Visual Basic.NET - Add two numbers (I/O from file) - vb.net

Question

Following code should sum two numbers from file "input.txt" and write the sum to "output.txt". Compilation is succesfull, but "output.txt" is still empty after running program. What am I doing wrong?
Imports System.IO
Public Class test
Public Shared Sub Main()
Dim scan as StreamReader = new StreamReader("input.txt")
Dim writer as StreamWriter = new StreamWriter("output.txt", True)
Dim input as String
input = scan.ReadLine()
Dim ab() as String = Split(input)
Dim res as Integer = Val(ab(0))+Val(ab(1))
writer.writeLine(res)
writer.close()
End sub
End class

Answer 1

Your code works properly for me, so as long as your input file is formatted properly (i.e. a single line with two numbers separated by spaces, like "1 2") and you have the necessary OS permissions to read and write to those files, then it should work for you too. However, it's worth mentioning that there are several issues with your code that would be good to correct, since the fly in the face of typical best-practices.
First, you should, as much as possible, turn Option Strict On. I know that you have it Off because your code won't compile with it On. The following line is technically misleading, and therefore fails with Option Strict On:
Dim res As Integer = Val(ab(0)) + Val(ab(1))
The reason if fails is because the Val function returns a Double, not an integer, so, technically, depending on the contents of the file, the result could be fractional or could be too large to fit in an Integer. With Option Strict Off, the compiler is essentially automatically fixing your code for you, like this:
Dim res As Integer = CInt(Val(ab(0)) + Val(ab(1)))
In order to set the res variable equal to the result of the calculation, the more capable Double value must be converted down to an Integer. When you are forced to put the CInt in the code yourself, you are fully aware that the conversion is taking place and what the consequences of it might be. When you have Option Strict Off and it inserts the conversion behind-the-scenes, then you may very well miss a potential bug.
Secondly, the Val function is old-school VB6 syntax. While it technically works fine, it's provided mainly for backwards compatibility. The new .NET equivalent would be to use Integer.Parse, Integer.TryParse or Convert.ToInt32.
Thirdly, you never close the scan stream reader. You could just add scan.Close() to the end of your method, but is better, when possible, to create Using blocks for any disposable object, like this:
Using scan As StreamReader = New StreamReader("test.txt")
Using writer As StreamWriter = New StreamWriter("output.txt", True)
Dim input As String
input = scan.ReadLine()
Dim ab() As String = Split(input)
Dim res As Integer = Integer.Parse(ab(0)) + Integer.Parse(ab(1))
writer.WriteLine(res)
End Using
End Using
Lastly, as Hans pointed out, it's not good to rely on the current directory. It's always best to specify full paths for your files. There are different methods in the framework for getting various folder paths, such as the user's desktop folder, or the download folder, or the temp folder, or the application folder, or the current application's folder, or the folder of the current running assembly. You can use any such method to get your desired folder path, and then use Path.Combine to add the file name to get the full file path. For instance:
Dim desktopFolderPath As String = Environment.GetFolderPath(Environment.SpecialFolder.DesktopDirectory)
Dim inputFilePath As String = Path.Combine(desktopFolderPath, "input.txt")
Dim outputFilePath As String = Path.Combine(desktopFolderPath, "output.txt")