<asp:ObjectDataSource ID="ObjQuestionRatings" runat="server" SelectMethod="LoadQuestions" TypeName="QuizNetOnline.Logic.QuizNetSystem">
            <asp:ControlParameter ControlID="cmbTopics" Name="TopicCodeID" PropertyName="SelectedValue" Type="String" />

public virtual List<Question> LoadQuestions(string topicCode)
      return LoadQuestions(null, topicCode, null, null, null, null, null, null, new SqlWrapper(), null);

What is wrong with above:

Name="TopicCodeID" is incorrect, the parameter name needs to be the same name as the function parameter name which is 'topicCode'