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.

Answer 2

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.