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
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.
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