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How would I let 1 .aspx page use another .aspx page's code?

I have 2 aspx files in my project. The first.aspx page has some content on it and when I click on a button, it will launch a frame (second.aspx that only has code to show a calendar) on the same page.
Now once that calendar(second.aspx) loads on first.aspx, I want to click a link on the calendar that will .show() a hidden DIV on the first.aspx page.
How do I access code cross pages? In other words, how can I write some code in second.aspx that will affect first.aspx.

What you're asking for is not really possible. You're probably approaching it the wrong way. What you should do is turn your calendar page into a user control so that it can be used seamlessly in first.aspx.
Here is how to get started with user controls in asp.net:
After you turn it into a user control there are different approaches to getting access to the properties of the user control from your page. Here is one approach using the FindControl method.
Hope that helps.

The easiest solution would be to show and hide your div with jquery. Simple give your div a class like:
<div class="myCalendarDiv" style="display:none" />
And your Button should look like this:
<asp:Button id="myButton" OnClientClick="return ShowCalendar();" runat="server" />
<script type="text/javascript">
function ShowCalendar() {
$(".myCalendarDiv").show();
return false;
}
</script>
Another way would be instead of creating a seprate webpage for the calendar, as proposed you can use a jquery dialog, or make a usercontrol and embedd it on the same page.

(Posted on behalf of the OP).
So since I was dealing with an Iframe, I found out that you can target the parent window which would be first.aspx.
I used "window.parent.MYFUNCTION();" to call my JavaScript function on first.aspx and show the div.

C# ASP.NET Custom Control not showing up

I'm trying to construct a custom control for ASP.NET
I started by creating a Web Application in VS2010 and creating a new .ascx page. The page is called "TestBox" and it's just a single TextBox control with "This is a test" as the text.
I built the project and then included the DLL in another website in order to make sure I would be able to move controls. Based on a tutorial I found here I added the following line of code to the top of the page:
<%# Register TagPrefix="TestControl" Namespace="TestControl" Assembly="TestControl" %>
Then I added this to the page itself:
<TestControl:TestBox ID="TestBox1" runat="server" />
The code compiles and the page loads without throwing up any errors, but when it loads it's completely blank. By introducing a deliberate runtime error, I determined that the TextBox is definitely being loaded, but the control itself still isn't showing up.
Am I missing something?
Code for the TestControl:
<%# Control Language="C#" AutoEventWireup="true" CodeBehind="TestBox.ascx.cs" Inherits="TestControl.TestBox" %>
<asp:TextBox ID="TextBox1" runat="server" ontextchanged="TextBox1_TextChanged">This is a test</asp:TextBox>
I haven't touched the Designer code or the .cs code in any way.
EDIT: Figured it out. I had declared a namespace for the .CS file but not the .ASPX file itself.

The answer was that I had to add a namespace to the ASPX file itself and not just the underlying code file. I forgot to add Class="TestControl.TestBox" to the page declaration.

Using a custom control more than once on the same page / form .net

I have been building a custom control for some time now and overcome a number of hurdles. One challenge I have yet to resolve is the ability to use a custom control more than once on the same page.
I have a custom control that functions well on its own, but when two of the same controls are placed on the page the second control is able to control the first one. My guess is that the first one (control) is the first object and the second one is the same object. How can I make sure in the code that if I use the same control more than once on a page it will behave as two separate controls. Are there any specific things I should look at to make sure it allows it to be on a page more than once.
Thanks in advance.

When you add multiple instances of a control, be sure to give them different IDs. Then when writing any code that will interact with them, reference them by that ID.
<%# Register Src="controls/myControl.ascx" TagName="myControl" TagPrefix="uc1" %>
<asp:Content ID="Content1" ContentPlaceHolderID="MainPlaceHolder" runat="server">
<uc1:myControl ID="ctlFirst" runat="server">
<uc1:myControl ID="ctlSecond" runat="server">
</asp:Content>
Then in the code behind:
ctlFirst.SomeProperty = true;
ctlSecond.SomeProperty = false;

Place a couple of instances of your custom control onto an ASPX page then view the HTML source and have a look at all the element IDs generated on each of the control instances. ASP.NET will automatically mangle the IDs of your control's children, prefixing them with the ID of the parent control. If you're outputting raw HTML, this might not happen. If there are any duplicate IDs, then that may be the cause of your problem, particularly if you're using client-side logic to manipulate the controls on the page.
Also, make sure that you're not using any session or application variables in your controls.

Controls on aspx page can't be accessed from code-behind

i have the following problem:
i work in web application and only today. any control i put on the page, when trying to access it from behind code. it doesn't appear at all. what are the probabilities and reasons for this problem.
note:
i work from a thin client. they make some maintenance on their server today. and after that i find this problem.
the previous controls on my page can be accessed normally with out any problems.

See if the controls have an ID this might sound stupid but perhaps yesterday someone
created the view with a snippet label *tab *tab and forgot to add an id
Below result of label *tab *tab ..
<asp:Label Text="text" runat="server" />
Should be
<asp:Label Text="text" ID="lblInfo" runat="server" />
Else check the attributes of the page / controls / codebehindfile

make sure the CodeBehindFile attributes are set correctly

see if run at server attribute is properly set on new controls

Can you change a master page's ContentPlaceHolder's Content Page Asynchronously in .NET?

From what I've already read this appears to be impossible, but I wanted to see if anyone out there has a secret trick up their sleeve or at least a definitive "no".
Supposedly a master page is really just a control for a content page to use, not actually the "master" of a content page. If I wanted to go from one content page, to another content page with the same master page, I would just say
Response.Redirect("PageB.aspx");
But this would immediately cause a postback, flickering the page, which is the crappy pre-ajax way of doing things.
In this current project, I'm trying to see if I could figure out how to change the current content page of a ContentPlaceHolder in the master page asynchronously, when a button is clicked on the master page.
Is this possible, if so how?

I don't know if you can between pages (.aspx) but it can definitely be done using UserControls.
ASP.Net pages each have their own URL so what you're trying to do is to go from one URL to another without any postback, that's just not how it's supposed to work.
Using user controls (.ascx):
Create a page that uses the MasterPage and use something like this in the content
<ajax:UpdatePanel ...>
<ContentTemplate>
<asp:PlaceHolder ...>
</ContentTemplate>
</ajax:UpdatePanel>
Search for UpdatePanel and tweak its settings to do what you want, then learn how to swap user controls in a placeholder.

No, you cannot because a master page is actually a control rendered on a particular aspx page, rather than actually containing the aspx page as it deceptively appears to be programmatically and in design view.
More Info:
You could however use a variety of other controls to simulate this effect. The asp:MultiView control is one example, each "page" could be made in a single view and placed in an update panel, thus allowing it to be switched asynchronously. Alternatively you could define each page in a separate user control and put those in an update panel, asynchronously switching the visible property on those controls as needed.
There are really a lot of different ways to achieve an effect similar to changing the master page's content placeholder.