用了selectedValue C＃套装组合项目 [英] C# Set combo item with selectedValue

问题描述

我是动态创建一个像这样的组合框：

I am dynamically creating a combobox like this:

public Control GenerateList(Question question)
{
    // Get a list with answer possibilities
    List<QuestionAnswer> answers = question.GetAnswers();

    // Get a collection of given answers
    Collection<QuestionnaireAnswer> givenAnswers = question.GetFilledAnswers();

    ComboBox cmb = new ComboBox();
    cmb.Name = "cmb";
    cmb.DataSource = answers;
    cmb.DisplayMember = "Answer";
    cmb.ValueMember = "Id";

    // Check an answer is given to the question
    if (givenAnswers != null && givenAnswers.Count > 0)
    {
        cmb.SelectedValue = givenAnswers[0].AnswerId;

    }

    cmb.DropDownStyle = ComboBoxStyle.DropDownList;
    cmb.SelectedIndexChanged += new EventHandler(cmb_SelectedIndexChanged);
    cmb.Leave += new EventHandler(cmb_Leave);

    return cmb;
}

现在的问题是，当执行 cmb.SelectedValue = givenAnswers [0] .AnswerId; cmb.SelectedValue总是空。

The problem is,when executing cmb.SelectedValue = givenAnswers[0].AnswerId; cmb.SelectedValue is always null.

在调试和我探讨答案（数据源）我看到ID（ValueMember）是exactle一样AnswerId（if语句）。两者有相同的类型（长）和相同的价值，但停留的SelectedValue空。

When debugging and I explore answers (the datasource) I see that Id (ValueMember) is exactle the same as AnswerId (in the if statement). Both have the same type (long) and the same value, but SelectedValue stays null.

有什么事我不知道？

编辑：

它看起来像组合框保持为空。当我把 cmb.SelectedValue = givenAnswers [0] .AnswerId; 与 cmb.SelectedIndex = 0; 我得到一个ArgumentOutOfRangeException。而这个问题的答案集合数是2，所以数据源不为空...非常strenge呵呵

It looks like the combobox stays empty. When I replace cmb.SelectedValue = givenAnswers[0].AnswerId; with cmb.SelectedIndex = 0; I get an ArgumentOutOfRangeException. This while the answers collections count is 2. So the datasource isn't null... Very strenge huh?

解决方案：

的SelectedValue，SelectedIndex的，性能的SelectedItem不能设置直到控件添加到窗体。后该控件添加到窗体中，selectedValue，则 - 索引和-Item属性进行设置。

The SelectedValue, SelectedIndex, SelectedItem properties can't be set until the control is added to the form. After the control is added to the form, the selectedValue, -Index and -Item properties can be set.

推荐答案

解决方案：

的SelectedValue，SelectedIndex的，性能的SelectedItem不能设置直到控件添加到窗体。之后，控制被添加到形式中，selectedValue，则 - 索引和-Item属性可设置

The SelectedValue, SelectedIndex, SelectedItem properties can't be set until the control is added to the form. After the control is added to the form, the selectedValue, -Index and -Item properties can be set.

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