AutoPostBack: true vs false

Question

Before I begin, I have already seen this question about a very similar topic (as well as this one and this one), none of which answer my question completely. I already understand the concepts presented in these questions/answers, but I have more questions.

A) What happens if you have multiple controls with AutoPostBack="false" and you change a number of them before a postback? Take the following brief example (assume that everything else needed for the page is written correctly and trivially; e.g., Page_Load):

Default.aspx:

<asp:DropDownList ID="ddlFoo" runat="server" 
    OnSelectedIndexChanged="ddlFoo_Changed" AutoPostBack="false" >
    <asp:ListItem Text="a" />
    <asp:ListItem Text="b" />
    <asp:ListItem Text="c" />
</asp:DropDownList>
<asp:DropDownList ID="ddlBar" runat="server" 
    OnSelectedIndexChanged="ddlBar_Changed" AutoPostBack="false" >
    <asp:ListItem Text="1" />
    <asp:ListItem Text="2" />
    <asp:ListItem Text="3" />
</asp:DropDownList>
<asp:Button ID="btnQux" runat="sever" Text="Click for PostBack" OnClick="btnQux_Click"

Default.aspx.cs:

protected void ddlFoo_Changed(object sender, EventArgs e)
{
    Response.Write("ddlFoo changed to " + ddlFoo.Text + ". ");
}
protected void ddlBar_Changed(object sender, EventArgs e)
{
    Response.Write("ddlBar changed to " + ddlBar.Text + ". ");
}
protected void btnQux_Changed(object sender, EventArgs e) { }

Now, say you change ddlFoo to 3 and then ddlBar to b. Then, you click btnQux. You get the following output from Response.Write after clicking:

ddlBar changed to b. ddlFoo changed to 3. 

Why does this happen? Do the OnSelectedIndexChanged methods get put into a stack to be called once a postback happens?

B) Why does my webpage load much more quickly when I use this approach and set AutoPostBack="false" for most of my controls? To be specific, I did this for a CheckBox, a DropDownList, and a TextBox in a GridView, which retrieved ~1200 rows and 27 columns of data and took 10s in VS2008 debug mode versus 310s before. Why would the load/refresh time be so much faster?

EDIT: I released the code earlier this afternoon, and there was no significant difference between the load time of the old (AutoPostBack="true") and new (AutoPostBack="false") versions. I think that perhaps the debugger was doing something extra, which caused the large jump in load time. A better way to rephrase question B) might be this then: What could the debugger have been doing to cause this large jump in load time?

Answer 1

Warning: I'm no ASP.NET expert... If this turns out to be garbage, I'll delete it :)

A) I believe you will see the new values of all the controls, whenever the postback ends up happening, including all the change events, just as you described. The values have changed, after all - the AutoPostBack just affects the timing (and whether the postback occurs at all, of course).

B) There's more Javascript in the HTML delivered with AutoPostBack = True on all the controls, but not enough to make that enormous difference. As noted in your edit, it looks like that was a transient issue anyway - we can't really explain transient issues without more diagnostics.