c# - ComboBox will not update its display list unless you change selections first

Question

Update: I checked the answer before I fully tested it still does not work. I updated the code below so you should just be able to paste in to a empty WinForms project and it should compile.

UPDATE: I have found that if I change the selected item on the ComboBox to any other item, it now behaves as expected (in my code below I would switch from test1 to test2). As I have not received any answers yet, I change the question to this.

Why do I have to change to a different item in the combo box before it will show the changes I make to the underlying data-source?

Here is a quick test case of what is happening.

1. Change test1 to test1asdf text in txtBroken
2. click off to commit change
3. text in combo box does not update.
4. Change combo box to test2
5. change test2 to test2asdf text in txtBroken
6. click off to commit change
7. text in combo box immediately shows 'test2asdf' still displays test1 for first item in the drop-down
8. change to test1
9. combo box displays test1 text box displays test1asdf
10. update text box to test1asd
11. combo box immediately displays test1asd

Other than behind the scenes changing the selected item on load and changing it back (this seems like such a hack) how can I fix this?

---

I have a combo box databound to a BindingSource bound to a List<Holder> it has Holder.Name as its display value. I also have a text box bound to Holder.Name but if I change the text in the text box it will not change what is displayed in the combo box. Changing selected items and changing back will show the updated text in the text box, but will still have the old value displayed in the combo box. How do I make the item in the combo box update?

using System;
using System.ComponentModel;
using System.Windows.Forms;

namespace Sandbox_Form
{
    public class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            lstBroken = new BindingList<Holder>();
            lstBroken.Add(new Holder("test1"));
            lstBroken.Add(new Holder("test2"));
            bsBroken = new BindingSource(lstBroken, null);
            cmbBroken.DataSource = bsBroken;
            cmbBroken.DisplayMember = "Name";
            cmbBroken.SelectedIndex = 0;
            txtBroken.DataBindings.Add("Text", bsBroken, "Name");
            txtBroken.TextChanged += new EventHandler(txtBroken_TextChanged);

        }

        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new Form1());
        }

        void txtBroken_TextChanged(object sender, EventArgs e)
        {
            ((Control)sender).FindForm().Validate();
        }
        private BindingSource bsBroken;
        private BindingList<Holder> lstBroken;
        private ComboBox cmbBroken;
        private TextBox txtBroken;
        private Label label1;
        /// <summary>
        /// Required designer variable.
        /// </summary>
        private System.ComponentModel.IContainer components = null;

        /// <summary>
        /// Clean up any resources being used.
        /// </summary>
        /// <param name="disposing">true if managed resources should be disposed; otherwise, false.</param>
        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        #region Windows Form Designer generated code

        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InitializeComponent()
        {
            this.cmbBroken = new System.Windows.Forms.ComboBox();
            this.txtBroken = new System.Windows.Forms.TextBox();
            this.label1 = new System.Windows.Forms.Label();
            this.SuspendLayout();
            // 
            // cmbBroken
            // 
            this.cmbBroken.DropDownStyle = System.Windows.Forms.ComboBoxStyle.DropDownList;
            this.cmbBroken.FormattingEnabled = true;
            this.cmbBroken.Location = new System.Drawing.Point(12, 32);
            this.cmbBroken.Name = "cmbBroken";
            this.cmbBroken.Size = new System.Drawing.Size(94, 21);
            this.cmbBroken.TabIndex = 0;
            // 
            // txtBroken
            // 
            this.txtBroken.Location = new System.Drawing.Point(13, 60);
            this.txtBroken.Name = "txtBroken";
            this.txtBroken.Size = new System.Drawing.Size(93, 20);
            this.txtBroken.TabIndex = 1;
            // 
            // label1
            // 
            this.label1.AutoSize = true;
            this.label1.Location = new System.Drawing.Point(13, 13);
            this.label1.Name = "label1";
            this.label1.Size = new System.Drawing.Size(41, 13);
            this.label1.TabIndex = 2;
            this.label1.Text = "Broken";
            // 
            // Form1
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(284, 262);
            this.Controls.Add(this.label1);
            this.Controls.Add(this.txtBroken);
            this.Controls.Add(this.cmbBroken);
            this.Name = "Form1";
            this.Text = "Form1";
            this.ResumeLayout(false);
            this.PerformLayout();

        }

        #endregion

        private void cmbWorks_SelectedIndexChanged(object sender, EventArgs e)
        {

        }
    }
    public class Holder
    {
        public Holder(string name)
        {
            Name = name;
        }
        private string _Name;
        public string Name
        {
            get { return _Name; }
            set
            {
                _Name = value;
            }
        }
    }
}

If I bind to a List<String> instead using Holder.Name it works as expected (this is just a simple mock-up, the real class has more than just a name so a list of strings will not work). I think this is a clue to what is wrong but I don't know what it is. Using an Observable instead of a list makes no difference.

Answer 1

Use a BindingList instead of a List. It was designed to address such issues. Dinesh Chandnani, a member of the .NET Client Team, states the following in a blog post:

BindingList<T> is the new generic implementation of IBindingList which fires ListChanged event when items are added/removed/inserted/etc. from the list. bindingSource hooks on to these events and is thus “aware” of these changes and can notify controls bound thos this BindingSource.

I was able to reproduce the issue you described in your updated entry, but didn't quite reproduce the original issue without tweaking the code slightly.

By using a BindingList<Holder> I was able to get an immediate response when focus left the textbox. It's possible to get instant updates by using an overloaded method when adding a new data binding. I also set the BindingSource's DataSource directly since using a null dataMember in the overloaded constructor was not yielding the expected behavior.

Here's the code I ended up with based off your sample code:

public partial class Form1 : Form
{
    private BindingSource bs;
    private BindingList<Holder> bList;

    public Form1()
    {
        InitializeComponent();

        bList = new BindingList<Holder>();
        bList.Add(new Holder("test1"));
        bList.Add(new Holder("test2"));

        bs = new BindingSource();
        bs.DataSource = bList;

        cmb.DataSource = bs;
        cmb.DisplayMember = "Name";
        cmb.ValueMember = "Name";

        // updates when focus leaves the textbox
        txt.DataBindings.Add("Text", bs, "Name");

        // updates when the property changes
        //txt.DataBindings.Add("Text", bs, "Name", false, DataSourceUpdateMode.OnPropertyChanged);
    }
}

Comment out the first txt binding and uncomment the one below it to see the DataSourceUpdateMode.OnPropertyChanged in action.

Here are some BindingList resources:

• BindingSource and BindingList Of T - DataBinding made simple!
• BindingSource - A Closer Look...
• Databinding - Bindinglist, BindingSource and BusinessObjects - Part 1
• Databinding - BindingList, BindingSource, and BusinessObjects: Part 2
• Behind the Scenes: Improvements to Windows Forms Data Binding in the .NET Framework 2.0, Part 2

  ---

  EDIT: in reply to the issues you're facing with your updated code, please make these changes:

1) Replace bsBroken = new BindingSource(lstBroken, null); with:

bsBroken = new BindingSource();
bsBroken.DataSource = lstBroken;

Or in one line: bsBroken = new BindingSource() { DataSource = lstBroken };

This yields the expected behavior with an immediate response to changes (I also mentioned this before above). Do not use the overload that accepts a dataMember and set it to null. Doing so gives the buggy behavior you're experiencing.

2) After doing the above, I see no need for the txtBroken_TextChanged event. Comment out the event handler assignment to test, but you should be able to remove it completely.