Items collection cannot be modified when the DataSource property is set.
This exception
“Items collection cannot be modified when the DataSource property is set.”
occurs when you trying to add some text like “Select” or “None” in combo box (drop down list) in .NET framework window form control. There is no such way to add items in combo box’s items property after you set DataSource. To avoid this you can add one more item in your datasource. For example if you are trying to bind drop down list through DataSet then add one more row into DataSet. There is one more workaround for
“Items collection cannot be modified when the DataSource property is set.”
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