ComboBox auto DropDownWidth regardless of DataSource type-CodePudding

I would like to resize any ComboBox dropdown width based on the longest string in the dropdown items. I want this to work on any ComboBox with items which means one whose items are strings, some object with DisplayMember set, or a DataTable. I found some code which works for strings, but not when a DisplayMember is set

static void resizeCombo(ComboBox cb)
{
    cb.DropDownWidth = cb.Items.Cast<string>().Max(x => TextRenderer.MeasureText(x, cb.Font).Width);
}

In the following three examples, the first which is just strings will work, but the following two don't work (cb.Items.Cast<string>() cast to string fails), and demonstrate that the DisplayMember can vary when bound to sources of different classes, so I can't just use "Name" for example

var c = new string[] { "Name1", "Name2" };
comboBox.DataSource = c.ToList();
resizeCombo(comboBox);

var c1 = new Class1[] { new Class1() { ID = 1, Name1 = "Name1" }, new Class1() { ID = 2, Name1 = "Name2" } };
comboBox1.DisplayMember = "Name1";
comboBox1.ValueMember = "ID";
comboBox1.DataSource = c1.ToList();
resizeCombo(comboBox1);

var c2 = new Class2[] { new Class2() { ID = 2, Name2 = "Name1" }, new Class2() { ID = 2, Name2 = "Name2" } };
comboBox2.DisplayMember = "Name2";
comboBox2.ValueMember = "ID";
comboBox2.DataSource = c2.ToList();
resizeCombo(comboBox2);

I could reflect the DisplayMember, and find the strings by name, and it may solve the List<class> case, but not DataTable.

I am looking for a method to get all the strings in the ComboBox regardless of how they are added. Is there one?

CodePudding user response：

Combobox has GetItemText method that returns the string representation of item.

This should work:

    static void resizeCombo(ComboBox cb)
    {
        if (cb.Items.Count == 0) cb.DropDownWidth = cb.Width;
        else
        {
            int maxWidth = int.MinValue;
            for (int i = 0; i < cb.Items.Count; i  )
            {
                maxWidth = Math.Max(maxWidth, TextRenderer.MeasureText(cb.GetItemText(cb.Items[i]), cb.Font).Width);
            }
            if (cb.Items.Count > cb.MaxDropDownItems) maxWidth  = SystemInformation.VerticalScrollBarWidth                    
            cb.DropDownWidth = maxWidth;
        }
    }

To test with DataTable:

        DataTable t = new DataTable();
        t.Columns.Add(new DataColumn("ID", typeof(int)));
        t.Columns.Add(new DataColumn("Name2", typeof(string)));

        t.Rows.Add(new object[] { 1, "Somename" });
        t.Rows.Add(new object[] { 2, "Some other name" });

        comboBox2.DisplayMember = "Name2";
        comboBox2.ValueMember = "ID";
        comboBox2.DataSource = t;
        resizeCombo(comboBox2);

NB! For this to work, do not use the resize function in a form's constructor. Use it from the Load event or similar, once the form is already up and running.