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I couldn’t find a quick reference to .NET string formatting using the String.Format() function, so I created this one (which has also spawned this String Formatting FAQ).

When I started working with the .NET framework, one thing puzzled me. I couldn’t find sprintf(). sprintf() is the C function that takes an output buffer, a format string, and any number of arguments, and builds a string for you. For example:

char szError[256];
sprintf(szError, “Error %d occurred.\n”, nError);

This would write “Error 12 occurred.” into the szError buffer (assuming nError was 12). It’s a basic part of C programming and most C++ programmers still use it though better functionality is available in the STL because sprintf is simple to use and clear. The STL equivalent would be:

str << “Error ” << nError << ” occurred.” << endl;

Or something close to that. It’s type-safe, and more OO than sprintf, but not as easy to read and not as easy to localize.

The .NET framework handles strings very nicely – but it takes some getting used to. The rough equivalent of sprintf in .NET is the static String.Format function, which takes a format string and some arguments, and generates an output string. (This is a nice improvement over sprintf since there’s no chance you’ll overflow the output buffer). For example:

string errorString = String.Format(“Error {0} occurred.”, nError);

Teeming with metadata, the .NET environment doesn’t need the format string to say what type of data you’re formatting, just where you want it. (A common sprintf bug is supplying the wrong data type – there’s no protection from using %s instead of %d and having your program crash when sprintf is called).

The {0} in the string above is replaced with the value of nError, but what if you want to specify the number of digits to use? Or the base (hexadecimal etc)? The framework supports all this, but where it seemed confusing is that it’s not the String.Format function that does the string formatting, but rather the types themselves.

Every object has a method called ToString that returns a string representation of the object. The ToString method can accept a string parameter, which tells the object how to format itself – in the String.Format call, the formatting string is passed after the position, for example, “{0:##}”

The text inside the curly braces is {index[,alignment][:formatString]}. If alignment is positive, the text is right-aligned in a field the given number of spaces; if it’s negative, it’s left-aligned.

Strings

There really isn’t any formatting within a string, beyond it’s alignment. Alignment works for any argument being printed in a String.Format call.

Numbers

Basic number formatting specifiers:

Custom number formatting:

The group separator is especially useful for formatting currency values which require that negative values be enclosed in parentheses. This currency formatting example at the bottom of this document makes it obvious:

Dates

Note that date formatting is especially dependant on the system’s regional settings; the example strings here are from my local locale.

The ‘U’ specifier seems broken; that string certainly isn’t sortable.

Custom date formatting:

Enumerations

Some Useful Examples

String.Format(“{0:$#,##0.00;($#,##0.00);Zero}”, value);

This will output “$1,240.00″ if passed 1243.50. It will output the same format but in parentheses if the number is negative, and will output the string “Zero” if the number is zero.

String.Format(“{0:(###) ###-####}”, 8005551212);

This will output “(800) 555-1212″.

Reference：http://blog.stevex.net/string-formatting-in-csharp/