尝试在SelectionChanged方法中更改文本框文本时出现WPF TargetInvocationException

Question

So my code gives me a TargetInvocationException

link to screenshot of exception details

This is my code

public partial class MainWindow
    {
        Double totalPrice = 0;
        public MainWindow()
        {
            InitializeComponent();
        }

        private void txtDiscount_SelectionChanged(object sender, RoutedEventArgs e)
        {
           Double newTotalPrice = 0;
           if (!String.IsNullOrEmpty(txtDiscount.Text))
           {
               newTotalPrice = totalPrice - Double.Parse(txtDiscount.Text);

           }
           txtFullAmount.Text = newTotalPrice.ToString();

        }

The last line is the one thats making it crash, Im trying to assign a textbox to change whenever a different textbox changes.

The funny part is that this works perfectly if I change the title instead

this.Title = newTotalPrice.ToString();

Edit: Also my original code works if used with a button instead.

Edit <TextBox Background="#ecf0f1" IsReadOnly="True" Text="" Name="txtFullAmount" SelectionChanged="txtDiscount_SelectionChanged" HorizontalAlignment="Left" Height="23" Margin="505,511,0,0" TextWrapping="Wrap" VerticalAlignment="Top" Width="85"/>

Answer 1

I think you set a SelectedIndex in your XAML. In this case, the SelectionChanged event is raised before the window is completly initialized. Before the initialisation, the member variables for the controls are always null .

The easiest way is to check if the TextBox exists and return if it is null :

private void txtDiscount_SelectionChanged(object sender, RoutedEventArgs e)
{
    if(txtFullAmount == null) return;
    //your code
}

The assignment to the Title works, because this exists already at this moment.