ASP.NET Web Forms DropDownList has a SelectedValue which is invalid because it does not exist in the list of items

Question

First of all there has been questions ( DropDownList has a SelectedValue which is invalid because it does not exist in the list of items , DropDownList "has a SelectedValue

Answer 1

Because there is no any datasource when your page is reloaded (postback happened), you should add SqlDataSource control in your .aspx file even in reloading page recognize DropDownList's DataSource. Then You can use InsertCommand, UpdateCommand, DeleteCommand and SelecCommand to obtain your DB.