C# 为什么不能设置动态创建的组合框的 selectedValue? [英] C# Why isn't it possible to set the selectedValue of a dynamically created combobox?
问题描述
我在类中创建了一个组合框,并想为该组合框设置选定的值.但是当我这样做时, selectedValue 保持为空,当我尝试设置 selectedIndex 时,我得到一个 ArgumentOutOfRangeException.
I create a combobox in a class and want to set the selected value for that combobox. But when I do that, the selectedValue stays null and when I try to set the selectedIndex I get a ArgumentOutOfRangeException.
代码:
public Control GenerateList(Question question)
{
// Clear the local givenAnswer collection
_givenAnswer.Clear();
// Get a list with answer possibilities
List<QuestionAnswer> answers = question.GetAnswerSort();
// Get a collection of given answers
Collection<QuestionnaireAnswer> givenAnswers = question.GetGivenAnswers();
_givenAnswer = givenAnswers;
ComboBox cmb = new ComboBox();
cmb.Name = "cmb";
cmb.DisplayMember = "Answer";
cmb.ValueMember = "Id";
cmb.DataSource = answers;
cmb.Dock = DockStyle.Top;
// Check an answer is given to the question
if (givenAnswers != null && givenAnswers.Count > 0)
{
cmb.Tag = givenAnswers[0].AnswerId;
cmb.SelectedValue = givenAnswers[0].AnswerId; // answerId = 55, but SelectedValue stays null
}
cmb.SelectedIndex = 1; // For testting. This will throw a ArgumentOutOfRangeException
cmb.DropDownStyle = ComboBoxStyle.DropDownList;
cmb.SelectedIndexChanged += new EventHandler(cmb_SelectedIndexChanged);
return cmb;
}
我希望有人能向我解释发生了什么,以便我理解为什么它不起作用.
I hope someone can explain to me what is happening so I can understand why it isn't working.
这是一个完整的小程序,可以说明我的问题.正如你所看到的,它没有设置 SelectedValue,它保持为 null
Here is a complete little program what illustrates my problem. As you can see it doesn't set the SelectedValue, this stays null
namespace Dynamic_Create_Combo
{
public partial class Form1 : Form
{
public Form1()
{
InitializeComponent();
GenerateControls gc = new GenerateControls();
Control c = gc.GenerateCombo();
this.SuspendLayout();
this.Controls.Add(c);
this.ResumeLayout(true);
}
}
public class GenerateControls
{
public Control GenerateCombo()
{
// Create datasource
Collection<Car> cars = new Collection<Car>();
Car c = new Car();
c.Id = 1;
c.Name = "Car one";
cars.Add(c);
Car c1 = new Car();
c1.Id = 2;
c1.Name = "Car two";
cars.Add(c1);
Car c2 = new Car();
c2.Id = 2;
c2.Name = "Car three";
cars.Add(c2);
ComboBox cmb = new ComboBox();
cmb.DropDownStyle = ComboBoxStyle.DropDownList;
cmb.DataSource = cars;
cmb.DisplayMember = "Name";
cmb.ValueMember = "Id";
cmb.SelectedValue = 2;
return cmb;
}
}
public class Car
{
private int _id;
private string _name;
public int Id
{
get { return _id; }
set { _id = value; }
}
public string Name
{
get { return _name; }
set { _name = value; }
}
}
}
推荐答案
您已将值成员设置为Id",但您正尝试使用AnswerId"作为所选值.
You've set the value member to be "Id" but you're trying to use "AnswerId" as the selected value.
没有更多细节,很难说为什么设置 SelectedIndex
会抛出 ArgumentOutOfRangeException
- 也许组合框忽略了所有没有Id"属性的值,因此没有给你任何值,所以选择索引 1 是不可能的?
Without more details, it's hard to say why setting SelectedIndex
is throwing an ArgumentOutOfRangeException
- perhaps the combobox is ignoring all values which don't have an "Id" property, thus giving you no values, so selecting index 1 is impossible?
好的,所以看起来它只是在它变得可见时才真正进行绑定 - 或者在过程的某个阶段.我已经尝试了一些方法来加速这一过程,但它们似乎没有帮助.您可以做的是推迟您的选择:
Okay, so it looks like it's only actually doing the binding when it becomes visible - or at some stage in the process. I've tried a few things to accelerate this, but they don't appear to help. What you can do is defer your selection:
EventHandler visibleChangedHandler = null;
visibleChangedHandler = delegate {
cmb.SelectedIndex = 2;
cmb.VisibleChanged -= visibleChangedHandler; // Only do this once!
};
cmb.VisibleChanged += visibleChangedHandler;
这是一个丑陋的解决方法,但它至少应该可以帮助您暂时开始.
It's an ugly workaround, but it should at least help you to get going for the moment.
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