Rashmi Pandit Rashmi Pandit - 3 months ago 15
C# Question

Setting selected item in a ListBox without looping

I have a multi-select listbox which I am binding to a DataTable. DataTable contains 2 columns description and value.

Here's the listbox populating code:

DataTable copytable = null;
copytable = GlobalTable.Copy(); // GlobalTable is a DataTable
copytable.Rows[0][0] = "--ALL--";
copytable.Rows[0][1] = "--ALL--";

breakTypeList.DataSource = copytable;
this.breakTypeList.DisplayMember = copytable.Columns[0].ColumnName; // description
this.breakTypeList.ValueMember = copytable.Columns[1].ColumnName; // value
this.breakTypeList.SelectedIndex = -1;


I am setting description as the DisplayMember and value as the ValueMember of the ListBox. Now depending on what the value is passed I need to set the selected item in the ListBox.

Here's my code:

ListBox lb = c as ListBox;
lb.SelectedValue = valuePassedByUser;


which is not working. Hence I have to resort to the code below (where I loop through all the items in the list box)

for (int i = 0; i < lb.Items.Count; i++)
{
DataRowView dr = lb.Items[i] as DataRowView;
if (dr["value"].ToString() == valuePassedByUser)
{
lb.SelectedIndices.Add(i);
break;
}
}


I would like to know what is missing/ erroneous in my code. Why is lb.SelectedValue = valuePassedByUser; selecting incorrect items?

Answer

Ok ... here comes hard-to-digest answer which I realized only yesterday. It's my mistake though that I didn't mention one important thing in my question because I felt it is irrelevant to problem at hand:

The data in the data table was not sorted. Hence I had set the listbox's Sorted property to true. Later I realized When the listbox's or even combo box's sorted property is set to true then the value member does not get set properly. So if I write:

lb.SelectedValue = valuePassedByUser;

it sets some other item as selected rather than settting the one whose Value is valuePassedByUser. In short it messes with the indexes.

For e.g. if my initial data is:

Index   ValueMember	DisplayMember
1          A            Apple
2          M            Mango
3          O            Orange
4          B            Banana

And I set sorted = true. Then the listbox items are:

Index   ValueMember	DisplayMember
1          A            Apple
2          B            Banana
3          M            Mango
4          O            Orange

Now if I want to set Banana as selected, I run the stmt:

lb.SelectedValue = "B";

But instead of setting Banana as selected, it sets Orange as selected. Why? Because had the list not been sorted, index of Banana would be 4. So even though after sorting index of Banana is 2, it sets index 4 as selected, thus making Orange selected instead of Banana.

Hence for sorted listbox, I am using the following code to set selected items:

private void SetSelectedBreakType(ListBox lb, string value)
{
    for (int i = 0; i < lb.Items.Count; i++)
    {
        DataRowView dr = lb.Items[i] as DataRowView;
        if (dr["value"].ToString() == value)
        {
            lb.SelectedIndices.Add(i);
            break;
        }
    }
}
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