How to I find the space a combo box needs to show all list items.

Question

I have a WinForms combo box, I wish to set the width of the column box so that any selected item can be shown in full. (I do not know what items will be in the combo box at the time of writing the software)

However when I call Combobox.PreferredSize, it does not seem to take into account the items that are in the drop down list.

Answer 1

You can use the System.Drawing.Graphics.MeasureString method. Take a look at this answer for more details on how to find the widest item in the list.

private void ResizeComboBox(ComboBox comboBox) {
  var maxItemLength = 0;
  // using the ComboBox to get a Graphics object:
  using (var g = Graphics.FromHwnd(comboBox.Handle)) {
    foreach (var item in comboBox.Items.Cast<string>()) {
      var itemLength = g.MeasureString(item, comboBox.Font);
      maxItemLength = Math.Max((int) itemLength.Width, maxItemLength);
    }
  }
  // correction for the drop down arrow
  maxItemLength += 15;
  comboBox.Width = maxItemLength;
}

Answer 2

Using System.Drawing.Graphis.MeasureString or the other (faster) alternative TextRenderer.MeasureText will do the trick in measuring the width of a string in a given font. Simply get the maximum width of all the items in the list of items and set the width of the control to that maximum.

The algorithm for doing so is:

using (Graphics g = comboBox.CreateGraphics())
{
    float maxWidth = comboBox.Width;

    foreach(string s in comboBox.Items)
    {
        SizeF size = g.MeasureString(s, comboBox.Font);
        if (size.Width > maxWidth)
            maxWidth = size.Width;
    }
}

comboBox.Width = maxWidth;