Calculated value on textChanged event can't be updated to database

Question

I have invoice form bound to a bindingsource and bindingnavigator. In the form I have unit price, quantity and total textboxes. The total textbox text property is set to total.text = quantity.text * unitprice.text in the textChanged event of both quantity and unitprice text boxes. The result is displayed in the total textbox. But when i hit save button on the bindingnavigator it is set to null and not saved to database.

The code on save button is bindingsource.endEdit() bindingsource.update() When I used msgbox to see the value of total textbox before bindingsource.endEdit() it is the correct value. but after the code bindingsource.endEdit() it is null. I checked the databinding property and it is correct. What is the problem.