WPF - Cannot convert text from TextBox to Double [duplicate]

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• What is a NullReferenceException and how do I fix it? 18 answers

I am having trouble converting the text from a TextBox to Double. In Win Forms it was fairy simple to convert it using the ToDouble() method, however now in WPF I am having issues with this.

First of all in the Main Window I have an int declared to help Id-ing other controls, this int is incremented only when pressing a button in order to add extra controls. Also I have a double[] declared to pass information from textboxes.

This is the int and double declaration:

public partial class MainWindow : Window
    {
        int i = 0;
        double[] Fi;

Now in the button pressed method I am trying to pass the information to the array from the textbox:

Fi[i] = double.Parse(textBox15.Text);
            MessageBox.Show("" + Fi[i]);

This is the error message I get when debugging the code:

An unhandled exception of type 'System.NullReferenceException' occurred in WpfApplication2.exe

Additional information: Object reference not set to an instance of an object.

The textbox is not null since I am inputing value to it from the keyboard, in this stage i = 0. Any advise on this matter? It is the 3rd attempt to convert, I have also user so far:

Fi[i] = Convert.ToDouble(textBox15.Text);

also giving the same error.