"I see stuff like {0,-8:G2} passed in as a format string. What exactly does that do?" -- Very Confused String Formatter

The above format can be translated into this:

"{<argument index>[,<alignment>][:<formatString><zeros>]}"

argument index: This represent which argument goes into the string.

String.Format("first = {0};second = {1}", "apple", "orange");

String.Format("first = {1};second = {0}", "apple", "orange");

 

gives the following strings:

 

"first = apple;second = orange"

"first = orange;second = apple"

 

alignment (optional): This represent the minimal length of the string.

Postive values, the string argument will be right justified and if the string is not long enough, the string will be padded with spaces on the left.

Negative values, the string argument will be left justied and if the string is not long enough, the string will be padded with spaces on the right.

If this value was not specified, we will default to the length of the string argument.

 

String.Format("{0,-10}", "apple");      //"apple     "

String.Format("{0,10}", "apple");       //"     apple"

format string (optional): This represent the format code.

Numeric format specifier is available here. (e.g. C, G...etc.)
Datetime format specifier is available here.

Enumeration format specifier is available here.

Custom Numeric format specifier is available here. (e.g. 0. #...etc.)

 

Custom formatting is kinda hard to understand. The best way I know how to explain something is via code:

 

int pos = 10;

int neg = -10;

int bigpos = 123456;

int bigneg = -123456;

int zero = 0;

string strInt = "120ab";

 

String.Format("{0:00000}", pos);      //"00010"

String.Format("{0:00000}", neg);      //"-00010"

String.Format("{0:00000}", bigpos);   //"123456"

String.Format("{0:00000}", bigneg);   //"-123456"

String.Format("{0:00000}", zero);     //"00000"

String.Format("{0:00000}", strInt);   //"120ab"

String.Format("{0:#####}", pos);      //"10"

String.Format("{0:#####}", neg);      //"-10"

String.Format("{0:#####}", bigpos);   //"123456"

String.Format("{0:#####}", bigneg);   //"-123456"

String.Format("{0:#####}", zero);     //""

String.Format("{0:#####}", strInt);   //"120ab"

While playing around with this, I made an interesting observation:

 

String.Format("{0:X00000}", pos);      //"A"

String.Format("{0:X00000}", neg);      //"FFFFFFF6"

String.Format("{0:X#####}", pos);      //"X10"

String.Format("{0:X#####}", neg);      //"-X10"

 

The "0" specifier works well with other numeric specifier, but the "#" doesn't. Umm... I think the "Custom Numeric Format String" probably deserve a whole post of it's own. Since this is only the "101" post, I'll move on to the next argument in the format string.

 

 

zeros (optional): It actually has a different meaning depending on which numeric specifier you use.

 

int neg = -10;

int pos = 10;

// C or c (Currency): It represent how many decimal place of zeros to show.

String.Format("{0:C4}", pos);      //"$10.0000"

String.Format("{0:C4}", neg);      //"($10.0000)"

 

// D or d (Decimal): It represent leading zeros

String.Format("{0:D4}", pos);      //"0010"

String.Format("{0:D4}", neg);      //"-0010"

 

// E or e (Exponential): It represent how many decimal places of zeros to show.

String.Format("{0:E4}", pos);      //"1.0000E+001"

String.Format("{0:E4}", neg);      //"-1.0000E+001"

 

// F or f (Fixed-point): It represent how many decimal places of zeros to show.

String.Format("{0:F4}", pos);      //"10.0000"

String.Format("{0:F4}", neg);      //"-10.0000"

 

// G or g (General): This does nothing

String.Format("{0:G4}", pos);      //"10"

String.Format("{0:G4}", neg);      //"-10"

 

// N or n (Number): It represent how many decimal places of zeros to show.

String.Format("{0:N4}", pos);      //"10.0000"

String.Format("{0:N4}", neg);      //"-10.0000"

 

// P or p (Percent): It represent how many decimal places of zeros to show.

String.Format("{0:P4}", pos);      //"1,000.0000%"

String.Format("{0:P4}", neg);      //"-1,000.0000%"

 

// R or r (Round-Trip): This is invalid, FormatException is thrown.

String.Format("{0:R4}", pos);      //FormatException thrown

String.Format("{0:R4}", neg);      //FormatException thrown

 

// X or x (Hex): It represent leading zeros

String.Format("{0:X4}", pos);      //"000A"

String.Format("{0:X4}", neg);      //"FFFFFFF6"

 

// nothing: This is invalid, no exception is thrown.

String.Format("{0:4}", pos));      //"4"

String.Format("{0:4}", neg));      //"-4"

In summary, there are four types of behaviour when using this <zeros> specifier:

Leading Zeros: D, X

Trailing Zeros: C, E, F, N, P

Nothing: G

Invalid: R, <empty>

 

Now, that we've gone through the valid specifiers, you can actually use this in more than just String.Format(). For example, when using this with Byte.ToString():

 

Byte b = 10;

b.ToString("D4");      //"0010"

b.ToString("X4");      //"000A"

 

Wow... this was way longer than I expected. The BCL team is having blog day today, I need to get back to posting something for the BCLWeblog.

<Editorial Comment>

One of the lesson I learnt from an earlier post is that, readers are not interested in a post that doesn't give you more information than what MSDN provides. Instead, readers are more interested in seeing stuff that are not available on MSDN. So when I was doing research to post about this topic, I found that MSDN actually talks about exactly what the {0,-8:G2} format does. It is just not easy to find nor centrally located.

For example, in the ToString MSDN Doc, the "Remarks" section covered some basic rules on what a "format string" is. In the String.Format MSDN Doc, the "Remarks" section actually have a pretty detail explaination of what the above format does. Furthermore, MSDN provides a format string overview as well as a the table that specifies all the values that are allowed.

This puts me in an interesting position when writing about this topic. MSDN actually have lots of info that cover it. But since I have also heard more than one person being confused about this topic, I decided to post a summary of the documents and more examples. Do you think this is useful? Should I just stick to posting exclusively on non-MSDN topics?

</Editorial Comment>