See the Standard and Custom Numeric Format Strings. I think you're looking for ".################":
.00005.ToString(".################") // returns ".00005"
It's somewhat hackish, yes, but it works, as long as you don't have more than that many places after the decimal. If you do, you might want to use a StringBuilder to build out about 330 #s in your string (double.Epsilon is on the order of E-324).
