You have a control that supports data binding (such as a DropDownList or a DataGrid) which is binded to a DataSource (such as a database table, an XML file or an array). If you try to add new items to it after you binded to a DataSource, you will get the error Items collection cannot be modified when the DataSource property is set.
This error is normal since controls that are binded to a DataSource cannot have their items changed or allow new items to be manually added, using a method for example. There is no way around this, thus if you want to add additional items to the control, you will need to add them to the DataSource. For example if the DataSource is an array, you will have to add a new value inside the array for each new item in the control, or if it’s a database table, you will need a new row.