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

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?

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.

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.

In addition to the value you wish to print, the {0} {1}, etc., you can specify a format. For example, {0,4} will be a value that is padded to four spaces.

There are a number of built-in format specifiers, and in addition, you can make your own. For a decent tutorial/list see String Formatting in C#. Also, there is a FAQ here.

For future reference, in Visual Studio you can try placing the cursor in the method name (for example, WriteLine) and press F1 to pull up help on that context. Digging around should then find you String.Format() in this case, with lots of helpful information.

Note that highlighting a selection (for example, double-clicking or doing a drag-select) and hitting F1 only does a non-context string search (which tends to suck at finding anything helpful), so make sure you just position the cursor anywhere inside the word without highlighting it.

This is also helpful for documentation on classes and other types.

It's a placeholder for the first parameter, which in your case evaluates to "wordpad.exe".

If you had an additional parameter, you'd use {1}, etc.

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).

Nowadays, your method is not used very often in C#. Today, most programmers use the string interpolation ($) which is available in C# 6.

Your code is equal to

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

This is what we called Composite Formatting of the .NET Framework to convert the value of an object to its text representation and embed that representation in a string. The resulting string is written to the output stream.

> The overloaded Console.WriteLine Method (String, Object)Writes > the text representation of the specified object, followed by the > current line terminator, to the standard output stream using the > specified format information.