Develop Reference

verify content control's ContentPlaceHolderID attribute in the content page

I am a new ASP.NET developer. I am making development using C#. I removed some default contents from that I am getting these error notification as attached in screen shot can anybody look at and give me a solution also some one inform me from where .NET pages control header and footer so I can make modification there.
Here is the complete error heading text:
"Cannot find ContentPlaceHolder 'MainContent' in the master page '/Site.Master', verify content control's ContentPlaceHolderID attribute in the content page."

Remove asp:Content ContentPlaceHolderID="MainContent" from your child pages or Add <asp:ContentPlaceHolder ID="MainContent" runat="server"> into your Master page.

There is no ContentPlaceHolder named 'MainContent' in your master page,
A content place holder should be in your Master page like this:
<asp:ContentPlaceHolder ID="MainContent" runat="server">
</asp:ContentPlaceHolder>
And child pages like this:
<asp:Content ID="Content2" ContentPlaceHolderID="MainContent" runat="server">
</asp:Content>

When you have a child page that inherits from a master page ContentPlaceHolder should match between them.
So if you masterpage declare that every child should fill its Box1 Box2 and Box3containers every
child page that is linked to it (MasterPageFile="~/MasterPages/Site.master") must contain this references <asp:Content runat="server" ContentPlaceHolderID="Box1"> even if empty.
In your case you have declared a box name that Master page doesn't has
update
looking at your code I can suspect that you have linked the master page in a wrong way.
Change the attribute of your page and be sure that the path is correct
MasterPageFile="~/MasterPages/Site.master"
if it doesn't work try this
If the page is empty delete it and recreate aspx page with master page, after you select the master page the child page will be created with the right contentplaceholders.

Error: Extender controls may not be registered before PreRender

I am trying to add Ajax Controls to my *.aspx pages.
I just added
<asp:CalendarExtender TargetControlID="UserName" runat="server">
</asp:CalendarExtender>
And getting the error. Any solution?
Please don't tell me to read the similar questions on stackoverflow.com. None worked for me.

As discussed in the chat, you should never put the :
<asp:ToolkitScriptManager ID="ScriptManager1" runat="server">
</asp:ToolkitScriptManager>
Inside a LayoutTemplate, as in the Login control for you case, see http://forums.asp.net/post/5101565.aspx.

Changing ScriptManager's attribute globally

In each projects we have several pages which have the following tag
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
Now we would like to change those to (about 80% of Pages)
<asp:ScriptManager ID="ScriptManager1" EnableScriptGlobalization="true" EnableScriptLocalization="true" runat="server">
</asp:ScriptManager>
Is there any way to do that from a single source like Web.config or Global.asax or using any HTTPHandler.

If you have a BasePage or a Master page then you could find the controls before the render event and modify it's properties

You could simply use a search-and-replace regex to do this job. :)

I have made an user control and placed the ajax calendar code in that. After that I replaced the Ajax calendar in the page with the user control.
Now for changing globally I can change it from the single user control.

Register Client Script with Dynamically Loaded Control in Ajax Call

I have a page that contains a custom tab control. When you click different tabs, it does a an ajax callback. During that ajax call, the code dynamically loads a different control based on which tab was clicked, then adds it to the appropriate tab. So basically I have some code that does a switch statement, uses LoadControl(), then adds the control in.
The issue I'm having is that none of the javascript within each of those controls that gets loaded is getting registered on the page. Based on this thread:
RegisterClientScriptBlock within AJAX method call
I thought I just had to switch from using Page.ClientScript.RegisterClientScriptBlock to ScriptManager.RegisterClientScriptBlock. I did that, but still nothing. Am I misunderstanding something about the ScriptManager? I want the javascript to be registered from within the dynamically loaded control, which happens to be loaded during an AJAX call.
Thanks in advance.

When load Update panel after Postback, you need to register the scripts located in the affected area again:
<asp:ScriptManager ID="ScriptManager1" EnablePartialRendering="true" runat="server">
<Scripts>
<asp:ScriptReference Path="~/Resources/js/registeragain.js" />
</Scripts>
</asp:ScriptManager>
<asp:UpdatePanel ID="UpdatePanel1" runat="server">
<ContentTemplate>
// AREA
</ContentTemplate>
</asp:UpdatePanel>
registeragain.js
Sys.Application.add_init(AppInit);
function AppInit(sender) {
Sys.WebForms.PageRequestManager.getInstance().add_endRequest(End);
}
function End(sender, args) {
//Aqui se vuelven a llamar los scripts necesarios despues de que carge el Update Panel
}

I had the same issue when loading web controls within an UpdatePanel. The problem is that the JavaScript code that is dynamically loaded into the page is not automatically parsed.
I have solved it from the information in this article
Just reuse the InlineScript control that is provided and wrap it around your <script /> tags. It will do the rest for you.

Loading a control From a master Page

The "Header control" we use holds a jquery reference. When I attempt to leverage any jquery functionality from the Master page it fails because the jquery reference has not been added yet.
Is there a way to force the Header Control that is embedded into the Master Page to load it's resources before the Master Page attempts to reference them?

I'd include the jquery reference in the head of the master page itself.
Or if you don't want the jquery on every page then you can do this in your master page:
<head runat="server">
<asp:ContentPlaceHolder ID="head" runat="server">
</asp:ContentPlaceHolder>
</head>
And then do this on the aspx page that needs jquery:
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script>
</asp:Content>
Either way you do it accomplishes the same thing. It includes the jquery file in the "head" of your HTML. And that's the easiest way to ensure jquery works properly.

You can use the ScriptManager in the template master to register your javascript inclue file for JQuery.
You can use
ScriptManager.RegisterStartupScript or
ScriptManager.RegisterClientScriptInclude
Another way is in the template master, in the oninit method use this code
Page.Header.Controls.Add(new LiteralControl(" <script src="jquery" type="text/javascript"></script> "));

You could simply move the JQuery script reference into the aspx of the master page itself (at the top, of course). Then, any controls would be able to access JQuery.