Question

textBox values across postback do NOT maintain in Viewstate.

http://msdn.microsoft.com/en-us/library/ms972976.aspx

When the ASP.NET Web page is posted back in the load postback data
stage, the Page class sees that one of the posted back form fields
corresponds to the IPostBackDataHandler interface. There is such a
control in the hierarchy, so the TextBox’s LoadPostData() method is
invoked, passing in the value the user entered into the TextBox
("Hello, World!"). The TextBox’s LoadPostData() method simply assigns
this passed in value to its Text property. the values are identified
via posted back form field values, and assigned in the LoadPostData()
method for those controls that implement IPostBackDataHandler.

Now lets talk about TextBox.OnTextChanged Method

It says there that:

Note

A TextBox control must persist some values between posts to the
server for this event to work correctly. Be sure that view state is
enabled for this control.

Question:

So I guess that for certain events, it does write to the Viewstate…

How does it tell ASP.NET to "Start tracking changes into the Viewsstate"?

Answer 1

When you enable ViewState for a TextBox, it will simply be saved there.

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ASPX Page Example

 <asp:TextBox ID="tb" runat="server" EnableViewState="true"></asp:TextBox>

C# (Code-behind) Example

tb.EnableViewState = true;

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Your quote talks about the fact that this value is not needed when restoring the Form on Postback.

So you can turn off ViewState for a TextBox and it will still retain its value. But OnChanged will not work.