C＃，将String转换为double的最佳方法是什么？

Question

I've been looking at already answered solutions to this questions but none of them seem to work with my code. I've only just started coding in C# and I'm confused how my code isn't working.

using System;

namespace dt
{
  class Averager 
  {
    static void Main() 
    {
      var total = 0.0;
      int runningNumbers = 0;

      while(true) 
      {
        Console.Write("Enter a number or type \"done\" to see the average: ");

        if(Console.ReadLine().ToLower() == "done") 
        {
          var average = (total / runningNumbers);
          Console.Write("Average: " + average);
          break;
        }
        else 
        {
          var tempNew = Double.Parse(Console.ReadLine());
          total += tempNew;
          runningNumbers += 1;
          continue;
        }

      }
    }
  }
}

In my code above, I'm trying to convert an inputted string to a double. When I run the program, it loops once and the the program freezes. If I type in the console, it gives me this error:

Unhandled Exception:

System.FormatException: Input string was not in a correct format.
at System.Number.ParseDouble (System.String value, System.Globalization.NumberStyles options, System.Globaliz
at System.Double.Parse (System.String s, System.Globalization.NumberStyles style, System.Globalization.Number
at System.Double.Parse (System.String s, System.IFormatProvider provider) [0x0000c] in at dturato.Averager.Main () [0x00058] in <249eeff07c4a4744a6b024a0c9b6c23b>:0
[ERROR] FATAL UNHANDLED EXCEPTION: System.FormatException: Input string was not in a correct format.
at System.Number.ParseDouble (System.String value, System.Globalization.NumberStyles options, System.Globaliz
at System.Double.Parse (System.String s, System.Globalization.NumberStyles style, System.Globalization.Number
at System.Double.Parse (System.String s, System.IFormatProvider provider) [0x0000c] in at d.Averager.Main () [0x00058] in <249eeff07c4a4744a6b024a0c9b6c23b>:0**

This , from what I've researched so far, means the String isn't converting to a double for some reason. If anyone could help or give any solutions that would be great, thanks.

Answer 1

You're calling ReadLine twice! The first time is after the prompt. The second is in the else clause of your if statement. The second one is likely null and causes the error when trying to convert to double. You need to store the value of your input to a variable:

using System;

namespace dt
{
  class Averager 
  {
    static void Main() 
    {
      var total = 0.0;
      int runningNumbers = 0;

      while(true) 
      {
        Console.Write("Enter a number or type \"done\" to see the average: ");
        string input = Console.ReadLine();

        if(input.ToLower() == "done") 
        {
          var average = (total / runningNumbers);
          Console.Write("Average: " + average);
          break;
        }
        else 
        {
          var tempNew = Double.Parse(input);
          total += tempNew;
          runningNumbers += 1;
          continue;
        }

      }
    }
  }
}

Answer 2

The error tells you the exact problem you're having.

Input string was not in a correct format.

You need to make sure the input into the parse method is valid. In most cases, you will find a TryParse variant of the Parse methods. In this case, double.TryParse does just that.

using System;

public class Example
{
   public static void Main()
   {
      string value;
      double number;

      value = Double.MinValue.ToString();
      if (Double.TryParse(value, out number))
         Console.WriteLine(number);
      else
         Console.WriteLine("{0} is outside the range of a Double.", 
                           value);

      value = Double.MaxValue.ToString();
      if (Double.TryParse(value, out number))
         Console.WriteLine(number);
      else
         Console.WriteLine("{0} is outside the range of a Double.",
                           value);
   }
}
// The example displays the following output:
//    -1.79769313486232E+308 is outside the range of the Double type.
//    1.79769313486232E+308 is outside the range of the Double type.   

To reference back to you code, you're initial problem is you're reading the input multiple times: Try this

using System;

namespace dt
{
  class Averager 
  {
    static void Main() 
    {
      var total = 0.0;
      int runningNumbers = 0;

      while (true)
        {
            // Ask for user for a new input
            Console.Write("Enter a number or type \"done\" to see the average: ");
            var line = Console.ReadLine();

            // Try to parse the input
            double value;
            if(double.TryParse(line, out value)) {
                total += value;
                runningNumbers += 1;
                continue;
            } else if(line.ToLower() == "done")
            {
                var average = (total / runningNumbers);
                Console.Write("Average: " + average);
                break;
            } else {
                Console.Write("Invalid input. Please try again.");
                continue;
            }

        }
    }
  }
}

Answer 3

I would just stuff it into a double right off the bat -- using "Convert.toDouble(yourValue). It appread that you are already doing your checking to see if you will be getting a numeric value or a string so why not skip the var and make it a double straight away?

using System;

 namespace dt{
    class Averager
        static void Main(){
  var total = 0.0;
  int runningNumbers = 0;

  while(true) 
  {
    Console.Write("Enter a number or type \"done\" to see the average: ");
    var test = Console.Readline();
    if(test.ToLower() == "done") 
    {
      var average = (total / runningNumbers);
      Console.Write("Average: " + average);
      break;
    }
    else 
    {
      double tempNew = Convert.toDouble(test);
      total += tempNew;
      runningNumbers += 1;
      continue;
    }

   }
  }
 }
}