Use the static method String.Format for form double string format in C#.
For three decimal places −
String.Format("{0:0.000}", 987.383); String.Format("{0:0.000}", 987.38); String.Format("{0:0.000}", 987.7899);
For thousands separator −
String.Format("{0:0,0.0}", 54567.46); String.Format("{0:0,0}", 54567.46);
To format string −
using System; class Demo { public static void Main(String[] args) { Console.WriteLine("Three decimal places..."); Console.WriteLine( String.Format("{0:0.000}", 987.383)); Console.WriteLine( String.Format("{0:0.000}", 987.38)); Console.WriteLine(String.Format("{0:0.000}", 987.7899)); Console.WriteLine("Thousands Separator..."); Console.WriteLine(String.Format("{0:0,0.0}", 54567.46)); Console.WriteLine(String.Format("{0:0,0}", 54567.46)); } }