how to check string i sempty or not

Question

days textbox1 calendarimage(when i clik the month that day will be in textbox1)

payable textbox2

wagesamount textbox3 textbox4 when i type the wages amt in textbox3 that amt is divided by textbox1 and multipled by textbox2.i get the answer no problem.

but my problem when the textbox3 vlaue is empty the error messages shows input string was not in correct format.

my code in textbox3 changed has follows

double days = Convert.ToDouble( textbox1.text );

double payable = Convert.ToDouble( textbox2.text );

double a = Convert.ToDouble( textbox3.text );

textbox4.Text = Convert.ToDouble((a / days) * payable ).ToString();

how to avoid this error.from the below code how to change the code and get the answer correct.

please help me.urgent.

Satyapriya Nayak · Answer

If your problem is solved please click the checkbox(This is correct answer) Thanks

Kunal Vaishya · Answer

double days = Convert.ToDouble(Convert.ToString(textbox1.text.trim()) ); double payable = Convert.ToDouble(Convert.ToString(textbox2.text.trim()) ); double a = Convert.ToDouble(Convert.ToString(textbox3.text.trim()) ); textbox4.Text = Convert.ToDouble((a / days) * payable ).ToString();