What does {0} mean when found in a string in C#?

Question

In a dictionary like this:

Dictionary<string, string> openWith = new Dictionary<string, string>();

openWith.Add("txt", "notepad.exe");
openWith.Add("bmp", "paint.exe");
openWith.Add("dib", "paint.exe");
openWith.Add("rtf", "wordpad.exe");

Console.WriteLine("For key = "rtf", value = {0}.", openWith["rtf"]);

The output is:

For Key = "rtf" value = wordpad.exe

What does the {0} mean?

Answer 1

You are printing a formatted string. The {0} means to insert the first parameter following the format string; in this case the value associated with the key "rtf".

For String.Format, which is similar, if you had something like

//            Format string                    {0}           {1}
String.Format("This {0}.  The value is {1}.",  "is a test",  42 ) 

you'd create a string "This is a test. The value is 42".

You can also use expressions, and print values out multiple times:

//            Format string              {0} {1}  {2}
String.Format("Fib: {0}, {0}, {1}, {2}", 1,  1+1, 1+2) 

yielding "Fib: 1, 1, 2, 3"

See more at http://msdn.microsoft.com/en-us/library/txafckwd.aspx, which talks about composite formatting.