即使设置了AutoPostBack，DropDownList也不会回发

Question

Obscure solution at end of post

Using asp.net page with C# codebehind, I have successfully built and populated a DropDownList.

What I would like to do is capture a new value selected from the dropdownlist (preferrably using a postback to my codebehind). The codebehind can then update other things on the page based on this newly selected dropdownlist value.

My first attempt was to use

<asp:DropDownList ID="myDDL" runat="server" AutoPostBack="true" OnSelectedIndexChanged="foo"></asp:DropDownList>

with the C# method

public void foo(object sender, EventArgs e)
{
    DropDownList ddl = sender as DropDownList;
    string myValue = "";

    if (ddl != null)
    {
        myValue = ddl.SelectedValue;
        // Do some stuff
    }
}

This did not work. When the selected index was changed, it simply reloaded the page, but the IsPostBack flag was always false.

So I sifted through SO and tried a number of different tactics. Most recently, I tried to register the client-side onChange event in the codebehind and turned off the AutoPostBack.

in the ASP.Net page:

<asp:DropDownList ID="myDDL" runat="server" AutoPostBack="false"></asp:DropDownList>

in the codebehind:

myDDL.Attributes.Add("onChange", "doSomeStuff(this);"); // Done on databind.

I added the client-side javascript to call the page's __doPostBack function

<script language="javascript" type="text/javascript">
    function doSomeStuff(ddl) {
        var ddlVals = document.getElementById(ddl.id);

        __doPostBack(ddlVals, '');
    }
</script>

This also failed, although I thought it was going somewhere when I saw the javascript execute properly.

Looking in the codebehind, though, it's still not working. When I put a breakpoint in the Page_Load IsPostBack is false! But it's supposed to be a postback!? It was posted back using __doPostBack and (seperately) automatically with AutoPostBack="true"

So I dug deeper.

According to this MSDN article (http://msdn.microsoft.com/en-us/library/ms178141(v=VS.85).aspx) based on the results on the page load I'm doing a "Server Transfer" rather than the desired Postback (IsPostBack is false, PreviousPage is, as expected, the same page that should be posting back, IsCallback is false, and IsCrossPagePosting is false).

What could be going on to hyjack the AutoPostBack and the __doPostBack to make it look and act like a "Server Transfer"?

What can I set/check on the parent control /page to make sure it's allowing postbacks?

EDIT:

The Page_Load looks something like:

private SpecialDataObject _someData;
private string foobar;
public void Page_Load(object sender, EventArgs e)
{
    //set some variables.
    this.foobar = "blah";

    LoadSomeUnrelatedData();

    if (!IsPostBack)
    {
        if (_someData == null)
        {
            LoadDataWithoutBinding();
        }
        BindMyData();
    }
}

With a breakpoint at //set some variables the Page.IsPostBack is always false even after AutoPostBack.

EDIT 2:

The answer was in the Server Transfer. In a distant control loaded from the master page, the URL is inspected and rerouted before it hits the page, effectively negating my postback. I didn't see it before because I had added the breakpoints only in the target page.

Answer 1

I would check to make sure that you don't have validation somewhere interfering with the postback. To check this, set CausesValidation to false on the DropDownList.

Answer 2

Are you resetting the value of your dropdown in your PageLoad?

Also you may want to think about using an UpdatePanel so that it doesnt reload the whole page.

Answer 3

Is it inside an UpdatePanel? If yes, set ChildrenAsTriggers="true"

Answer 4

Based on the attempts you've mentioned, as well as the comments regarding the update panel , I attempted a few things.

By setting the data source on the load event, you will only do it once:

protected void Page_Load(object sender, EventArgs e)
{
    if (!Page.IsPostBack)
    {
        //set up data here
    }
}

You may use the Page.IsPostBack code and the method of yours to get what you want:

if (Page.IsPostBack)
{
    //do page reload logic in here
}

protected void foo(object sender, EventArgs e)
{
    //get your selected value here
}

(Note: Both of the postback conditionals are in the page load event)

EDIT:

Here's the whole setup, its basic, but you get the idea:

As you can see when I made a selection from cat to dog, it recognized that there was a postback, so it skipped the databind and set t. I could only assume there's something else here that I'm not seeing if you can't get it to ever return true for you on postback.