Add comma to numbers every three digits using C#

Question

I want to add comma to decimal numbers every 3 digits using c#.
I wrote this code :

double a = 0; 
a = 1.5;
Interaction.MsgBox(string.Format(\"{0:#,###0}         


        

Answer 1

 double a = 1.5;
Interaction.MsgBox(string.Format("{0:#,###0.#}", a));