When you add a control to a page from the page editor, then this control is persisted in the data base. You cannot access it inside the Visual Studio. If you want to add items to a ComboBox then you can use one of the available data sources in the Toolbox from the right. Drop a Data widget to the page, configure it and then set its ID to the DataSourceID property of the ComboBox. For more information, please visit our online documentation http://www.sitefinity.com/4.0/documentation/User-guide/displaying-content-using-widgets.aspx
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