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?
It's a placeholder in the string.
For example,
string b = "world.";
Console.WriteLine("Hello {0}", b);
would produce this output:
Hello world.
Also, you can have as many placeholders as you wish. This also works on String.Format:
string b = "world.";
string a = String.Format("Hello {0}", b);
Console.WriteLine(a);
And you would still get the very same output.
It's a placeholder for a parameter much like the %s format specifier acts within printf.
You can start adding extra things in there to determine the format too, though that makes more sense with a numeric variable (examples here).