File

In the VB.NET language, files are written and read. With the File type, we perform efficient manipulations of files, including reads, writes and appends.

Background

The System.IO namespace deals with files. StreamReader is often the simplest way to read in a text file. And StreamWriter writes out text.

StreamReader

We often employ a using-construct to create a StreamReader object. StreamReader is ideal for looping over the lines in a file. It can also result in less memory usage.

Imports System.IO

Module Module1

    Sub Main()
        ' Step 1: create StreamReader for the file.
        Using reader As StreamReader = New StreamReader("C:\programs\file.txt")
            ' Step 2: begin do-loop.
            Do
                ' Step 3: call ReadLine.
                Dim line As String = reader.ReadLine
                ' ... Exit the loop if Nothing.
                If line Is Nothing Then
                    Exit Do
                End If
                Console.WriteLine(line)
            Loop
        End Using
    End Sub

End Module
Thank you
Friend

ReadAllText

We use this function to read an entire text file into a String. Please note that this function is implemented in terms of the other methods in the System.IO namespace.

Imports System.IO

Module Module1
    Sub Main()
        Dim value As String = File.ReadAllText("C:\file.txt")
        Console.WriteLine(value.Length)
    End Sub
End Module
40

ReadAllLines

This Function reads in a text file and places each line in the file in a String array. The array is like any other array, and can be converted or modified.

Imports System.IO

Module Module1

    Sub Main()
        ' Step 1: read the file into an array.
        ' ... Create the required file if needed.
        Dim array As String() = File.ReadAllLines("file.txt")

        ' Step 2: use For-Each loop.
        For Each line As String In array
            ' Use the line.
            Dim length As Integer = line.Length
        Next
    End Sub

End Module

List

Often we use Lists of strings. But the File.ReadAllLines method returns a String array. You can convert the result of File.ReadAllLines into a List with ToList.

Imports System.IO

Module Module1

    Sub Main()
        ' Create a list reference variable.
        ' ... Then read all lines in the file.
        ' ... Convert the array to a List.
        Dim list As List(Of String) =
            File.ReadAllLines("file.txt").ToList
        Console.WriteLine(list.Count)
    End Sub

End Module
5

Line count

There are many ways to acquire the line count of a file. In this example, we see that you can actually load the file into an array, and then get the Length of that array.

Imports System.IO

Module Module1

    Sub Main()
        ' Get the length of the file.
        ' ... Not the fastest way to get the line count.
        Dim length As Integer = File.ReadAllLines("file.txt").Length
        Console.WriteLine(length)
    End Sub

End Module
5

WriteAllLines

It is possible to take a String array and write it as lines to a file. We use File.WriteAllLines, a shared method. In this example, the output file will contain three lines.

Imports System.IO

Module Module1

    Sub Main()
        ' Create an array of three elements.
        Dim array As String() = New String() {"cat", "dog", "arrow"}
        ' Write the array to a file.
        File.WriteAllLines("file.txt", array)
    End Sub

End Module
cat
dog
arrow

ReadAllBytes

We can invoke File.ReadAllBytes to get a Byte array from a file. Here we read in a WEBP file and print its Length and first byte value.

Imports System.IO

Module Program
    Sub Main()
        ' Read in binary file with ReadAllBytes.
        Dim webpFile() As Byte = File.ReadAllBytes("C:\programs\test.webp")
        Console.WriteLine("Length: {0}", webpFile.Length)
        Console.WriteLine("First byte: {0}", webpFile(0))
    End Sub
End Module
Length: 822
First byte: 82

With clear code, such as the Using statement, we read and write (mutate) files. Often we handle text files. But these functions can also act on images or binary data.