Reading Datafile in As Few Lines As Necessary

Question

Reading Datafile in As Few Lines As Necessary

Stephen Woehr 0

I know the DataReader method, but I thought there was a way using "ReadLine" or something like that.

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3 answers

Your answer

Answer 1

DataReader is for reading rows from a data source such as a database. It returns an unbuffered, forward-only stream, and rows are read by calling ExecuteReader() and then Read().

If the goal is to read a text file line by line in VB, LineInput exists, but it is provided for backward compatibility and may affect performance. For non-legacy applications, My.Computer.FileSystem provides better performance.

If the goal is to read only as much of a file as needed, File.ReadLines is the better fit. It returns the lines of a file as IEnumerable<string>, and enumeration can begin before the whole collection is returned. That makes it more efficient than reading the entire file first when working with large files.

Answer 2

Jay Pham (WICLOUD CORPORATION) 4,435 Microsoft External Staff Moderator

Hi @Stephen Woehr ,

Adding a concrete VB example on top of what was already mentioned. File.ReadLines is the one you are looking for. It returns an IEnumerable(Of String), so lines are pulled only as you enumerate them.

Imports System.IO

' Read only the first 10 lines, nothing beyond that is read from disk
For Each line As String In File.ReadLines("C:\data\myfile.txt").Take(10)
    Console.WriteLine(line)
Next

' Or stop on a condition
For Each line As String In File.ReadLines("C:\data\myfile.txt")
    If line.StartsWith("END") Then Exit For
    ' process line
Next

If you prefer controlling each read yourself, StreamReader.ReadLine() does the same job.

Using reader As New StreamReader("C:\data\myfile.txt")
    Dim line As String = reader.ReadLine()
    Do While line IsNot Nothing
        ' process line
        line = reader.ReadLine()
    Loop
End Using

The key difference to watch for: File.ReadAllLines loads the entire file into memory before you can touch the first line, so that is the one to avoid for your scenario.

I hope this helps! If my answer was helpful so far, you can follow this guidance to provide some feedback. This also help others find the solution easier.

Jay Pham (WICLOUD CORPORATION) 4,435 Reputation points Microsoft External Staff Moderator

2026-08-07T01:17:28.4666667+00:00

Hi @Stephen Woehr ,

Have you seen my answer in this post? If you have any question, feel free to reach out. I'm happy to support you.
Jay Pham (WICLOUD CORPORATION) 4,435 Reputation points Microsoft External Staff Moderator

2026-08-11T01:23:49.9566667+00:00

Hi @Stephen Woehr ,

If you're still interested or have any questions, feel free to reach out whenever it works best for you.

Answer 3

Senthil kumar 2,410

Hi @Stephen Woehr

 Dim lstData As List(Of String) = File.ReadAllLines("d:\data\datafile.txt").Take(10).ToList()


 lstData.ForEach(Function(x)
                     Console.WriteLine(x)
                 End Function)

this will data load to list.

Thanks.

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