Adding Multiple Rows to Datatable

Recently i have been trying to use Datatable to store my input data temporary but iam not able to do so. when i click on add new record button it overwrites my first column instead of adding a new row. I am using C# to code my webforms.
My point is that for a particular customer got more than one sales record. So i just wanted to store it in the datatable till the user finishes entering all the data. Thus Datatable will show the user whatever they had entered and if they had made any mistakes they can change it right way.

If anyone has any solutions to this then please to do so....

IsNumeric and ISNull Function

I am just wondering y there is no function such as IsNumeric and IsNull in C#. It makes life bit difficult while doing critical programming.
It just lets me to wonder.........?????