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.

Answer 1

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.

Answer 2

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. ;)

Answer 3

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();

Answer 4

You can use helper method like this:

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

Answer 5

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

Answer 6

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.