How to set default input value in .Net Console App?

Question

How can you set a default input value in a .net console app?

Here is some make-believe code:

Console.Write("Enter weekly cost: ");
string input = Console.ReadLine("135"); // 135 is the default. The user can change or press enter to accept
decimal weeklyCost = decimal.Parse(input);

Of course, I don't expect it to be this simple. I am betting on having to do some low-level, unmanaged stuff; I just don't know how.

EDIT

I know I can replace no input with the default. That's not what I am asking about. I am trying to LEARN what's involved in achieving the behavior I described: giving the user an editable default. I'm also not worried about input validation; my question has nothing to do with that.

Dumfound

You can code this as the answer suggested - the user will not care about the coding technic. For the theoretical question if there is a way to do it with readline - probably not (at least not documented). – Cosentino

But - I see were you're trying to go, we are looking for a solution that will enable the user to change the default text. – Cosentino

I know it can't be done with .ReadLine(). But, I know there is a way to do this. – Dumfound

Answer 1

I believe that you will have manage this manually by listening to each key press:

Quickly thown together example:

   // write the initial buffer
   char[] buffer = "Initial text".ToCharArray();
   Console.WriteLine(buffer);

   // ensure the cursor starts off on the line of the text by moving it up one line
   Console.SetCursorPosition(Console.CursorLeft + buffer.Length, Console.CursorTop - 1);

   // process the key presses in a loop until the user presses enter
   // (this might need to be a bit more sophisticated - what about escape?)
   ConsoleKeyInfo keyInfo = Console.ReadKey(true);
   while (keyInfo.Key != ConsoleKey.Enter)
   {

       switch (keyInfo.Key)
       {
            case ConsoleKey.LeftArrow:
                    ...
              // process the left key by moving the cursor position
              // need to keep track of the position in the buffer

         // if the user presses another key then update the text in our buffer
         // and draw the character on the screen

         // there are lots of cases that would need to be processed (backspace, delete etc)
       }
       keyInfo = Console.ReadKey(true);
   }

This is quite involved - you'll have to keep ensure the cursor doesn't go out of range and manually update your buffer.

Inhume

I don't think this is what is meant by the question. – Antiquary

Actually this is definitely the best answer so far. – Dumfound

Throw this into an Extension method so you could call Console.ReadLine("135"); Can Extension methods be overloads of existing methods? If not, give it a new name. – Lichenin

@Dennis - You can't add extension methods to static classes because you can't create instances of them. Maybe SuperConsole.ReadLine("default")? There are a lot of cases to handle, but coding this out and packaging it into a DLL should be worth it. – Dumfound

I got "static types cannot be used as parameters" when I tried to create an extension method for Console. So maybe that wouldn't work & maybe that's a new SO question. – Lichenin

Answer 2

Here's a simple solution:

public static string ConsoleReadLineWithDefault(string defaultValue)
{
    System.Windows.Forms.SendKeys.SendWait(defaultValue);
    return Console.ReadLine();
}

It's not complete however. Some characters in the SendWait input string have special meaning so you have to escape them (eg. +, (, ), etc.) See: http://msdn.microsoft.com/en-us/library/system.windows.forms.sendkeys.aspx for a complete description.

Cable

Answer 3

I went ahead and completed Matt's implementation approach:

    public static string ReadInputWithDefault(string defaultValue, string caret = "> ")
    {
        Console.WriteLine(); // make sure we're on a fresh line

        List<char> buffer = defaultValue.ToCharArray().Take(Console.WindowWidth - caret.Length - 1).ToList();
        Console.Write(caret); 
        Console.Write(buffer.ToArray());
        Console.SetCursorPosition(Console.CursorLeft, Console.CursorTop);

        ConsoleKeyInfo keyInfo = Console.ReadKey(true);
        while (keyInfo.Key != ConsoleKey.Enter)
        {
            switch (keyInfo.Key)
            {
                case ConsoleKey.LeftArrow:
                    Console.SetCursorPosition(Math.Max(Console.CursorLeft - 1, caret.Length), Console.CursorTop);
                    break;
                case ConsoleKey.RightArrow:
                    Console.SetCursorPosition(Math.Min(Console.CursorLeft + 1, caret.Length + buffer.Count), Console.CursorTop);
                    break;
                case ConsoleKey.Home:
                    Console.SetCursorPosition(caret.Length, Console.CursorTop);
                    break;
                case ConsoleKey.End:
                    Console.SetCursorPosition(caret.Length + buffer.Count, Console.CursorTop);
                    break;
                case ConsoleKey.Backspace:
                    if (Console.CursorLeft <= caret.Length)
                    {
                        break;
                    }
                    var cursorColumnAfterBackspace = Math.Max(Console.CursorLeft - 1, caret.Length);
                    buffer.RemoveAt(Console.CursorLeft - caret.Length - 1);
                    RewriteLine(caret, buffer);
                    Console.SetCursorPosition(cursorColumnAfterBackspace, Console.CursorTop);
                    break;
                case ConsoleKey.Delete:
                    if (Console.CursorLeft >= caret.Length + buffer.Count)
                    {
                        break;
                    }
                    var cursorColumnAfterDelete = Console.CursorLeft;
                    buffer.RemoveAt(Console.CursorLeft - caret.Length);
                    RewriteLine(caret, buffer);
                    Console.SetCursorPosition(cursorColumnAfterDelete, Console.CursorTop);
                    break;
                default:
                    var character = keyInfo.KeyChar;
                    if (character < 32) // not a printable chars
                        break;
                    var cursorAfterNewChar = Console.CursorLeft + 1;
                    if (cursorAfterNewChar > Console.WindowWidth || caret.Length + buffer.Count >= Console.WindowWidth - 1)
                    {
                        break; // currently only one line of input is supported
                    }
                    buffer.Insert(Console.CursorLeft - caret.Length, character);
                    RewriteLine(caret, buffer);
                    Console.SetCursorPosition(cursorAfterNewChar, Console.CursorTop);
                    break;
            }
            keyInfo = Console.ReadKey(true);
        }
        Console.Write(Environment.NewLine);

        return new string(buffer.ToArray());
    }

    private static void RewriteLine(string caret, List<char> buffer)
    {
        Console.SetCursorPosition(0, Console.CursorTop);
        Console.Write(new string(' ', Console.WindowWidth - 1));
        Console.SetCursorPosition(0, Console.CursorTop);
        Console.Write(caret);
        Console.Write(buffer.ToArray());
    }

Notes:

• Works for only one line of input
• You can define what stands before the editable text area (caret parameter)
• Use at your own risk, there may still be some IndexOutOfBound-problems. ;)

Anthropomorphism

Answer 4

Or... Just test the value entered, if it's empty put the default value in input.

Cosentino

Answer 5

There's a much better way to do this now, check out Readline on nuget: https://www.nuget.org/packages/ReadLine

1. install-package Readline
2. var input = ReadLine.Read("Enter weekly cost: ", "135");

I like to use the console to write interactive tests, and having default values can really help things.

Salman

Slight difference here: the @default is just a default value, not an automatically inserted value that can be erased by the user – Beware

a good framework to use... but only .net standard 2.0... not available for .net framework... with .net framework I get an error on trying install that package – Stroke

Sorry. I was wrong with my statement. I find out, I need at least .NET Framework 4.6.1 but in my project I used 4.5.2. That's the reason, why I couldn't use that library/nuget-package. – Stroke

Answer 6

1. Add Reference to Assembly Library "System.Windows.Forms" to your Project
2. Add SendKeys.SendWait("DefaultText") immediately after your Console.WriteLine command and before your Console.ReadLine command

 

string _weeklycost = "";
Console.WriteLine("Enter weekly cost: ");
System.Windows.Forms.SendKeys.SendWait("135");
_weeklycost = Console.ReadLine();

Ratliff

Answer 7

Simple solution, if user inputs nothing, assign the default:

Console.Write("Enter weekly cost: ");
string input = Console.ReadLine();
decimal weeklyCost = String.IsNullOrEmpty(input) ? 135 : decimal.Parse(input);

When dealing with user inputs, you should expect that it might contain errors. So you could use TryParse in order to avoid an exception, if the user has not input a number:

Console.Write("Enter weekly cost: ");
string input = Console.ReadLine(); 
decimal weeklyCost;
if ( !Decimal.TryParse(input, out weeklyCost) ) 
    weeklyCost = 135;

This would be considered best-practice for handling user input. If you need to parse many user inputs, use a helper function for that. One way of doing it is to use a method with a nullable and return null if parsing failed. Then it is very easy to assign a default value using the null coalescing operator:

public static class SafeConvert
{
    public static decimal? ToDecimal(string value)
    {
        decimal d;
        if (!Decimal.TryParse(value, out d))
            return null;
        return d;
    }
}

Then, to read an input and assign a default value is as easy as:

decimal d = SafeConvert.ToDecimal(Console.ReadLine()) ?? 135;

Antiquary

You left his fictitious parameter to ReadLine in place. – Terminator

Answer 8

You can use helper method like this:

public static string ReadWithDefaults(string defaultValue)
{
    string str = Console.ReadLine();
    return String.IsNullOrEmpty(str) ? defaultValue : str;
}

Irizarry

Answer 9

I have found a solution that does exactly what you want. Thanks to LagDaemon.ReadLine, which is a fork of package used in Lunster's answer, we can do this:

ReadLine.Send("default value");
var input = ReadLine.Read("Enter something: ");

Console.WriteLine("Received: " + input);
Thread.Sleep(-1);

Showcase: GIF showing how you can use this answer

This solution also supports shortcuts like left and right arrow to move the cursor and handles text overflow when you reach the terminal's right border.

Warning: this solution does not replace/extend the official Console.ReadLine method and should still be considered as "hacky".

Neolatin