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Adding ASP.NET User Controls at runtime via <script runat="server">

I am having trouble executing a control inside the <script runat="server"> tags in an *.aspx page.
The control works when it is defined declaratively in the HTML section of the page, but I am unable to make it work when placed within the script tag itself.
At the beginning of the page I register my control with:
<%# Register assembly="App_Web_exemple.ascx.cc671b29" namespace="Moncontrol" tagprefix="moncontrol" %>
Then, in the HTML, I call it (successfully) with the following declaration:
<moncontrol:exemple ISBN="9782894646151" runat="server" />
When I try to add it programmatically within the <script runat="server">, however, I am unable to execute it. I tried with the tags <asp:Text /> and <asp:Literal />, as follows, but that also doesn't doesn’t work.
In the HTML part:
<asp:Text id="TestControl" runat="server" />
In the script part
TestControl.Text = "<moncontrol:exemple ISBN=\"9782894646151\" runat=\"server\" />";

To clarify, what you're looking to do is programmatically add a User Control to your Web Forms page at runtime. There are a few ways of accomplishing this.
Before we begin, it's worth noting that the code you wrote likely "works" insomuch that it compiles and doesn't throw a runtime error. But it's also not executing the control. I suspect if you look at your HTML, you'll find the control declaration being output as a string literal (i.e., unprocessed by the server). It is then disregarded by the browser since it doesn't know what the <moncontrol:exemple /> tag represents. That's obviously not what you want.
Establishing a Control Container
Regardless of which approach you take, you'll want to start with some type of container on your page that you can add the control to, such as a Panel. If you don't want the container to output any wrapper markup, you can use a Placeholder:
<asp:Placeholder id="ControlContainer" runat="server" />
This serves a similar purpose as your current Text control, except its only purpose is to provide a container that you will add your user control to. From ASP.NET's perspective, however, this can be any type of server control, including a <script runat="server">, as per your request. More on that later.
Programmatically Creating the Control
Next, you're going to create the control programmatically. This is where we run into various options. The most universal approach is to use ParseControl() method (reference). This looks something like this:
Control control = Page.ParseControl("<%# Register assembly=\"App_Web_exemple.ascx.cc671b29\" namespace=\"Moncontrol\" tagprefix=\"moncontrol\" %><moncontrol:exemple ISBN=\"9782894646151\" runat=\"server\" />");
That will parse the control using the same method that processes the declarative syntax on the page, and return a Control object with your Exemple control as the first control in its Controls collection.
I find that syntax a bit sloppy, however, since it's representing a .NET object and its properties as a string literal. Given that, there are some cleaner approaches. In this case, it appears that your control is being compiled into an assembly and, therefore, likely has a Code Behind defined which inherits from UserControl. If so, you should be able to simply do something like:
Exemple control = new Exemple();
And then set the properties on it programmatically, the way you would in any other C# object. Much cleaner!
If your control was instead being compiled dynamically by the server, then you'd instead use the Reference directive with the LoadControl() method, as described in the MSDN article How to: Create Instances of ASP.NET User Controls Programmatically. I don't believe that method will work for you, however.
Adding the Control Instance to the Page
Regardless of which approach you take, the next step is the same: you then add the control you've programmatically added to your page by adding it to the Controls collection of the target container. E.g.,:
ControlContainer.Controls.Add(control);
Note: You can technically just add this to the Page class's Control collection, too, but that doesn't give you any control over where on the page it is placed; having a PlaceHolder control (or equivalent) lets you specify exactly where you want the control to appear.
I hope this helps. There are a couple of caveats depending on how you wrote and compiled your control, but this should give you the basic structure needed to address your problem.

Create User Control is .Net without predictable ClientID Mode

I am creating a fieldset user control which makes it easier to use in my web application.
However, since it is a user control it needs a unique ID to create the control and the child controls inside of it
I have a ASP Placeholder control to allow the user control to accept child controls, but creates a longer and longer ClientID that isn't really necessary as this is standard .NET programming.
Is there a way to turn off this automatic referencing within the my user control and still retain the referencing in the parent object which the fieldset is in? (It is in a masterpage by the way...)
What I am asking is --- is there a way to create a usercontrol without the child elements of the usercontrol having their ID based on the parent usercontrol... I notice that the ASP UpdatePanel has this ability and want to replicate that same functionality.
Much appreciate your feedback.

There are a few ways to deal with this there is a good resource at 4guysfromrolla
However in summary some of your options are :-
The first is that you could make all your client id modes static - have a look at the ClientIDMode in web.config
Don't make predictable client id modes but use the Id of them in your code such as
var txt = document.getElementById('<%=TextBoxID.ClientID%>');
So here you would inject the client Id using ASP.NET directly into your HTML.
Override the client ID to be static for each property - this is where you can specify in .NET what you want the client ID to be foreac control you are using.
<asp:TextBox ID="txtName" runat="server" ClientIDMode="Static" />
Hope this helps!

Dynamically Change Label Text in SharePoint Rendering Template

If I did not have a rendering template I could add the following to my aspx page:
<asp:label ID="myLabel" runat="server" />
Then this in my code behind:
myLabel.Text = "Hello World";
But since my label is inside of a <SharePoint:RenderingTemplate> I am not able to access it the normal way.
Is there another way?
Edit: Ive found a number of articles like this one talking about creating a .dll for every rendering template. Is it really that complicated to output a string to a page? I should clarify that I am open to all ideas. I do not need code behind. I simply need to output a dynamic string to the template.

You can just create your own label control and work with it in any way you like.
There is an example here: Extending the Label Control, but it's basically like you would extend any other control.
What I would do then: Either set the .Text in that custom control directly, or what would be better is to use resource files. Then you could create your won property on the label like so:
<Custom:label ID="myLabel" specialresource="myLabel" runat="server" />
And in your class you could just handle the specialresource property and e.g. set the .Text to whatever you get from the resource.

ASP.Net: User control with content area, it's clearly possible but I need some details

I have seen two suggestions for my original question about whether it is possible to define a content area inside a user control and there are some helpful suggestions i.e.
Passing in content to ASP.NET user control
and
ASP.NET User Control inner content
Now, I like the theory of the latter better than the former just for aesthetic reasons. It seems to make more sense to me but the example given uses two variables content and templateContent that the answerer has not defined in their example code. Without these details I have found that the example does not work. I guess they are properties of the control? Or some such?
EDIT - DETAILS: What I am trying to do
I have need of an ASP.Net user control that conceals some content in a panel inside a placeholder and asks for the input of a code in a visible panel.
Essentially the user will put their code into the provided textbox in Panel A and submit it, it will be checked and, if it is valid, panel B and the locked content will be displayed.
I have done a test where the content was hard coded into panel B but as soon as I need to make the content a generic input it fails. If it were just text or somesuch then I could make it a property of the control, but as it is, in fact, another User Control I am having some difficulty getting this into the "hidden" panel.
Any other workable solutions are also welcome.
EDIT NOTE: The solution I'm trying to implement this in 2.0 I did find a 3.5 solution which I cannot use.
The former example seems workable but I'd prefer to go with the latter if someone could fill in the blanks for me.
Thanks.

Okay, so this is disturbingly easy but many of the tutorials on the web that talk about this kind of thing push to do extravagant things that require the control to parse ListItems or such.
So this solution is purely so that you can build a control that, for whatever reason, has a placeholder in it that could have anything inside it (kind of like a content area on a Master page). In this instance it happens to be because the Panel containing the placeholder is hidden until appropriate input actions have taken place in another panel.
First, you need to add this:
[ParseChildren(true,"Content")]
[PersistChildren(false)]
just above the part of the control which looks like this:
public partial class MyControl : System.Web.UI.UserControl
then in the control scoped declarations at the head of the control you want to declare thus:
private Control _content;
[PersistenceMode(PersistenceMode.InnerProperty)]
public Control Content { get { return _content; } set { _content = value; } }
Finally you need to place the content into the placeholder like this:
phContent.Controls.Add((Control)_content);
That last line goes into the Page_Init event. For reference "phContent" is the name of the place holder where you want the content to appear, like this:
<asp:Panel ID="pnlLockable" runat="server" Visible="False">
<asp:Placeholder runat="server" ID="phContent" />
</asp:Panel>
On the front end the resulting implementation looks like this:
<uc:MyControl runat="server" ID="lockit1">
<Content>
//...your stuff goes here...
</Content>
<uc:MyControl>
Note that I presume that what is inbetween the Content Tags is a root control. This is because I nested another user control in there. I imagine if you put whatever content you want within a panel or placeholder it should be fine.

Also you can read "How to: Create Templated ASP.NET User Controls". Really helpful.

Building Dynamic UI with C#?

I need to expose some input fields based on what properties I find for particular types in an assembly.
I'm not sure how common an approach like that is. Perhaps there are easier ways. Maybe on the client side instead of server.
If anyone knows of a good way of doing something like this, I would appreciate the help.
Create input controls accordingly and simple add control to some div container? I'm not sure if it would be more complex than that.
I'll need to somehow add css classes to the controls as I build them so they get placed nicely; that might get tricky.

This all sounds like standard asp.net development. Any good tutorial should be able to help you. For the asp server controls, you use the CssClass property to set the class for the control.
Here is the asp.net tutorial from the W3C Schools.

I assume you will use reflection to figure out what properties entity has, then you would based on the type of the property create an input field. You would have to dynamically create control to handle input in code behind. Make sure you give that control and id. You will have to recreate these controls on the post back. This looks to me like dynamic property editor. There might be some free ones, google for it.

If the UI doesn't have to be completely dynamic you could include all the controls in the markup with any optional ones set to Visible="false". Then, selectively enable the appropriate controls in your code-behind. For example:
Default.aspx
<asp:Button ID="EvenButton" runat="server" Text="Even" Visible="false" />
<asp:Button ID="OddButton" runat="server" Text="Odd" Visible="false" />
Default.aspx.cs
protected void Page_Load(object sender, EventArgs e)
{
String msg = "A message to count";
if (msg.Length % 2 == 0)
{
// Enable the Even Button
EvenButton.Visible = true;
}
else
{
OddButton.Visible = true;
}
}
The advantage of this method is that you can lay things out with the appropriate CSS easily in the markup. If, on the other hand, your UI is much more dynamic than this, you'll probably have to resort to dynamically creating controls in the code-behind and adding them to the page via calls to Controls.Add(). This way, however, is harder to layout. And you have to deal with things like re-wiring any event handlers on each postback.
Hope that helps.

I ended up leveraging jQuery.
I laid out a simple markup with the basic layout I would need.
For creating controls dynamically, I did it all in javascript using jQuery methods.
This of course requires that you return some data set to the UI intelligently enough to render it.