Can't save my combobox items to mystringcollection in appsettings

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C#

I'm developing a win app and somewhere i have to insert items to the combobox. But when i restart the app, the items are not there anymore I used a string collection for saving them. Of course it reads the items but it doesn't save the settings in it. Here is my code Thanks for your helps.

in my Form_load

private void Services_Form_Load(object sender, EventArgs e)
{
if(Properties.Settings.Default.combo_items!=null)
{
foreach (object items in Properties.Settings.Default.combo_items)
{
comboBox1.Items.Add(items);
}
}
}

in my form_closing

private void Services_Form_FormClosing(object sender, FormClosingEventArgs e)
{
if (Properties.Settings.Default.combo_items != null)
{
Properties.Settings.Default.combo_items.Clear();
foreach (string items in combo_Combobox_1.Items)
{
Properties.Settings.Default.combo_items.Add(items);
}
} Properties.Settings.Default.Save();
}
I have also add and remove some strings to the value of string collection at first.<

What I have tried:

I have also add and remove some strings to the value of string collection at first.