尝试访问组合框wpf中的值时出现Null错误

Question

我正试图根据组合框中选择的内容进行一些计算，但我一直得到System.NullReferenceException："对象引用未设置为对象实例。"作为错误。

这是我的代码，错误出现在if语句的行中：

private void TokenWorthtxtBox_TextChanged(object sender, TextChangedEventArgs e)
{
if (BookCondition.SelectedItem == null || BookGenre.SelectedItem == null || BookFormat.SelectedItem == null || BookExamBoard.SelectedItem == null)//this is where the error appears
{
errormessage.Text = "Please select values from the drop down boxes.";
}
else
{
int tokenPrice = BookPriceCalculation(BookTitleTxtBox.Text, BookCondition.SelectedItem.ToString(), BookGenre.SelectedItem.ToString(), BookFormat.SelectedItem.ToString(), BookExamBoard.SelectedItem.ToString());
DisplaySuggestedTokentxtBlock.Text = $"Suggested Token: {tokenPrice}";
}
}

Answer 1

它在第一个"if"中控制的对象可以为null，因此它们以"？"结尾如果你添加它，你的问题就可以解决。

if (BookCondition?.SelectedItem == null || BookGenre?.SelectedItem == null || BookFormat?.SelectedItem == null || BookExamBoard?.SelectedItem == null)
{
errormessage.Text = "Please select values from the drop down boxes.";
}