Really Dynamic Master Pages

• Download demo project in C# - 3.83 KB
• Download demo project in VB.NET - 3.77 KB
• Download demo with nested master pages (C#, VS2010) - 23.7 KB

Introduction

Master pages are a very cool feature in ASP.NET 2.0. They allow you to easily create a consistent look and feel throughout your website. They also provide an easy way to set/change the master pages at runtime. This is achieved by setting the MasterPageFile property of a page in the PreInit event. This, however, requires that the master pages have ContentPlaceHolders which the Content controls on the page reference through the ContentPlaceHolderID property. This article focuses on creating ContentPlaceHolders, Contents and linking them together programmatically.

Background

When I first used ASP.NET 2.0, one of the first features I used were master pages. I soon learned how to switch between such at runtime. Some time later, I was working on a small project, where the goal was to create as many things dynamically as possible (kind of a web application framework). I had a thought about using master pages that are stored in a DB (as metadata) and then later building them dynamically. I soon found out that this task is not as easy as it looks. It requires more than just the knowledge of how to create controls through code. I did some research on how to properly do this and I wanted to share this technique, hence this article was born.

Using the Code

The sample provided contains two pages and a master page. Its only purpose is to demonstrate how things are done. On the start page (Default.aspx), you are to enter a number, press a button and let the magic begin. Despite the visually not very attractive result on the next page (only some text can be seen), the magic still does happen in the background. Let's see what happens. For this, you should have some knowledge about the ASP.NET page lifecycle.

The most important step is to create the content on the page. You will be surprised to find that no creation of Content classes is necessary. Instead you should use the AddContentTemplate method of the Page class. Chances are you never heard of this method, it is because it has the [EditorBrowsable(EditorBrowsableState.Never)] attribute applied and thus isn't visible for IntelliSense. MSDN has some information about it, you can read it here. You need two parameters, the first being the name of the ContentPlaceHolder it will be linked to (you can only set the ContentPlaceHolderID attribute in the markup), the other an ITemplate. If you have never worked with templated controls, you should find this a bit complicated, but it really isn't. However explaining this still goes beyond the reach of this article. In this example, all the content is the same, but that's not obligatory, then again easier to implement and satisfactory for demonstration purposes.

protected void Page_PreInit(object sender, EventArgs e)
{
    //...same check as in the constructor of the Master Page 
    for (int i = 1; i < num + 1; i++)
        base.AddContentTemplate("ContentPlaceHolder" + i.ToString(),
            new CompiledTemplateBuilder(new BuildTemplateMethod(this.Build)));
}

Using constructors in ASP.NET is a very uncommon scenario, but in this case there's no other place to put the code needed, because the master page's Init event is already too late and there's no PreInit event. You just need to add the names of the ContentPlaceHolder controls to be created later to the ContentPlaceHolders collection of the Master class. Note that you must use the names in lowercase.

public MasterPage()
{
   object cphnum = HttpContext.Current.Session["cphnum"];
   if (cphnum == null || !(cphnum is int))
      return;
   num = (int)cphnum;
   for (int i = 1; i < num + 1; i++)
      base.ContentPlaceHolders.Add("contentplaceholder" + i.ToString()); 
}

Finally you should instantiate the ContentPlaceHolders and add them to the control hierarchy of the page. This is also achieved in a for loop, so that the required number of controls gets created. For this purpose, you must use the ContentTemplates collection the master pages base, which as of type is an ITemplate.

protected void Page_Init(object sender, EventArgs e)
{
    //...
    for (int i = 1; i < num + 1; i++)
    {
    ContentPlaceHolder cph = new ContentPlaceHolder();
    cph.ID = "ContentPlaceHolder" + i.ToString();
    PlaceHolder1.Controls.Add(cph);
    ((ITemplate)base.ContentTemplates["ContentPlaceHolder" +
        i.ToString()]).InstantiateIn(cph);
    }
}

Now that you know the basics, you can easily extend the sample to be more useful than its current form.

Nesting master pages 

Creating nesting master pages with this technique is also supported, however a slight modification of the code is needed. Theoretically a nested master page is nothing more then both a master page with it's own ContentPlaceHolders and a regular page with Contents to fill in it's parent's placeholders. Note that content needs to exist on the content page before the master can use it to populate it's placeholder, however the init events fire in opposite order, so the main master's one fires before the nested one's, resulting in a NullReferenceException (or when checked for, in empty content). And the constructor is too early to use for this, because then it would ruin the synergy between the nested master and the content page, for similiar reasons. So another event must be used called FrameworkInitialize(). This fires in the needed time window, and also in the correct order so it allows for deep nesting. A demo is also available for this scenario, see the list of downloads above. 

History 

• 12.02.2007 - Initial posting of the article
• 16.04.2007 - Added VB.NET demo
• 07.05.2012 - Added a section about nesting