How can you set a default input value in a .net console 开发者_如何学运维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.

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.

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.

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. ;)

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

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();

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.

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;

You can use helper method like this:

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