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

    My two cents:
    I think it's important to realize there is a difference between "Data" and "Formatting". In this example you are kind of trying to store both in a data-only file. This will, as you can tell from other answers, change the nature of the data. (In other words cause it to be converted to a string. A CSV file is a data only file. You can do some tricks here and there to merge formatting in with data, but to my way of thinking this essentially corrupts the data by merging it with non-data values: ie: "Formatting".
    If you really need to be able to store formatting information I suggest that, if you have time to develop it out, you switch to a file type capable of storing formatting info separately from the data. It sounds like this problem would be a good candidate for a XML Spreadsheet solution. In this way you can not only specify your data, but also it's type and any formatting you choose to use.