How to show long numbers in Excel?

Question

I have to build a C# program that makes CSV files and puts long numbers (as string in my program). The problem is, when I open this CSV file in Excel the numbers appear like

Answer 1

You can't. Excel stores numbers with fifteen digits of precision. If you don't mind not having the ability to perform calculations on the numbers from within Excel, you can store them as Text, and all of the digits will display.