C#

Just in case that still matters to anyone:

You can dynamically add settings through Settings.Default.Properties.Add(...) and have these also persisted in the local storage after saving (I had those entries reflected in the roaming file).

Nevertheless it seems that the dynamically added settings keep missing in the Settings.Default.Properties collecion after loading again.

I could work around this problem by adding the dynamic property before first accessing it. Example (notice that I "create" my dynamic setting from a base setting):

// create new setting from a base setting:
var property = new SettingsProperty(Settings.Default.Properties["<baseSetting>"]);
property.Name = "<dynamicSettingName>";
Settings.Default.Properties.Add(property);
// will have the stored value:
var dynamicSetting = Settings.Default["<dynamicSettingName>"];

I don't know if this is supported by Microsoft as the documentation is very rare on this topic.

Problem is also described here http://www.vbdotnetforums.com/vb-net-general-discussion/29805-my-settings-run-time-added-properties-dont-save.html#post88152 with some solution offered here http://msdn.microsoft.com/en-us/library/saa62613(v=VS.100).aspx (see Community Content - headline "How to Create / Save / Load Dynamic (at Runtime) Settings"). But this is VB.NET.

SettingsAttributeDictionary attrDict = new SettingsAttributeDictionary();
UserScopedSettingAttribute attr = new UserScopedSettingAttribute();
attrDict.Add(attr.GetType(), attr);

SettingsProperty p = new SettingsProperty("Testing", typeof(bool), b.Providers["LocalFileSettingsProvider"], false, false, SettingsSerializeAs.String, attrDict, false, false);
b.Properties.Add(p);

foreach(object item in listBox.Items)
    if((string) item == textbox.Text)
        // do your thing

if (listbox1.Items.Contains(textbox1.Text))
{
    // you found it
}