Tweaking a DataGridView ComboBoxColumn to allow editing.

Let me first say this. I love the DataGridView and I think it's an awesome and very versatile control. It's highly customizable, and you can create a damn near excel-like application around it. Having said that though, anyone who's ever used a DataGridView knows just how "quirky" the stupid thing can be. Things that look like they should be simple, require clever hacks to implement.

Recently, I was creating an in-house app here at work, in which I needed a DataGridView with a ComboBoxColumn but with one wrinkle. Out of the box, the ComboBoxColumn doesn't support editing. Meaning, it acts like a ComboBox that has the DropDownStyle set to DropDownList (which doesn't allow the user to enter new values). We, however, did need to have the ability for the user to enter new values. I thought that this would be a simple property that I'd be able to set on the ComboBoxColumn. Yea, well there is no such property, so queue clever hack!

The first thing you need to do is hook into the DataGridView's EditingControlShowing event. This event fires when the actual ComboBox is "dropped down". The interesting thing is that the EventArgs has a property on it "Control" that can be cast to a standard WinForms ComboBox.

To demonstrate, I've created a simple Windows App that has a button on it and a DataGridView. It also has a method that just returns a list of strings that contains all the days of the week (needed something just for testing). When the button is clicked, the DataGridView gets populated with a ComboBoxColumn with all the days. Here's the entire code:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Windows.Forms;

namespace DataGridViewComboBoxTesting
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            this.dataGridView1.EditingControlShowing += HandleEditShowing;
        }

        private void HandleEditShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
        {
            var cbo = e.Control as ComboBox;
            if (cbo == null)
            {
                return;
            }

            cbo.DropDownStyle = ComboBoxStyle.DropDown;
            cbo.Validating -= HandleComboBoxValidating;
            cbo.Validating += HandleComboBoxValidating;
        }

        private void HandleComboBoxValidating(object sender, CancelEventArgs e)
        {
            var combo = sender as DataGridViewComboBoxEditingControl;
            if (combo == null)
            {
                return;
            }
            if (!combo.Items.Contains(combo.Text)) //check if item is already in drop down, if not, add it to all
            {
                var comboColumn = this.dataGridView1.Columns[this.dataGridView1.CurrentCell.ColumnIndex]
 as DataGridViewComboBoxColumn;
                combo.Items.Add(combo.Text);
                comboColumn.Items.Add(combo.Text);
                this.dataGridView1.CurrentCell.Value = combo.Text;
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            var cboColumn = new DataGridViewComboBoxColumn
            {
                Name = "ComboBox",
                HeaderText = "Combo Box Column"
            };

            foreach (var day in this.GetListOfStrings())
            {
                cboColumn.Items.Add(day);
            }

            this.dataGridView1.Columns.Add(cboColumn);
        }

        public IEnumerable<string> GetListOfStrings()
        {
            foreach (var day in Enum.GetValues(typeof(DayOfWeek)))
            {
                yield return day.ToString();
            }
        }
    }
}

As you can see, in the HandleEditShowing method, we can get access the the underlying ComboBox itself, and set the DropDownStyle property right there. We then do a little more hackery so that when the user enters a new value, we can add it to all other ComboBox's in that column. We do that by hooking into the ComboBox's Validating event (first we unhook it in case we're already hooked into it, so that our event handler doesn't get called more than once).

        private void HandleEditShowing(object sender, DataGridViewEditingControlShowingEventArgs e)
        {
            var cbo = e.Control as ComboBox;
            if (cbo == null)
            {
                return;
            }

            cbo.DropDownStyle = ComboBoxStyle.DropDown;
            cbo.Validating -= HandleComboBoxValidating;
            cbo.Validating += HandleComboBoxValidating;
        }

The event handler then looks like this:

        private void HandleComboBoxValidating(object sender, CancelEventArgs e)
        {
            var combo = sender as DataGridViewComboBoxEditingControl;
            if (combo == null)
            {
                return;
            }
            if (!combo.Items.Contains(combo.Text)) //check if item is already in drop down, if not, add it to all
            {
                var comboColumn = this.dataGridView1.Columns[this.dataGridView1.CurrentCell.ColumnIndex] as DataGridViewComboBoxColumn;
                combo.Items.Add(combo.Text);
                comboColumn.Items.Add(combo.Text);
                this.dataGridView1.CurrentCell.Value = combo.Text;
            }
        }

Simple. We just check if it's already in the list, if it's not then add it to the combo box as well as add it to all other combo boxes.

Here's a link to the source code. In the code I also created a class called DataGridViewComboBoxColumnEx which basically inherits from DataGridViewComboBoxColumn and encapsulates all this hackery nicely. You just use it as you normally would but it allows editing.

Source code: http://www.box.net/shared/73bmzmv1r6