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}", a));

But it returns 2.
Where am I wrong ?
Please describe how can I fix it ?

Answer 1

There is a standard format string that will separate thousand units: N

float value = 1234.512;

value.ToString("N"); // 1,234.512
String.Format("N2", value); // 1,234.51

Answer 2

Here is how to do it:

 string.Format("{0:0,0.0}", a)