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VB.NET File Handling
These VB.NET examples show how to handle files. They use the System.IO namespace.Files are a form of persistent storage.
They are written and read. With the File type, we perform efficient and simple 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. Then we use a While-loop to keep reading lines. We call ReadLine in this loop.
Tip: StreamReader is ideal for looping over the lines in a file. This can also result in less memory usage.
Tip 2: To use the StreamReader, it is best to use the Using statement, which provides for system-level cleanup of resources.
While: One way to use the ReadLine method in a loop is to use the Do While (True) loop construct with an early exit.
Based on:
.NET 4.5
VB.NET program that uses StreamReader type
Imports System.IO
Module Module1
Sub Main()
' Create StreamReader for the file.
Using reader As StreamReader = New StreamReader("file.txt")
' Do While true loop.
Do While (True)
' Read a line.
Dim line As String = reader.ReadLine
' See if line is Nothing.
If line Is Nothing Then
Exit Do
End If
' Write line to screen.
Console.WriteLine(line)
Loop
End Using
End Sub
End Module
Output: requires file
Line 1
Line 2
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.
Thus: You could implement a function equivalent to ReadAllText with the StreamReader type.
VB.NET program that uses ReadAllText
Imports System.IO
Module Module1
Sub Main()
Dim value As String = File.ReadAllText("C:\file.txt")
Console.WriteLine(value.Length)
End Sub
End Module
Output: depends on file
40
ReadAllLines. This program calls the ReadAllLines method with an argument equal to the file name. Then the result of the method is stored in the String array variable.
Next: A For-Each loop is used on that String, providing each line in the file for usage elsewhere in code.
File.ReadLines: ReadLines is different from ReadAllLines. It reads one line in at a time. This uses less memory.
VB.NET program that uses ReadAllLines
Imports System.IO
Module Module1
Sub Main()
' Read the file into an array.
' ... Make sure to create the required file if isn't there.
Dim array As String() = File.ReadAllLines("file.txt")
Dim line As String
For Each line In array
' We now have the line so can use it.
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.
Then: You can use each line of the file in a List collection. You can apply all List functions.
VB.NET program that uses ToList method on file array
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
Output: depends on file
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.
Tip: You could instead use StreamReader code above, and then increment an integer on each successful call to the ReadLine method.
VB.NET program that gets line count
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
Output: depends on file
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.
VB.NET program that uses WriteAllLines
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
File contents: file.txt
cat
dog
arrow
Overview. We describe many of the File functions available. Some have been omitted. The System.IO namespace provides many other types that are separate from these shared methods.
File.ReadAllBytes: Useful for files not stored as plain text. You can open images or movies with this method.
File.ReadAllLines: Microsoft: "Opens a file, reads all lines of the file with the specified encoding, and closes the file."
File.ReadAllText: Returns the contents of the text file at the specified path as a string. Useful for plain text or settings files.
File.WriteAllBytes: Useful for files such as images that were created or mutated in memory.
File.WriteAllLines: Stores a string array in the specified file, overwriting the contents. Shown in an example.
File.WriteAllText: Writes the contents of a string to a text file. One of the simplest ways to persist text data.
File.AppendAllText: Use to append the contents string to the file at the path. It creates a new file if needed.
Paths. Files found in external storage have file paths. You can use the Path type to specify and determine volumes, folders and file extensions.
Recursive: Newer versions of the .NET Framework include a method that handles recursive directory file lists. We show an older solution.
Operations. Many operations are available in the System.IO namespace. A file may be copied. A file info may be received, allowing you to check sizes and dates without using the actual file.
FileInfo: It is often necessary to get the size of a file in bytes. The FileInfo type is often needed for this purpose.
WebClient. With WebClient, we use VB.NET to download files from the Internet. We process them on the local computer. We even write the downloaded data to the local disk.
Images. Image formats are complex. It is probably best to avoid processing them yourself. With the Image type and its properties, you can read JPG, PNG and other types of files.
XML. Data is usually best stored in a structured format. In cases where a database is not necessary, you can use XML files. We use XmlReader and XmlWriter.
XElement: The XElement type provides powerful XML parsing. It can load files from the disk or the network.
BinaryReader. Computers understand binary data well. But humans don't read it with the same ease. You can use the BinaryReader and BinaryWriter types to effectively load in binary data.
Compress. Files can be compressed in the VB.NET language. This requires a newer version of the .NET Framework that includes the System.IO.Compression namespace.
ZipFile: This class can be used to compress an entire directory. We can avoid writing the code ourselves.
Office. Sometimes, you may need to read data from an Excel spreadsheet or Word DOC into your VB.NET program. Office formats are complex.
Excel: This Excel tutorial reads in sheets from XLS and XLSX files. Performance is important here.
Word: This simple Word tutorial parses text from a Word document. The code is somewhat confusing.
With clear code, such as the Using statement, we read and write (mutate) files. In particular we focused on text files. But these methods can also act on images or binary data.