C# Set combo item with selectedValue

8
votes

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;
}

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

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.

Is there something I don't see?

EDIT:

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?

Solution:

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.

6
c#winformscombobox

6 Answers

8
votes

Solution:

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.

3
votes

I met this weird issue before, at last I gave up and used another way:

cmb.Items.FindByValue(givenAnswers[0].AnswerId).Selected = true;

It worked fine...Hope you good luck!

3
votes
cmb.SelectedIndex = cmb.FindStringExact("Desired Value")

The cmb.FindStringExact("Desired String") returns the index of the value you would like to select and the cmb.SelectedIndex sets the combobox to that index.

Thanks to Billious for showing me the light!

FYI - This is the VB.NET Winforms Version.

1
votes

Are you looking at the same property?

 cmb.ValueMember = "Id"; 
 ..
 cmb.SelectedValue = givenAnswers[0].AnswerId;

You're refering to another ValueMember then the Id you're posting into the SelectedValue. Besides that you might want to try to set your Display- and Value-member before databinding. It's faster.

0
votes

Make sure QuestionAnswer has public accessors corresponding (same name) to the Display/Value Members you use.

0
votes

I ran into the same problem, and found that my issue was I was treating SelectedValue as an integer, when in actual fact it was an object. The "FindByValue" solution from Danny Chen above doesn't work in WinForms, so I tried using "FindStringExact" and searched on the DisplayMember:

cmb.Items.FindStringExact(<Display string>)

Not an ideal solution, but it worked.