CodeBehind

While our first example was fine, we unfortunately broke one of the coding principles of ASP.NET: To separate markup and code. As you noticed, we added a scripting block (using <% %>), where we wrote a line of C# code to use the label. While this is just fine for a small and simple example like this, we would soon get a real mess with a bunch of C# code within an even bigger amount of HTML code. If you throw in some JavaScript and some CSS as well, it will soon become very chaotic to edit. That's why MS introduced CodeBehind, a technique which allows you to completely separate markup (HTML, CSS etc.) and code (C#, VB.NET etc.). So let's remove the script block (from <% to %>) and save the file.

Zoals we eerder bespraken, heeft VS een bestand toegevoegd met de naam Default.aspx.cs. Als je het niet kunt zien in de Solution Explorer, klik dan op het kleine plusteken links van het bestand Default.aspx. Open dit bestand. Als je nog niet eerder met .NET of een andere niet-web programmeertaal hebt gewerkt, kan het er op dit punt een beetje intimiderend uitzien. Het ziet er helemaal niet uit als HTML. Ik zal echter proberen de verschillende delen van de inhoud uit te leggen, en hopelijk zal je al snel zien dat CodeBehind een geweldige tool is om een beter overzicht van je werk te krijgen. Hier is een volledige lijst van het bestand zoals het er op dit moment uitziet

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;

public partial class _Default : System.Web.UI.Page 
{
    protected void Page_Load(object sender, EventArgs e)
    {

    }
}

For now, the top part is rather irrelevant to us. It's a list of namespaces being included with the using keyword, for usage in the file. Since this is an ASP.NET tutorial and not a dedicated C# tutorial, I won't explain this in depth. Next, we have the class. Classes are a part of the concept of Object Oriented programming, which has become very popular, especially with languages like Java and C#. OO is a very complex subject, which also won't be explained within this tutorial.

The name of this class is "_Default", and the : (colon) tells us that this class inherits from the Page class in the System.Web.UI namespace. This means that our page can already do a bunch of things, without any programming, because it inherits methods and properties from another class. All ASP.NET pages inherits from the Page class, or another class which inherits from the Page class.

The only method within this class is the Page_Load, which is called every time the page is loaded. Let's use that to our advantage, and set the text from this method. We can use the exact same line of code as before, but of course without the script block tags. Add the line of code between the { and } characters:

HelloWorldLabel.Text = "Hello, world!";

That's it. Run the project (F5), and have a look. The page looks exactly like before, but we have just used CodeBehind for the first time. But this example is starting to get a bit old, so in the next chapter, we will look into something a bit more interesting.