C# 4 and .NET 4

examining system.string ❘ 213
As you probably know, you need to specify the format in which you want a variable displayed when you
call Console.WriteLine(). Therefore, this section uses this method as an example, although most of the
discussion applies to any situation in which you want to format a string. For example, if you want to display
the value of a variable in a list box or text box, you will normally use the String.Format() method to
obtain the appropriate string representation of the variable. However, the actual format specifiers you use to
request a particular format are identical to those passed to Console.WriteLine(). Hence, you will focus
on Console.WriteLine() as an example. You start by examining what actually happens when you supply
a format string to a primitive type, and from this, you will see how you can plug format specifiers for your
own classes and structs into the process.
Chapter 2, “Core C#,” uses format strings in Console.Write() and Console.WriteLine() like this:
double d = 13.45;
int i = 45;
Console.WriteLine("The double is {0,10:E} and the int contains {1}", d, i);
The format string itself consists mostly of the text to be displayed, but wherever there is a variable to be
formatted, its index in the parameter list appears in braces. You might also include other information inside
the braces concerning the format of that item. For example, you can include:
➤
➤
The number of characters to be occupied by the representation of the item, prefixed by a comma. A
negative number indicates that the item should be left-justified, whereas a positive number indicates
that it should be right-justified. If the item actually occupies more characters than have been
requested, it will still appear in full.
A format specifier, preceded by a colon. This indicates how you want the item to be formatted. For
example, you can indicate whether you want a number to be formatted as a currency or displayed in
scientific notation.
The following table lists the common format specifiers for the numeric types, which were briefly discussed
in Chapter 2.
sPeCifier aPPlies To meaning eXamPle
C Numeric types Locale-specific monetary
value
D Integer types only General integer 4834
E Numeric types Scientific notation 4.834E+003
F Numeric types Fixed-point decimal 4384.50
G Numeric types General number 4384.5
N Numeric types Common locale-specific
format for numbers
P Numeric types Percentage notation 432,000.00%
$4834.50 (USA) £4834.50 (UK)
4,384.50 (UK/USA)
4 384,50 (continental Europe)
X Integer types only Hexadecimal format 1120 (If you want to display 0x1120, you
will have to write out the 0x separately)
If you want an integer to be padded with zeros, you can use the format specifier 0 (zero) repeated as many
times as the number length is required. For example, the format specifier 0000 will cause 3 to be displayed
as 0003, and 99 to be displayed as 0099, and so on.
It is not possible to give a complete list, because other data types can add their own specifiers. Showing how
to define your own specifiers for your own classes is the aim of this section.
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