Exponential Number Conversion Not Working Properly c#

HI,

This s My Exponential Number : 9.94092e+009

Exponential Number's Original Data is : 9940915145(Phone No)

I converting Exponential Number to String as per giving search result

 Double d = Convert.ToDouble(9.94092e+009);
            MessageBox.Show(d.ToString());

Output :

But I need to get original value after convert...what i did wrong in the flow

this is my problem anyone know the solution for this

Thanks & Regards,