how to get the sum of values in 2 textboxes that gets updated C#

Question

I have 2 textboxes that get updated regularly, via serial port each of them displays data on 1 line, I want to get the sum of these values from these textboxes and put the sum into another text box.

Code:

value 1 and 2 are type string
textbox1.text = value1;
textbox2.text = value2;

double value3 = convert.ToDouble(value1) + convert.ToDouble(value2);
textbox3.text = value3.ToString();

Output textbox1:

100, after sometime updates to 200, then 300

Output textbox2:

50, after sometime updates to 100, then 150

What I'm getting, output in textbox3:

150, after some time updates to 300, then 450

The value I should get is

100+200+300+50+100+150

Answer 1

solved my own problem, ill post here in case someone else might be in the same position

used two lists got the sums and added the sums

List<double> totalvalueList = new List<double>();
totalvalueList.Add(Convert.ToDouble(value1));
            double totalval = totalvalueList.Sum();