ASP.net通过从表中预加载其数据来加速dropdownlist加载

Question

I have a grid view that uses dropdownlist to select employees ID but displaying names.

 <EditItemTemplate>
 <asp:DropDownList ID="DropDownList5" runat="server" AppendDataBoundItems="True" DataSourceID="SqlDataSourceEmployees" DataTextField="name" DataValueField="empID" SelectedValue='<%# Bind("employee") %>'>
                                <asp:ListItem></asp:ListItem>
                            </asp:DropDownList>
  </EditItemTemplate>

This works fine, but the SqlDataSourceEmployees is called the moment user clicks on the dropdownlist, which causes quite annoying delay, since as I understand first it fires a SQL command (simple SELECT empID,NAME FROM EMPLOYEES WHERE department=@department ) and then the list is populated. It would be much better to bind the dropDowList to something that is already in memory, especially that I don't have to worry that the data in the list would change after page has been loaded.

I've thought about loading it to the DataTable on PageLoad and then binding such table to dropDownList but the list cannot find mentioned above table. I've even put the DataTable as public method of the webpage:

public partial class PlannersCurrentRoster : System.Web.UI.Page
{

 private DataSet.employeesDataTable employeesTable;

public DataSet.employeesDataTable EmployeesTable
{
    get { return employeesTable; }
    set { employeesTable = value; }
}

protected void Page_Load(object sender, EventArgs e)
{

    DataSetTableAdapters.employeesTableAdapter TA = new DataSetTableAdapters.employeesTableAdapter();
    DataSet.employeesDataTable empTable = TA.GetDataByDepartment(lblDepartment.Text);
    EmployeesTable = empTable;

but then changing the bind of the list

<EditItemTemplate>
                            <asp:DropDownList ID="DropDownList5" runat="server" AppendDataBoundItems="True" DataSourceID="EmployeesTable" DataTextField="name" DataValueField="empID" SelectedValue='<%# Bind("employee") %>'>
                                <asp:ListItem></asp:ListItem>
                            </asp:DropDownList>

fails to find the "EmployesTable".

EDIT:

Following the solution below I've tried:

protected void GridView5_RowUpdating(object sender, GridViewUpdateEventArgs e)
{ ((DropDownList)GridView5.Rows[e.RowIndex].FindControl("DropDownList5")).DataSource = EmployeesTable;
    ((DropDownList)GridView5.Rows[e.RowIndex].FindControl("DropDownList5")).DataBind();

}

Which doesn't speed up things a bit, I'm sure that the DDL still takes data from SQL source (when I was trying to remove it, I had an error stating that SelevtedValue is invalid)

So I've tried to assign it one step earlier, during editing event

 protected void GridView5_RowEditing(object sender, GridViewEditEventArgs e)
{
    ((DropDownList)GridView5.FindControl("DropDownList5")).DataSource = EmployeesTable;
    ((DropDownList)GridView5.FindControl("DropDownList5")).DataBind();


}

but then it fails to find dropdownlist5

EDIT: I give up. After reading this article I've simply changed SQLDataSource type to DataReader which indeed improved performance. Or maybe its the placebo effect for my tired mind.

Answer 1

You can't do it the way you have it since the your employeesTable variable is destroyed as soon as the page is served. Each postback then gets a new instance. If the list is not unique, instead store it into the Cache object. You can then set a timeout for it as well.

If it is based on data for the page, you can either store it in session which could carry it across pages but can degrade performance if you have a large number of objects storing in session across users.

If it's not too large you could store it in ViewState instead. It will then be serialised to the client. The downside is, the data can bloat the HTML sent to the client.

In your case, since the datatable seems to be dependent upon the a text field, it may be better to use the viewstate. In your case though, complexity is added due to the fact you need to know when the text value has changed as well so you can negate the data.

The following is a crude example of ViewState but you can also adapt for Session and the Cache.

private string SelectedDepartmentText
{
    get
    {
        if(ViewState["SelectedDepartmentText"] != null)
            return ViewState["SelectedDepartmentText"].ToString();
        else
            return string.Empty;
    }
    set{ViewState["SelectedDepartmentText"] = value;}
}

public DataSet.employeesDataTable EmployeesTable
{
     get
    {
           if(!SelectedDepartmentText.Equals(lblDepartment.Text))
           {
               // if the SelectedDepartmentText isn't the same as the lblDepartment.Text, go fetch it
               DataSetTableAdapters.employeesTableAdapter TA = new DataSetTableAdapters.employeesTableAdapter();
               ViewState["EmployeesTable"] =TA.GetDataByDepartment(lblDepartment.Text);
               // save the lblDepartment.Text value to the viewstate for next time.
               SelectedDepartmentText = lblDepartment.Text;
               return ViewState["EmployeesTable"];
           }
           else
           {
               // let's see if we have something already and return it
               if(ViewState["EmployeesTable"] != null)
                   return (DataSet.employeesDataTable)ViewState["EmployeesTable"];
               else
                {

                // if we don't, let's get it, this also handles an empty string for the                     
                // lblDepartment.Text
            DataSetTableAdapters.employeesTableAdapter TA = new DataSetTableAdapters.employeesTableAdapter();
                        // store it in the viewstate
            ViewState["EmployeesTable"] =TA.GetDataByDepartment(lblDepartment.Text);
                        // and return whatever we got back
            return (DataSet.employeesDataTable)ViewState["EmployeesTable"];
                }
           }
           return null;
    }
    set{ ViewState["EmployeesTable"] = value;}
}