(How)
C^# You Are - 6 May 2007

This week's questions are all about formatting values as strings.

1. How do you specify a precision when formatting a float value as a string?  Answer

   The numeric precision is specified as a number after the format specifier.  Depending on the specifier used the value is truncated, rounded or left unchanged if the precision is less than the actual value.

   int value = 123;
   
   Console.WriteLine(String.Format("{0:d4}", value));  //Prints 0123
   Console.WriteLine(String.Format("{0:d2}", value));  //Prints 123
   
   Console.WriteLine(String.Format("{0:f4}", value));  //Prints 0123.0000
   Console.WriteLine(String.Format("{0:f2}", value));  //Prints 123.00
   
   Console.WriteLine(String.Format("{0:g4}", value));  //Prints 123
   Console.WriteLine(String.Format("{0:g2}", value));  //Prints 12.e+02
   

2. How do you add thousands separators to an integral value when formatting as a string?  Answer

   The general number (n) specifier is used to add thousands separators to the number.

   int value = 12345678;
   Console.WriteLine(String.Format("{0:n}", value));     //Prints: 12,345,678.00
   Console.WriteLine(String.Format("{0:n0}", value));   //Prints: 12,345,678
   

3. How do you ensure that a number is at least 3 digits in size (using zeroes to fill in the number) when formatting as a string?  Answer

   The d3 specifier will ensure that the number is at least 3 digits in size and add zeros to ensure that it is accurate.  An alternative is to use the 0 specifier to specify the exact zeroes to use.

   int value = 123;
   Console.WriteLine(String.Format("{0:000}", value));    //Prints: 123
   Console.WriteLine(String.Format("{0:0000}", value));  //Prints: 0123

4. How do you make a date/time value format itself using the local time format settings?  Answer

   The date/time will automatically format itself using the local time format settings.  However if the time is in UTC then the UTC value will be shown in the local time format but it will still represent the UTC time value.

   Console.WriteLine(String.Format("{0}", DateTime.Now));      //Prints: May 6, 2007 4:32:00 PM
   Console.WriteLine(String.Format("{0}", DateTime.UtcNow));  //Prints: May 6, 2007 8:32:00 PM

5. How do you make a date/time value format itself in a manner that it can be round-tripped back to the original value?  Answer

   Most date/time formats can be round-tripped back from a string due to the flexibility of the parser.  However to truly ensure the value gets translated properly you should use a culture-neutral format like R which uses a standards-based format.  Technically the format uses a culture invariant format but it should still translate properly.

   Console.WriteLine(String.Format("{0:R}", DateTime.Now));  //Prints: Sun, 06 May 2007 16:32:00 GMT

   For completeness, use the R specifier for other numeric values.

6. How do you display the milliseconds of a date/time value when formatting it as a string?  Answer

   Technically you can't (at least based on the documentation).  Instead the documentation refers to fractional seconds which just happen to line up with the milliseconds.  Use the f specifier to indicate a digit of the milliseconds.  You can specify up to 7 of them.

   DateTime dt = DateTime.Now;   //Milliseconds = 906
   Console.WriteLine(String.Format("{0}, {1:fffffff}", dt.Millisecond,dt));  //Prints: 906, 9062500

7. How do you format a float value as a monetary value when converting to a string?  Answer

   Currency is identified using the Cor c specifiers.  It uses the current locale settings to format the currency value.

   float cost = 3.57F;
   Console.WriteLine(String.Format("{0:c}", cost));   //Prints $3.57

8. How do you display an integral value in hexadecimal when formatting as a string?  Answer

   Use the X for upper-case hex digits or x for lower-case digits. 

   int value = 123456;
   Console.WriteLine(String.Format("{0:X}", value));  //Prints: 1E240
   Console.WriteLine(String.Format("0x{0:X}", value));  //Prints: 0x1E240
   

   Ironically .NET does not support any additional base specifiers such as binary or octal.

9. How do you right-justify an integral value to 10 characters?  Answer

   Use the alignment specifier to specify a value of 10.  The alignment specifier follows the index of the specifier but before the format specifier and preceded by a comma.

   int value = 123456;
   Console.WriteLine(String.Format("{0,4}", value));   //Prints: 123456
   Console.WriteLine(String.Format("{0,9}", value));   //Prints:       123456

10. How do you format an integral value as a normal number but in parenthesis when it is negative?  Answer

    Formatting supports three different sections of specifiers (separated by semicolons).  The first section represents positive numbers, the second negative numbers and the third zero.  Therefore you can use different formatting for each of the three possible values.

    int value = 123;
    int value2 = -123;
    Console.WriteLine(value.ToString("###;(###);Zero"));   //Prints: 123
    Console.WriteLine(value2.ToString("#;(###);Zero"));     //Prints: (123)

    Notice that in the last example is used the ToString method instead of String.Format.  String.Format does not support (from what I can tell) multiple sections.  Instead it will simply print out the text and move on.