What is ToString("N0") format?

Checkout the following article on MSDN about examples of the N format. This is also covered in the Standard Numeric Format Strings article.

Relevant excerpts:

//       Formatting of 1054.32179:
//          N:                     1,054.32 
//          N0:                    1,054 
//          N1:                    1,054.3 
//          N2:                    1,054.32 
//          N3:                    1,054.322 

When precision specifier controls the number of fractional digits in the result string, the result string reflects a number that is rounded to a representable result nearest to the infinitely precise result. If there are two equally near representable results:

• On the .NET Framework and .NET Core up to .NET Core 2.0, the runtime selects the result with the greater least significant digit (that is, using MidpointRounding.AwayFromZero).
• On .NET Core 2.1 and later, the runtime selects the result with an even least significant digit (that is, using MidpointRounding.ToEven).

This is where the documentation is:

http://msdn.microsoft.com/en-us/library/dwhawy9k.aspx

The numeric ("N") format specifier converts a number to a string of the form "-d,ddd,ddd.ddd…", where "-" indicates a negative number symbol if required, "d" indicates a digit (0-9) ...

And this is where they talk about the default (2):

http://msdn.microsoft.com/en-us/library/system.globalization.numberformatinfo.numberdecimaldigits.aspx

      // Displays a negative value with the default number of decimal digits (2).
      Int64 myInt = -1234;
      Console.WriteLine( myInt.ToString( "N", nfi ) );

Here is a good start maybe

Double.ToString()

Have a look in the examples for a number of different formating options Double.ToString(string)