOF(1)
For x As Single = 0.99 To 0 Step -0.01
Seek(1, CInt(FileLength * x))
Dim S As String = LineInput(1)
S = LineInput(1)
filePosition = Seek(1)
If filePosition < 50000 Then
filePosition = 1
Exit For
End If
V = Split(S, ",")
Dim MessageTime As Date = CDate(V(3) & " " & Mid$(V(4), 1, 8))
Dim Diff As Integer = DateDiff(DateInterval.Minute, MessageTime, CDate(RequestedStartTime))
If Diff >= 2 Then
Exit For
End If
Next
But I don't want to use FileOpen, I want to use a FileStream.
Any help is greatly appreciated!

Answer 1

This is a more or less direct conversion of your code, where we use FileStream.Position to specify where in the file to read:
Using streamReader As System.IO.StreamReader = System.IO.File.OpenText(CurrentFullPath)
For x As Single = 0.99 To 0 Step -0.01
streamReader.BaseStream.Position = CLng(streamReader.BaseStream.Length * x)
Dim S As String = streamReader.ReadLine()
'... etc.
Next
End Using

Answer 2

what bout something like this (C# version):
using (var file = System.IO.File.OpenText(filename))
{
while (!file.EndOfStream)
{
string line = file.ReadLine();
//do your logic here
//Logical test - if true, then break
}
}
EDIT: VB version here (warning - from a C# dev!)
Using file as FileStream = File.OpenText(filename)
while Not file.EndOfStream
Dim line as string = file.ReadLine()
''//Test to break
''//exit while if condition met
End While
End Using

Answer 3

I normally prefer vb.net, but C#'s iterator blocks are slowly winning me over:
public static IEnumerable<string> SkimFile(string FileName)
{
long delta = new FileInfo(FileName).Length / 100;
long position = 0;
using (StreamReader sr = new StreamReader(FileName))
{
while (position < 100)
{
sr.BaseStream.Seek(position * delta, SeekOrigin.Begin);
yield return sr.ReadLine();
position++;
}
}
}
Put it in a class library project and use it from vb like this:
Dim isMatch as Boolean = False
For Each s As String in SkimFile("FileName.txt")
If (RequestedDate - CDate(s.SubString(3,11))).Minutes > 2 Then
isMatch = True
Exit For
End If
Next s
(I took some liberties with you criteria (assumed fixed-width values rather than delimited) to make the example easier)

Answer 4

There's an example on MSDN.
Edit in response to comment:
I must admit I'm a bit confused, as you seemed insistant on using a buffered FileStream, but want to read a file a line at a time? You can do that quite simply using a StreamReader. I don't know VB, but in C# it would be something like this:
using (StreamReader sr = File.OpenText(pathToFile))
{
string line = String.Empty;
while ((line = sr.ReadLine()) != null)
{
// process line
}
}
See http://msdn.microsoft.com/en-us/library/system.io.file.aspx.