Auto-width of ComboBox's content

You can't use it directly.

Do a trick

First iterate through all items of your combobox, check for the width of every items by assigning the text to a label. Then, check width every time, if width of current item gets greater than previous items then change the maximum width.

int DropDownWidth(ComboBox myCombo)
{
    int maxWidth = 0;
    int temp = 0;
    Label label1 = new Label();

    foreach (var obj in myCombo.Items)
    {
        label1.Text = obj.ToString();
        temp = label1.PreferredWidth;
        if (temp > maxWidth)
        {
            maxWidth = temp;
        }
    }
    label1.Dispose();
    return maxWidth;           
}

private void Form1_Load(object sender, EventArgs e)
{
    comboBox1.DropDownWidth = DropDownWidth(comboBox1);
}

---

OR

As suggested by stakx, you can use TextRenderer class

int DropDownWidth(ComboBox myCombo)
{
    int maxWidth = 0, temp = 0;
    foreach (var obj in myCombo.Items)
    {
        temp = TextRenderer.MeasureText(obj.ToString(), myCombo.Font).Width;
        if (temp > maxWidth)
        {
            maxWidth = temp;
        }
    }
    return maxWidth;
}

Here is very elegant solution. Just subscribe your combobox to this event handler:

 private void AdjustWidthComboBox_DropDown(object sender, EventArgs e)
        {
            var senderComboBox = (ComboBox)sender;
            int width = senderComboBox.DropDownWidth;
            Graphics g = senderComboBox.CreateGraphics();
            Font font = senderComboBox.Font;

            int vertScrollBarWidth = (senderComboBox.Items.Count > senderComboBox.MaxDropDownItems)
                    ? SystemInformation.VerticalScrollBarWidth : 0;

            var itemsList = senderComboBox.Items.Cast<object>().Select(item => item.ToString());

            foreach (string s in itemsList)
            {
                int newWidth = (int)g.MeasureString(s, font).Width + vertScrollBarWidth;

                if (width < newWidth)
                {
                    width = newWidth;
                }
            }

            senderComboBox.DropDownWidth = width;
        }

This code was taken from the codeproject: Adjust combo box drop down list width to longest string width. But I have modified it to work with comboboxes filled with any data (not only strings).

obj.ToString() doesn't work for me, I suggest to use myCombo.GetItemText(obj). This works for me:

private int DropDownWidth(ComboBox myCombo)
{
    int maxWidth = 0, temp = 0;
    foreach (var obj in myCombo.Items)
    {
        temp = TextRenderer.MeasureText(myCombo.GetItemText(obj), myCombo.Font).Width;
        if (temp > maxWidth)
        {
            maxWidth = temp;
        }
    }
    return maxWidth + SystemInformation.VerticalScrollBarWidth;
}