C# ObjectType对象为null，即使向其传递了值

我的表单的SelectIndexchange有问题

例如，我在countryComboBox中添加了国家A和国家B

我现在可以使用countryComboBox的selectIndexChanged选择一个国家。 选择后，我可以添加状态。添加州后，它将显示在countryStateListBox中，然后我可以选择一个州将城镇添加到其中。城镇将显示在城镇列表框中

我遇到的问题是，如果我选择国家B，然后返回国家A，我可以在CountryStates列表框中查看州，但不能在townListBox中查看每个州的城镇。虽然我检查了内存并正确添加了它，但无法在townListbox上正确查看它

基本上，一旦我回到以前添加内容的国家，countryStateListBox\u SelectIndexChanged就会生效

countryStateListBox中的行\u SelectIndexChanged

CountryState state = country.CountryStates.Find(x => x.CountryStateName.Equals(countryStateListBox.SelectedItem));

这就是我的问题所在

比特：

country.CountryStates.Find（x=>x.CountryStateName.Equals（countryStateListBox.SelectedItem））

具有值，但未将其传递给对象“状态”。虽然它以前有用 换到另一个国家。有没有办法解决这个问题

我花了一整天的时间调试这个。 任何帮助都会很好的，泰

 private void countryComboBox_SelectedIndexChanged(object sender, EventArgs e)
    {           
        Country country = Countries.Find(x => x.CountryName.Equals(countryComboBox.SelectedItem));

        if (country != null)
        {
            countryStateListBox.Items.Clear();
            townListBox.Items.Clear();
            country.countryStateList.ForEach(x => countryStateListBox.Items.Add(x));
        }
    }




private void countryStateListBox_SelectedIndexChanged(object sender, EventArgs e)
    {
        Country country = Countries.Find(x => x.CountryName.Equals(countryComboBox.SelectedItem));
        CountryState state = country.CountryStates.Find(x => x.CountryStateName.Equals(countryStateListBox.SelectedItem));// problem here!

        if (state != null)
        {
            townListBox.Items.Clear();
            state.Towns.ForEach(x => townListBox.Items.Add(x));
        }
    }

---

您需要检测是用户触发了列表框上的更改，还是您通过编程触发了更改

private bool triggeredByUser = true ;

private void countryComboBox_SelectedIndexChanged(object sender, EventArgs e)
{           
    Country country = Countries.Find(x => x.CountryName.Equals(countryComboBox.SelectedItem));

    if (country != null)
    {
        triggeredByUser = false ;

        countryStateListBox.Items.Clear();
        //townListBox.Items.Clear();
        country.countryStateList.ForEach(x => countryStateListBox.Items.Add(x));

        //by default select the first state
        countryStateListBox.selectedIndex = 0 ;
        CountryState state = country.CountryStates.Find(x => x.CountryStateName.Equals(countryStateListBox.SelectedItem));

        //refresh the town list based on the country and state
        refreshTownList(country,state) ;

        triggeredByUser = true ;
    }
}




private void countryStateListBox_SelectedIndexChanged(object sender, EventArgs e)
{
    if(triggeredByUser)
    {
        Country country = Countries.Find(x => x.CountryName.Equals(countryComboBox.SelectedItem));
        CountryState state = country.CountryStates.Find(x => x.CountryStateName.Equals(countryStateListBox.SelectedItem));

        //refresh the town list based on the country and state
        refreshTownList(country,state) ;
    }
}

private refreshTownList(Country country,CountryState state)
{
    if (state != null)
    {
        townListBox.Items.Clear();
        state.Towns.ForEach(x => townListBox.Items.Add(x));
    }
}

---

未填充townListBox的问题是因为在您的countryComboBox\u SelectedIndexChanged方法中，有一行用于填充countryStateListBox，而不是townListBox。

---

在countryComboBox\u SelectedIndexChanged方法中添加一行填充townListBox。

我将尝试一下，看看会发生什么。感谢您的回复请记住Peter Trypsteen在其回答中提到的内容，这似乎也与“country.countryStateList.ForEach（x=>countryStateListBox.Items.\u Add（x））；”和'triggeredByUser=true；'我添加了

foreach（country.CountryStateList中的var项）{CountryState state=country.CountryStates.Find（x=>x.CountryStateName.Equals（item.ToString（））；if（state！=null）{townListBox.Items.Clear（）；state.Towns.foreach（x=>townListBox.Items.Add（x））；}

Hi Peter，我实际上尝试过在countryComboBox\u SelectedIndexChanged方法中用另一行代码填充townListBox。当您在ComBoBox中更改country时，它会起作用，因为您可以看到它会更改，但一旦您选择或单击countryStateListBox，townListBox将再次变为空