Changing seemingly unrelated code breaks combo box. Causes it to display object name instead of list of values

Question

I have a C#.net winforms application which uses Entity Framework code first to connect to SQL DB.

One of my combo boxes loses the list of values that its supposed to pull from an enumeration (list of the 12 months) and it just shows the name of the month list object.

It happened since I changed code which is completely unrelated to the combo and should not affect it (moved some unrelated code to another class) and it seems to have caused the issue above. Something similar happened previously when I added some new code it caused the same thing to happen. Eventually it fixed itself after i changed a linq query to return a "TBC" string when a certain value was null. Bearing in mind the code changed has nothing to do with the combo box in question. The combo is also populated before any of the unrelated code ever runs.

This is the code that populates the combo:

private void UpdateMonthCombo()
{
    ArrayList AL = new ArrayList();
    foreach (string cs in Enum.GetNames(typeof(MonthsEnum)))
    {
        MonthList aEnum = new MonthList((int) Enum.Parse(typeof(MonthsEnum), cs), cs);
        AL.Add(aEnum);
    }

    BindingSource source = new BindingSource();
    source.DataSource = AL;
    comboMonth.ValueMember = "_MonthNumber";
    comboMonth.DisplayMember = "_Name";
    comboMonth.DataSource = source;
    comboMonth.SelectedItem = null;
}

I'd love to know how this can happen and how to fix and prevent it. I'm sure you can tell that i'm relatively new to C#!

Any help is much appreciated.

This is MonthList class:

public class MonthList
{

    public string _Name { get; set; }
    public int _MonthNumber { get; set; }

    public MonthList(int MonthNumber, string Name)
    {
        _Name = Name;
        _MonthNumber = MonthNumber;
    }
    public enum MonthsEnum : int
    {
        January = 1,
        February = 2,
        March = 3,
        April = 4,
        May = 5,
        June = 6,
        July = 7,
        August = 8,
        September = 9,
        October = 10,
        November = 11,
        December = 12
    }       
}

Tried using this instead of the enumeration. Tested on separate form works as expected. But on my form it only displays the month numbers:

private void UpdateMonthCombo()
    {
        var monthList = CultureInfo.InvariantCulture.DateTimeFormat.MonthNames.Select((item, index) => new
        {
            Month = item,
            Value = index + 1
        });

        var result = monthList.Where(m => m.Month != "");

        comboMonth.DataSource = result.ToList();
        comboMonth.ValueMember = "Value";
        comboMonth.DisplayMember = "Month";
    }

I refactored my selectedindexchanged event on the combo box. It fixed the combobox. But now the main gridview is broken. It seems related to a method which adds a combobox column. When i comment out this method the gridview shows all columns (no combo column) but the original comboMonth becomes broken again showing just the month numbers:

private void AddProjectCombo()
    {
        gridBusinessKMs.Columns["Project"].Visible = false;
        DataGridViewComboBoxColumn cmbCol = new DataGridViewComboBoxColumn();

        cmbCol.DisplayStyle = DataGridViewComboBoxDisplayStyle.Nothing;
        cmbCol.HeaderText = "Project";
        cmbCol.Name = "Project ";
        cmbCol.Items.Add("True");

        using (var context = new MileageLogDBContext())
        {
            cmbCol.DataSource = (from p in context.Projects
                                 orderby p.ProjectDescription
                                 select p.ProjectDescription).ToList();
        }
        gridBusinessKMs.Columns.Add(cmbCol);
        gridBusinessKMs.Columns["Project "].DisplayIndex = 1;
    }

Answer 1

I believe that ArrayList holds references to objects and you could possibly be changing the object and thus the binding source. (see the answer to this question: ArrayList vs List<> in C#) If the binding source was changed then the value of DisplayMember may revert to the object's ToString method:

If the specified property does not exist on the object or the value of DisplayMember is an empty string (""), the results of the object's ToString method are displayed instead.

https://msdn.microsoft.com/en-us/library/system.windows.forms.listcontrol.displaymember(v=vs.110).aspx

I don't know what MonthsEnum or MonthList are doing but I tested the following code and it worked.

private void UpdateMonthCombo()
    {
        List<Tuple<int, string>> AL = new List<Tuple<int, string>>();

        string[] monthNames = System.Globalization.CultureInfo.CurrentCulture.DateTimeFormat.MonthNames;

        for (int i = 0; i < 12; i++)
        {
            AL.Add(new Tuple<int, string>(i + 1, monthNames[i]));
        }

        comboMonth.DataSource = AL;
        comboMonth.ValueMember = "Item1";
        comboMonth.DisplayMember = "Item2";
        comboMonth.SelectedItem = null;
    }