当我使用comboBoxCities.Visibility = System.Windows.Visibility.Hidden时，为什么程序会崩溃;

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问题描述

你好

我有一个组合框，我有一个名为 comboBoxCountry_SelectionChanged的事件，当我将以下代码添加到程序崩溃时，当它试图启动时" comboBoxCities.Visibility = System.Windows.Visibility.Hidden;"

I have a combobox and I have an event for it called comboBoxCountry_SelectionChanged when I add the following code to event the program crashes when its trying to start "comboBoxCities.Visibility = System.Windows.Visibility.Hidden;"

我已粘贴下面的c＃和xaml代码

I have pasted the c# and xaml code below

C＃代码


  private void comboBoxCountry_SelectionChanged(object sender, SelectionChangedEventArgs e)
  {
   comboBoxCities.Visibility = System.Windows.Visibility.Hidden;
   labelCity.Visibility = System.Windows.Visibility.Hidden;

   if (comboBoxCountry.SelectedItem.ToString().Contains("United States"))
   {
    comboBoxCities.Visibility = System.Windows.Visibility.Visible;
    labelCity.Visibility = System.Windows.Visibility.Visible;
   }

   else if (comboBoxCountry.SelectedItem.ToString().Contains("Canada"))
   {
    comboBoxCities.Visibility = System.Windows.Visibility.Visible;
    labelCity.Visibility = System.Windows.Visibility.Visible;
   }
  }

推荐答案


private void comboBoxCountry_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
  if (comboBoxCities == null)
    return;

  comboBoxCities.Visibility = System.Windows.Visibility.Hidden;
  labelCity.Visibility = System.Windows.Visibility.Hidden;

  if (comboBoxCountry.SelectedItem.ToString().Contains("United States"))
  {
    comboBoxCities.Visibility = System.Windows.Visibility.Visible;
    labelCity.Visibility = System.Windows.Visibility.Visible;
  }

  else if (comboBoxCountry.SelectedItem.ToString().Contains("Canada"))
  {
    comboBoxCities.Visibility = System.Windows.Visibility.Visible;
    labelCity.Visibility = System.Windows.Visibility.Visible;
  }
}

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