How do I covert the value of a textbox to double

1.For converting from text box text to double you should use double.TryParse() method like below:

double number =0.0;
double.TryParse(T.Trim(), out number);
var AB= number * Simulation.B)

2.You could also test the result of the TryParse and if the conversion is not OK to use a default value like below:

double number =0.0;
if(double.TryParse(T.Trim(), out number) == false)
   number = 1.0; //Default value!
//
var AB= number * Simulation.B)