Console Input and Command-Line Arguments

Reading from the keyboard

Console.ReadLine() reads one line of text typed by the user:

' illustrative: waits for keyboard input
Module Program
    Sub Main()
        Console.Write("What's your name? ")
        Dim name = Console.ReadLine()

        Console.Write("How old are you? ")
        Dim age As Integer
        If Integer.TryParse(Console.ReadLine(), age) Then
            Console.WriteLine($"Hi {name}, next year you'll be {age + 1}.")
        Else
            Console.WriteLine($"Hi {name}, that doesn't look like a number.")
        End If
    End Sub
End Module

A typical session:

What's your name? Ada
How old are you? 36
Hi Ada, next year you'll be 37.

Input is always a String: parse it with TryParse and handle bad input instead of crashing. Console.ReadKey() reads a single key press.

A simple menu loop

' illustrative: waits for keyboard input
Module Program
    Sub Main()
        Do
            Console.WriteLine("1) Say hello  2) Show time  Q) Quit")
            Dim choice = Console.ReadLine()
            If choice Is Nothing Then Exit Do     ' input ended (for example, Ctrl + Z)
            Select Case choice.Trim().ToUpper()
                Case "1"
                    Console.WriteLine("Hello!")
                Case "2"
                    Console.WriteLine(Date.Now.ToShortTimeString())
                Case "Q"
                    Exit Do
                Case Else
                    Console.WriteLine("Unknown choice")
            End Select
        Loop
    End Sub
End Module

ReadLine returns Nothing when the input stream ends — for example, when the user presses Ctrl + Z then Enter (Windows) or Ctrl + D (Linux/macOS), or when input is redirected from a file. Checking for it avoids a crash (or an endless loop).

Command-line arguments

Main can take the arguments as a string array:

' illustrative: run as  dotnet run -- 3 4
Module Program
    Function Main(args As String()) As Integer
        If args.Length <> 2 Then
            Console.Error.WriteLine("Usage: add A B")
            Return 1
        End If
        Console.WriteLine(CInt(args(0)) + CInt(args(1)))
        Return 0
    End Function
End Module

7

• Everything after -- in dotnet run -- ... is passed to your program.
• Declaring Main as a Function ... As Integer returns an exit code: 0 means success, anything else signals an error to scripts and CI pipelines.
• Console.Error.WriteLine writes to the error stream, where usage and error messages belong.