WPF application settings - resetting a single property

You can use the Settings.Default.Properties["MyProperty"].DefaultValue to obtain the default value for the property, and set the property value to that.

Found solution when reading .NET source code:

Settings.Default.PropertyValues["MyPropertyName"].SerializedValue = Settings.Default.Properties["MyPropertyName"].DefaultValue;
Settings.Default.PropertyValues["MyPropertyName"].Deserialized = false;

It's the PropertyValue that's need to be set in combinaison of Deserialized (the order matter) :

public void ResetOneSetting(string propertyName)
{
    SettingsPropertyValue propertyToReset = Settings.Default.PropertyValues.OfType<SettingsPropertyValue>().FirstOrDefault(p => p.Name == propertyName);
    if (propertyToReset != null)
    {
        propertyToReset.PropertyValue = propertyToReset.Property.DefaultValue;
        propertyToReset.Deserialized = false;
    }
}