ViewState: Misunderstood monster or fluffy sleeping kitten?

The former!

Let me explain something to guys. ViewState is more complicated than you think it is. That's why you all abuse it so much :) Let me demonstrate:

public class MyControl : Control
{
  public string Text
  {
    get { return (string) this.ViewState["Text"]; }
    set { this.ViewState["Text"] = value; }
  }

  protected override void OnLoad(object sender, EventArgs args)
  {
     if(!Page.IsPostBack)
     {
        this.Text = "Hello World!";
     }
  }

  public override void Render(HtmlTextWriter writer)
  {
    writer.WriteLine(this.Text);
  }
}

What is WRONG with this picture?

Hmmmmmmm???

No.. that's not it.

You don't know?

I told you that's not it.

Give up?

That's what I thought. Sorry for my rudeness but it seems like no one on the planet knows why this is wrong. I have to deal with it every day as a lead programmer. But hey, I don't blame you. I've done this sort of thing myself. It looks perfectly harmless. You want the default value of "Text" to be "Hello World!". And why wouldn't you anyway, "Hello World!" has been serving us programmers since 1812. What better way to honor its 194th birthday than to make it the default value on your Text property?

No problem. Let it shine I say! But that's not what is wrong.

What's wrong is HOW you make it the default value. You know that HTTP is a connectionless protocol right? Well yeah, that's why Session state and ViewState were invented. To help you maintain data across those stateless requests. Brilliant, it really is! "Hello World" has never been so happy.

So let me ask you... What is this line of code for?

if(!Page.IsPostBack)

I know. It's so you don't do unnecessary work, right?

ViewState will be so kind as to save the value for you. Therefore, you'd be wasting CPU cycles if you had to set it on every request, right? Sure. What's that you say? Oh... You're also worried about overwriting the value in case someone has changed it? I see. Well that's pretty smart... I mean, afterall, ViewState is loaded before OnLoad. We wouldn't want to overwrite any precious changes to that value. Very good! Now wait just a second here...

You were on the right track, but you failed to understand the REAL problem here. The problem isn't that you would be wasting CPU cycles if you reassigned the value on every request...

Oh no. The problem is much greater than that.

The problem is that you are completely unaware of the purpose of ViewState. Did I say that already? Sorry, really I am...

Here... There's no better way than to show you.

public class MyControl : Control
{
  public string Text
  {
    get { return this.ViewState["Text"] == null ?
            "Hello World" :
            (string) this.ViewState["Text"]; }
    set { this.ViewState["Text"] = value; }
  }

  public override void Render(HtmlTextWriter writer)
  {
    writer.WriteLine(this.Text);
  }
}

What have we got here. Hmm... No OnLoad anymore.
"Hello World" has moved up into the property. If the value in ViewState is NULL we return it. That means, dun dun dun! We have a default value! And we didn't have to set it, its just THERE.

Ok... so big deal. Who cares?

I'll tell you who cares... and its not a fluffy sleeping kitten.

In the 2nd control, the value "Hello World!" is not persisted into the hidden form field "_VIEWSTATE". In the first control, it is.

Its a very... subtle... difference. But its VERY IMPORTANT!!!

I have seen controls written the "bad" way that contained dozens of properties. The result is, when someone puts this nasty control on a page, even BEFORE THE FIRST POSTBACK, their viewstate is HUGELY BLOATED with crap. If you do it the 2nd way, its just as big as if you had ZERO properties. If some unlucky Joe Programmer uses your control within a DataGrid or repeater, they'll be taking a hit once for every data item they bind.

VIEWSTATE IS NOT TO BE TAKEN LIGHTLY :)

Here's the rule of thumb that you must live and die by: VIEWSTATE IS FOR TRACKING CHANGES TO THE FORM'S STATE. Thats CHANGES. It is NOT meant to store the default values of your properties. Why on earth should it store default values? THEY'RE DEFAULT VALUES for crying out loud. Don't do it! :)

I feel much better now. Thank you! :)