String format for Double in C#

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 −

Example

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));
   }
}