C# winforms combobox dynamic autocomplete

This code is write on your form load. It display all the Tour in database when user type letter in combo box. This code automatically suggest and append the right choice as user want.

            con.Open();
            cmd = new SqlCommand("SELECT DISTINCT Tour FROM DetailsTB", con);
            SqlDataReader sdr = cmd.ExecuteReader();
            DataTable dt = new DataTable();
            dt.Load(sdr);
            combo_search2.DisplayMember = "Tour";
            combo_search2.DroppedDown = true;

            List<string> list = new List<string>();
            foreach (DataRow row in dt.Rows)
            {
                list.Add(row.Field<string>("Tour"));
            }
            this.combo_search2.Items.AddRange(list.ToArray<string>());
            combo_search2.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
            combo_search2.AutoCompleteSource = AutoCompleteSource.ListItems;
            con.Close();

This was a major pain to get working. I hit a bunch of dead ends, but the final result is reasonably straight forward. Hopefully it can be of benefit to someone. It may need a little spit and polish that's all.

Note: _addressFinder.CompleteAsync returns a list of KeyValuePairs.

public partial class MyForm : Form
{
    private readonly AddressFinder _addressFinder;
    private readonly AddressSuggestionsUpdatedEventHandler _addressSuggestionsUpdated;

    private delegate void AddressSuggestionsUpdatedEventHandler(object sender, AddressSuggestionsUpdatedEventArgs e);

    public MyForm()
    {
        InitializeComponent();

        _addressFinder = new AddressFinder(new AddressFinderConfigurationProvider());

        _addressSuggestionsUpdated += AddressSuggestions_Updated;

        MyComboBox.DropDownStyle = ComboBoxStyle.DropDown;
        MyComboBox.DisplayMember = "Value";
        MyComboBox.ValueMember = "Key";
    }

    private void MyComboBox_KeyPress(object sender, KeyPressEventArgs e)
    {
        if (char.IsControl(e.KeyChar))
        {
            return;
        }

        var searchString = ThreadingHelpers.GetText(MyComboBox);

        if (searchString.Length > 1)
        {
            Task.Run(() => GetAddressSuggestions(searchString));
        }
    }

    private async Task GetAddressSuggestions(string searchString)
    {
        var addressSuggestions = await _addressFinder.CompleteAsync(searchString).ConfigureAwait(false);

        if (_addressSuggestionsUpdated.IsNotNull())
        {
            _addressSuggestionsUpdated.Invoke(this, new AddressSuggestionsUpdatedEventArgs(addressSuggestions));
        }
    }

    private void AddressSuggestions_Updated(object sender, AddressSuggestionsUpdatedEventArgs eventArgs)
    {
        try
        {
            ThreadingHelpers.BeginUpdate(MyComboBox);

            var text = ThreadingHelpers.GetText(MyComboBox);

            ThreadingHelpers.ClearItems(MyComboBox);

            foreach (var addressSuggestions in eventArgs.AddressSuggestions)
            {
                ThreadingHelpers.AddItem(MyComboBox, addressSuggestions);
            }

            ThreadingHelpers.SetDroppedDown(MyComboBox, true);

            ThreadingHelpers.ClearSelection(MyComboBox);
            ThreadingHelpers.SetText(MyComboBox, text);
            ThreadingHelpers.SetSelectionStart(MyComboBox, text.Length);
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex);
        }
        finally
        {
            ThreadingHelpers.EndUpdate(MyComboBox);
        }
    }

    private class AddressSuggestionsUpdatedEventArgs : EventArgs
    {
        public IList<KeyValuePair<string, string>> AddressSuggestions { get; private set; }

        public AddressSuggestionsUpdatedEventArgs(IList<KeyValuePair<string, string>> addressSuggestions)
        {
            AddressSuggestions = addressSuggestions;
        }
    }
}

ThreadingHelpers is just a set of static methods of the form:

    public static string GetText(ComboBox comboBox)
    {
        if (comboBox.InvokeRequired)
        {
            return (string)comboBox.Invoke(new Func<string>(() => GetText(comboBox)));
        }

        lock (comboBox)
        {
            return comboBox.Text;
        }
    }

    public static void SetText(ComboBox comboBox, string text)
    {
        if (comboBox.InvokeRequired)
        {
            comboBox.Invoke(new Action(() => SetText(comboBox, text)));
            return;
        }

        lock (comboBox)
        {
            comboBox.Text = text;
        }
    }

Take 2. My answer below didn't get me all the way to the desired result, but it may still be useful to somebody. The auto-select feature of the ComboBox was causing me major pain. This one uses a TextBox sitting over the top of a ComboBox, allowing me to ignore whatever appears in the ComboBox itself and just respond to the selection changed event.

1. Create Form
2. Add ComboBox
   • Set desired size and location
   • Set DropDownStyle to DropDown
   • Set TabStop to false
   • Set DisplayMember to Value (I'm using a list of KeyValuePairs)
   • Set ValueMember to Key
3. Add Panel
   • Set to same size as ComboBox
   • Cover ComboBox with the Panel (This accounts for the standard ComboBox being taller than the standard TextBox)
4. Add TextBox
   • Place TextBox over the top of the Panel
   • Align bottom of the TextBox with the bottom of Panel/ComboBox

Code behind

public partial class TestForm : Form
{
    // Custom class for managing calls to an external address finder service
    private readonly AddressFinder _addressFinder;

    // Events for handling async calls to address finder service
    private readonly AddressSuggestionsUpdatedEventHandler _addressSuggestionsUpdated;
    private delegate void AddressSuggestionsUpdatedEventHandler(object sender, AddressSuggestionsUpdatedEventArgs e);

    public TestForm()
    {
        InitializeComponent();

        _addressFinder = new AddressFinder(new AddressFinderConfigurationProvider());
        _addressSuggestionsUpdated += AddressSuggestions_Updated;
    }

    private void textBox1_PreviewKeyDown(object sender, PreviewKeyDownEventArgs e)
    {
        if (e.KeyCode == Keys.Tab)
        {
            comboBox1_SelectionChangeCommitted(sender, e);
            comboBox1.DroppedDown = false;
        }
    }

    private void textBox1_KeyDown(object sender, KeyEventArgs e)
    {
        if (e.KeyCode == Keys.Up)
        {
            if (comboBox1.Items.Count > 0)
            {
                if (comboBox1.SelectedIndex > 0)
                {
                    comboBox1.SelectedIndex--;
                }
            }

            e.Handled = true;
        }
        else if (e.KeyCode == Keys.Down)
        {
            if (comboBox1.Items.Count > 0)
            {
                if (comboBox1.SelectedIndex < comboBox1.Items.Count - 1)
                {
                    comboBox1.SelectedIndex++;
                }
            }

            e.Handled = true;
        }
        else if (e.KeyCode == Keys.Enter)
        {
            comboBox1_SelectionChangeCommitted(sender, e);
            comboBox1.DroppedDown = false;

            textBox1.SelectionStart = textBox1.TextLength;

            e.Handled = true;
        }
    }

    private void textBox1_KeyPress(object sender, KeyPressEventArgs e)
    {
        if (e.KeyChar == '\r')  // Enter key
        {
            e.Handled = true;
            return;
        }

        if (char.IsControl(e.KeyChar) && e.KeyChar != '\b') // Backspace key
        {
            return;
        }

        if (textBox1.Text.Length > 1)
        {
            Task.Run(() => GetAddressSuggestions(textBox1.Text));
        }
    }

    private void comboBox1_SelectionChangeCommitted(object sender, EventArgs e)
    {
        if (comboBox1.Items.Count > 0 &&
            comboBox1.SelectedItem.IsNotNull() &&
            comboBox1.SelectedItem is KeyValuePair<string, string>)
        {
            var selectedItem = (KeyValuePair<string, string>)comboBox1.SelectedItem;

            textBox1.Text = selectedItem.Value;

            // Do Work with selectedItem
        }
    }

    private async Task GetAddressSuggestions(string searchString)
    {
        var addressSuggestions = await _addressFinder.CompleteAsync(searchString).ConfigureAwait(false);

        if (_addressSuggestionsUpdated.IsNotNull())
        {
            _addressSuggestionsUpdated.Invoke(this, new AddressSuggestionsUpdatedEventArgs(addressSuggestions));
        }
    }

    private void AddressSuggestions_Updated(object sender, AddressSuggestionsUpdatedEventArgs eventArgs)
    {
        try
        {
            ThreadingHelper.BeginUpdate(comboBox1);

            ThreadingHelper.ClearItems(comboBox1);

            if (eventArgs.AddressSuggestions.Count > 0)
            {
                foreach (var addressSuggestion in eventArgs.AddressSuggestions)
                {
                    var item = new KeyValuePair<string, string>(addressSuggestion.Key, addressSuggestion.Value.ToUpper());
                    ThreadingHelper.AddItem(comboBox1, item);
                }

                ThreadingHelper.SetDroppedDown(comboBox1, true);
                ThreadingHelper.SetVisible(comboBox1, true);
            }
            else
            {
                ThreadingHelper.SetDroppedDown(comboBox1, false);
            }
        }
        finally
        {
            ThreadingHelper.EndUpdate(comboBox1);
        }
    }

    private class AddressSuggestionsUpdatedEventArgs : EventArgs
    {
        public IList<KeyValuePair<string, string>> AddressSuggestions { get; }

        public AddressSuggestionsUpdatedEventArgs(IList<KeyValuePair<string, string>> addressSuggestions)
        {
            AddressSuggestions = addressSuggestions;
        }
    }
}

You may or may not have issues with setting the DroppedDown property of the ComboBox. I eventually just wrapped it up in a try block with an empty catch block. Not a great solution, but it works.

Please see my other answer below for info on ThreadingHelpers.

Enjoy.