单击RadioButton时，对象引用未设置为对象的实例

Question

我有两个RadioButtons

 private void CarRadioButton_Checked(object sender, RoutedEventArgs e)
        {
            try
            {

                // When CarRadioButton is clicked. "Leasing" and "Sale" is added 
                   to Combobox

                ContractComboBox.Items.Clear();

                ContractComboBox.Items.Add("Leasing");
                ContractComboBox.Items.Add("Sale");


           }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }

        }

        private void TruckRadioButton_Checked(object sender, RoutedEventArgs e)
        {
           try
            {
                 // When TruckRadioButton is clicked. "Leasing" is added 
                   to Combobox

                ContractComboBox.Items.Clear();

                ContractComboBox.Items.Add("Leasing");

            }
            catch (Exception ex)
            {
             MessageBox.Show(ex.Message);
            }

        }

当我从ComboBox中选择一个项目时，我想做一些事情，如下所示：

  private void ContractComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
        {
            if (ContractComboBox.SelectedItem.ToString() == "Leasing")
            {
                PriceMonthTextBox.IsEnabled = true;
                PeriodTextBox.IsEnabled = true;
            }

            if (ContractComboBox.SelectedItem.ToString() == "Sale")
            {
                PriceMonthTextBox.IsEnabled = false;
                PeriodTextBox.IsEnabled = false;
            }

        }    

现在这是我的问题。如果我从ComboBox中选择一个项目，然后点击另一个RadioButton，我会收到错误：Object reference not set to an instance of an object.

我做的步骤导致例外：

1. CarRadioButton点击。
2. 从ComboBox
挑选“租赁”
3. 点击TruckRadioButton
4. 获取错误：Object reference not set to an instance of an object。

有什么想法吗？我认为它有点问题，但我无法发现它:(

Answer 1

为什么不通过对SelectedItem的简单检查来保护SelectedIndexChanged中的代码？

private void ContractComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    if (ContractComboBox.SelectedItem != null)
    {
        if (ContractComboBox.SelectedItem.ToString() == "Leasing")
        {
            PriceMonthTextBox.IsEnabled = true;
            PeriodTextBox.IsEnabled = true;
        }

        if (ContractComboBox.SelectedItem.ToString() == "Sale")
        {
            PriceMonthTextBox.IsEnabled = false;
            PeriodTextBox.IsEnabled = false;
        }
    }
}    

Answer 2

试试这个：

private void ContractComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
      if(ContractComboBox.SelectedIndex >= 0)
      {
        if (ContractComboBox.SelectedItem.ToString() == "Leasing")
        {
            PriceMonthTextBox.IsEnabled = true;
            PeriodTextBox.IsEnabled = true;
        }

        if (ContractComboBox.SelectedItem.ToString() == "Sale")
        {
            PriceMonthTextBox.IsEnabled = false;
            PeriodTextBox.IsEnabled = false;
        }
      }
}