SelectedIndex与comboboxes不成功 [英] SelectedIndex not successful with comboboxes

问题描述

我的Windows窗体有三个组合框。 Combobox2引用了combobox1的子表，Combobox3引用了combobox2的子表。



我希望三个组合框在用户选择值之前不显示selectedItem。



首次展示表格时我有这个设置：

My Windows form has three comboboxes. Combobox2 references a child table of combobox1, and Combobox3 references a child table of combobox2.

I want the three comboboxes not to show a selectedItem until the user selects a value.

I have this set when the Form is first shown:

private void Form1_Shown(Object sender, EventArgs e)
 {

     this.comboBox1.SelectedIndex = -1;
     MessageBox.Show("You are in the Form.Shown event.");

 }



private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
 {
     if ((isClosing == false)&&(comboBox1.SelectedValue!=null))
     {
         try
         {
             String cb1Value = comboBox1.SelectedValue.ToString();
               //fill child table code goes here

         }
         catch (Exception ex)
         { MessageBox.Show(ex.Message, ex.GetType().ToString() + " comboBox1_SelectedIndexChanged"); }
         try
         { this.comboBox2.SelectedIndex = -1; }//end try
         catch (Exception ex)
         {
             MessageBox.Show(ex.Message, ex.GetType().ToString() + "cb2 ind comboBox1_SelectedIndexChanged ");
         }//end catch
     }//end if
 }//end comboBox1_SelectedIndexChanged

但是当我选择combobox1时，combobox2仍然有一个选定的值！



此外，当这些选择完成时，我不断收到以下错误消息：

对象引用未设置为对象的实例。



有谁知道发生了什么？在此先感谢！

But combobox2 still has a selected value when I make a selection for combobox1!

In addition, I keep getting the following error message when these selections are made:
"Object reference not set to an instance of an object."

Does anyone know what is happening? Thanks in advance!

推荐答案

你的if语句中是否有拼写错误打开comboBox！_SelectedIndexChanged？你有 comboBox1.SelectedValue1 = null 我觉得它应该是 comboBox1.SelectedValue！= null 。

Is there a typo in your if statement that opens comboBox!_SelectedIndexChanged? You've got comboBox1.SelectedValue1=null and I'm thinking it should be comboBox1.SelectedValue!=null.

首先，我希望这不是另一个错字，但你有：

First, I hope this is not another typo, but you have:

if ((isClosing == false)&&(comboBox1.SelectedValue=null))

如果没有双等号，comboBox1.SelectedValue将被设置为null。



这将导致您的调用 String cb1Value = comboBox1.SelectedValue.ToString（）; 引发对象引用未设置为对象错误的实例。



至于comboBox2未被重置，我不确定。将SelectedIndex设置为-1应该这样做。虽然，你真的不需要它在try / catch块中。



设置SelectedIndex = -1将永远不会抛出错误，除非comboBox2已被设置为null某点。

Without the double equal sign, comboBox1.SelectedValue will be set to null.

That would result in your call String cb1Value = comboBox1.SelectedValue.ToString(); raising an object reference not set to an instance of an object error.

As far as comboBox2 not being reset, I'm not sure. Setting the SelectedIndex to -1 should do that. Though, you really don't need it in a try/catch block.

Setting SelectedIndex = -1 will never throw an error unless comboBox2 has been set to null at some point.

您可以设置SelectedIndex = -1，但总是提醒您，当您将其设置为-1时，即使似乎选择了一个项目，也无法从中获取其值SelectedItem因为它会返回null。



请在调用任何方法之前检查SelectedItem是否为null。



：rose：

You can set SelectedIndex = -1, but always remind you that when you set it to -1 even though one item will seem to be selected, you cannot get its value from SelectedItem as it will return you null.

Please check SelectedItem to be null before calling any method.

:rose:

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