C# 尝试在更改下拉菜单中的选择时返回主键

我有一个下拉列表，如下所示，我正在将priomary键作为DataValueEld取回

<asp:DropDownList ID="userNameDropDown" runat="server" DataSourceID="SqlDataSource1"
                        DataTextField="Name" DataValueField="PK_User" OnSelectedIndexChanged="DropDownList1_SelectedIndexChanged"
                        AutoPostBack="true">
                    </asp:DropDownList>
                </span>
                <asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%$ ConnectionStrings:ApplicationServices %>"
                    SelectCommand="SELECT [PK_User], [Name] FROM [User] ORDER BY CASE WHEN [LoginName] = @userLoggedIn THEN 1 ELSE 2 END, [Name]">
                    <SelectParameters>
                        <asp:QueryStringParameter Name="userLoggedIn" Type="String" />
                    </SelectParameters>
                </asp:SqlDataSource>

我发现以下错误，不明白原因：

A first chance exception of type 'System.InvalidCastException' occurred in App_Web_uhtjotwm.dll
System.InvalidCastException: Specified cast is not valid.
   at user_nonscrumstories.getRecentStoryPK() in c:\Documents and Settings\tunnelld\My Documents\Visual Studio 2010\WebSites\ZSRBlank18\user\nonscrumstories.aspx.cs

编辑：

---

问题出在

getRecentStoryPK

上，没有看到粘贴的代码与此错误相关异常抛出的确切行是什么？是返回null的

result=

行，还是

.AddWithValue

的

用户名下拉列表。SelectedItem.Value

实际上不是

int

。。。？

A first chance exception of type 'System.InvalidCastException' occurred in App_Web_uhtjotwm.dll
System.InvalidCastException: Specified cast is not valid.
   at user_nonscrumstories.getRecentStoryPK() in c:\Documents and Settings\tunnelld\My Documents\Visual Studio 2010\WebSites\ZSRBlank18\user\nonscrumstories.aspx.cs

private string getRecentStoryPK()
{
    //userNameDropDown.DataBind();
    SqlConnection conn = new SqlConnection();
    //create array to store results of select query
    int result = 0;
    conn.ConnectionString = ConfigurationManager.ConnectionStrings["ApplicationServices"].ConnectionString;
    try
    {
        //open SQL connection
        conn.Open();
        //create SQL command
        SqlCommand getRecentStoryPK = new SqlCommand("SELECT MAX([PK_NonScrumStory]) FROM [NonScrumStory] WHERE [UserId] = @userIdParam", conn);
        //create and assign parameters
        getRecentStoryPK.Parameters.AddWithValue("@userIdParam", Int32.Parse(userNameDropDown.SelectedItem.Value));
        result = (int)getRecentStoryPK.ExecuteScalar();
    }
    catch (Exception ex)
    {
        System.Diagnostics.Debug.Write(ex.ToString());
        result = -1;
    }
    finally
    {
        conn.Close();
    }
    //return populated string
    return result.ToString();
}