String.Format on steroids:

• Supports everything what String.Format does (http://msdn.microsoft.com/en-us/library/txafckwd.aspx)
• Supports expressions in format strings - useful for plural forms
• Supports named arguments

You want to display number of messages. How?

Display(string.Format("You have {0} messages.", messageCount));

It will display "You have 0 messages.", "You have 2 messages.", "You have 1 messages.". Wait... "1 messages"? Is it plural???

So you change your code to:

if(messageCount == 1)
{
	Display("You have 1 message.");
}
else
{
	Display(string.Format("You have {0} messages.", messageCount));
}

If you have to support only one language you are done. However, if you have to support multiple languages then you may have something like this:

if(messageCount == 1)
{
	Display(Resources.YouHave1Message);
}
else
{
	Display(string.Format(Resources.YouHaveMessages, messageCount));
}

Where resources are resolved based on current culture and for our example they are defined as follows:

English version works, but for Czech, you now get "Máte 2 zpráv." This is not correct - it should be "Máte 2 zprávy." (same applies for 3 and 4 messages).

So you add new resource:

And change code to:

if(messageCount == 1)
{
	Display(Resources.YouHave1Message);
}
else if(messageCount > 1 && messageCount < 5)
{
	Display(string.Format(Resources.YouHave2To4Messages, messageCount));
}
else
{
	Display(string.Format(Resources.YouHaveMessages, messageCount));
}

It works, but imagine adding more languages, each having it's own rules... crazy, heh?

And what about this?

Display(Format.Evaluate(Resources.YouHaveMessages, messageCount));

Where resources are defined as:

Now you get "You have no message.", "You have 1 message.", "You have 5 messages.", "Máte 2 zprávy.", "Nemáte žádnou zprávu." etc. Mission accomplished!

You have {0 {=0:no message}{=1:1 message}{else:{0} messages}}.

• Display "You have "
• Take value of first argument (message count in our case)
  • if its value is 0
    • display "no message"
  • if its value is 1
    • display "1 message"
  • else
    • evaluate {0} messages (yes, you can nest expressions!)
• Display "."

{0} messages

• Display value of first argument (message count)
• Display " messages"

{0 {=0:Nemáte žádnou zprávu}{else:Máte {0} {0 {=1:zprávu}{>1<5:zprávy}{else:zpráv}}}}.

• Take value of first argument
  • if its value is 0
    • display "Nemáte žádnou zprávu"
  • else
    • evaluate Máte {0} {0 {=1:zprávu}{>1<5:zprávy}{else:zpráv}}
• Display "."

Máte {0} {0 {=1:zprávu}{>1<5:zprávy}{else:zpráv}}

• Display "Máte "
• Display value of first argument
• Display " "
• Take value of first argument
  • if its value is 1
    • display "zpráv"
  • if its value is greater than 1 and less than 5
    • display "zprávy"
  • else
    • display "zpráv"

{0 {=1:...}}

If value of first argument is 1 then...

{0 {=-1:...}}

If value of first argument is -1 then...

{0 {={1}:...}}

If value of first argument is equal to value of second argument then...

{0 {>=1<5:...}}

If value of first argument is greater or equal to 1 and less than 5 then...

{0 {=1,=5:...}}

If value of first argument is 1 or 5 then...

Most String.Format format strings are compatible with Format. Padding, format providers etc. are supported.

There is howevever one difference - escapes:

• In String.Format you have to double { and } to escape them - {{escaped}} gives "{escaped}".
• In Format you have to add backslash - \{escaped\} gives "{escaped}".

This change was necessary to support nesting.

Method calls are pretty same as of String.Format, so it is possible to do simple find & replace:

String.Format(IFormatProvider provider, string format, params object[] arguments)
String.Format(string format, params object[] arguments)

vs.

Format.Evaluate(IFormatProvider provider, string format, params object[] arguments)
Format.Evaluate(string format, params object[] arguments)

Format does more than String.Format, so yes, it is slower.

If you plan to reuse format expression you can "pre-compile" it, so it doesn't need to be parsed again:

var format = Format.Parse("...");

...
format.Evaluate(...);
...

You do not have to remember what {0} or {1} mean. Just use overload taking ArgumentCollection as its input:

var arguments = new ArgumentCollection();

arguments.Add(1); // argument without name - accessible as {0}
arguments.Add("MESSAGE_COUNT", 10); // argument with name, accessible both as {1} and {MESSAGE_COUNT}

Format.Evaluate("You have {MESSAGE_COUNT {=0:no message}...", arguments);