SelectMethod为ObjectDataSource调用了两次

Question

as I am new to ASP.NET webforms and Entity framework, I am experimenting with a pet project.

During this I came across the following which I am trying to understand:

1. I have an ObjectDataSource (called EmployerObjectDataSource ) that is using a method of business logic layer (BLL) object for selecting the data - the method is GetEmployer
2. In the Page_PreRender callback of my page, I call a method populateFields to populate the fields within a FormView
3. In the populateFields I call EmployerObjectDataSource.Select() to get the Employer record.
4. If there are any records returned, then I populate the text boxes with the values from the returned record.

Here is the code:

    //Following Dmytro's comment, I will use Page_Load instead, however this 
    //does not resolve the problem
    //protected void Page_PreRender(object sender, EventArgs e)
    protected void Page_Load(object sender, EventArgs e)
    {
        _username = "Lefteris";
        _version = 1;

        if (!Page.IsPostBack)
        {
            populateFields();
        }
    }

    private bool populateFields()
    {
        //IEnumerable<Employer> empl = ((IEnumerable<Employer>)EmployerObjectDataSource.Select()).ToList();

        //The GetEmployer method of BLL is called here (as expected)
        List<Employer> empl = (List<Employer>)EmployerObjectDataSource.Select();

        System.Threading.Thread.Sleep(1000);
        if (empl.Count() == 1)
        {
            Employer employer = empl.First();

            //The GetEmployer method of BLL is called here (WHY????)
            ((RadTextBox)EmployerFormView.Row.FindControl("txtAme")).Text = employer.AME.ToString();
            ((RadTextBox)EmployerFormView.Row.FindControl("txtAfm")).Text = employer.EmplrAFM.ToString();
            ((RadTextBox)EmployerFormView.Row.FindControl("txtName")).Text = employer.EmplrLastName.ToString();
  ...

The GetEmployer is shown below:

    public List<Employer> GetEmployer(string username, short version)
    {
        DateTime today = DateTime.Today;
        List<Employer> employers = (ikaRepository.GetEmployers(username, today, version)).ToList<Employer>();

        Debug.Assert(employers.Count() <= 1, "This is a logical Error - Can we have more than one active Employer records per user?");
        return employers;
    }

Here is the question: When I attached the debugger, I saw that the GetEmployer method of the BLL is called twice. First time on the .Select() and second time when I try to get the value of the first field of the Employer record.

Thank you

Answer 1

I'm not good in English, I apologize, in advance.

You manually bound FormView to ObjectDataSource in code-behind. So you called Select method once in Page Loading phase (based on current codes).

Have you assigned "EmployerObjectDataSource" to EmployerFormView 's DataSourceID in your markups (ASPX contents), and bind FormView to ObjectDataSource in Page Event Handling phase (after Load phase), so it call Select method once again.

It is better, that you don't manually bind DataBoundControls (like FormView ) to DataSourceControls (like ObjectDataSource ). Instead, you can use SelectParameters inside ObjectDataSource markup if it needs to pass some data to BLL for selecting/filtering logics.

I think this situation it is what happened. I suggest you should write your markups too.

Another situation is binding one ObjectDataSource to more that one DataBoundControls . In this situation, every DataBoundControls will call ObjectDataSource 's Select method on it's binding-time. To handling this situation, you can use caching capabilities of ObjectDataSource .

I hope these explanations will be useful.

I suggest you to read these resources before using ASP.NET Data Controls:

1. ObjectDataSource on MSDN
2. FormView on MSDN
3. ASP.NET Page Life Cycle Overview on MSDN

Good luck!