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

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

Answer 2

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;

Answer 3

You can use helper method like this:

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

Answer 4

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.