Most of the time you get this error when you are trying to retrieve the selected value of a listbox (DropDownList) and no value is selected.
Even though apparently the first item in a DropDownList is always selected by default, this is done by the browser and ASP.NET is not aware of the selected item. To prevent this error from happening you need to define one of the items in your DropDownList as selected:
<asp:ListItem Value="1" Selected="True">Item 1</asp:ListItem>
Nathan Pakovskie is an esteemed senior developer and educator in the tech community, best known for his contributions to Geekpedia.com. With a passion for coding and a knack for simplifying complex tech concepts, Nathan has authored several popular tutorials on C# programming, ranging from basic operations to advanced coding techniques. His articles, often characterized by clarity and precision, serve as invaluable resources for both novice and experienced programmers. Beyond his technical expertise, Nathan is an advocate for continuous learning and enjoys exploring emerging technologies in AI and software development. When he’s not coding or writing, Nathan engages in mentoring upcoming developers, emphasizing the importance of both technical skills and creative problem-solving in the ever-evolving world of technology.
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