WinForms ComboBox data binding gotcha

Question

Assume you are doing something like the following

List<string> myitems = new List<string>
{
    "Item 1",
    "Item 2",
    "Item 3"
};

ComboBox box = new ComboBox();
box.DataSource = myitems;

ComboBox box2 = new ComboBox();
box2.DataSource = myitems

So now we have 2 combo boxes bound to that array, and everything works fine. But when you change the value of one combo box, it changes BOTH combo boxes to the one you just selected.

Now, I know that Arrays are always passed by reference (learned that when i learned C :D), but why on earth would the combo boxes change together? I don't believe the combo box control is modifying the collection at all.

As a workaround, don't this would achieve the functionality that is expected/desired

ComboBox box = new ComboBox();
box.DataSource = myitems.ToArray();

score 39 · Accepted Answer · edited Dec 24 '19 at 07:28

39

This has to do with how data bindings are set up in the dotnet framework, especially the BindingContext. On a high level it means that if you haven't specified otherwise each form and all the controls of the form share the same BindingContext. When you are setting the DataSource property the ComboBox will use the BindingContext to get a ConcurrenyMangager that wraps the list. The ConcurrenyManager keeps track of such things as the current selected position in the list.

When you set the DataSource of the second ComboBox it will use the same BindingContext (the forms) which will yield a reference to the same ConcurrencyManager as above used to set up the data bindings.

To get a more detailed explanation see BindingContext.

edited Dec 24 '19 at 07:28

Kols

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answered Aug 02 '08 at 17:18

Robert Höglund

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1

Seems like a lot of effort to provide no benefit and make data binding controls completely counter intuitive. – Belmiris Jun 01 '15 at 15:13
This explains why the problem occurs - but does not suggest any solution? – StayOnTarget Apr 24 '19 at 14:51

score 22 · Answer 2 · edited Oct 14 '12 at 12:35

A better workaround (depending on the size of the datasource) is to declare two BindingSource objects (new as of 2.00) bind the collection to those and then bind those to the comboboxes.

I enclose a complete example.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace WindowsFormsApplication2
{
    public partial class Form1 : Form
    {
        private BindingSource source1 = new BindingSource();
        private BindingSource source2 = new BindingSource();

        public Form1()
        {
            InitializeComponent();
            Load += new EventHandler(Form1Load);
        }

        void Form1Load(object sender, EventArgs e)
        {
            List<string> myitems = new List<string>
            {
                "Item 1",
                "Item 2",
                "Item 3"
            };

            ComboBox box = new ComboBox();
            box.Bounds = new Rectangle(10, 10, 100, 50);
            source1.DataSource = myitems;
            box.DataSource = source1;

            ComboBox box2 = new ComboBox();
            box2.Bounds = new Rectangle(10, 80, 100, 50);
            source2.DataSource = myitems;
            box2.DataSource = source2;

            Controls.Add(box);
            Controls.Add(box2);
        }
    }
}

If you want to confuse yourself even more then try always declaring bindings in the constructor. That can result in some really curious bugs, hence I always bind in the Load event.

Thank you for indicating that binding in the constructor results in some really curious bugs. I had a similar problem and I moved my code to the Load event and it showed the bound data. — bunggo, Oct 08 '13 at 19:47
I have found some of the weirdest unreported bugs with that. Worst case scenario being that a combobox refused to give its focus back to any other control :D — Quibblesome, Oct 09 '13 at 10:07

WinForms ComboBox data binding gotcha

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