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我有一个下拉列表，如下所示，我将主键作为 DataValueEield 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>

当我更改下拉菜单中的选择时，我试图从中检索主键：

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
    System.Diagnostics.Debug.WriteLine(userNameDropDown.SelectedItem.Value);
}

我收到以下错误，不明白为什么：

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：

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();
}
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