Display textbox value as decimal from Code behind as "10.20"

Hi,

Here is the simple code the show the text box values in the format 0.00

Let take textbox1, I am going to do some calculation and display the output in the format 0.00
Double a=10.2;
Double b=10;
Double sum=a+b;
textbox1.Text=sum.ToString("N2");

or

textbox1.Text=sum.ToString("#0.00");

Output : 20.20

textbox1.Text =sum.ToString("N2"); 

textbox1.Text =sum.ToString("#0.00");