Tuesday, February 10, 2009

Persisting Data in User Controls with ViewState

In ASP.NET, the ViewState is used to persist data across requests. Let's say, for example, I want to use a User Control that will show information based on a particular date. I could create a property in that control that looks like this:
private DateTime _Date;
public DateTime Date
{
set
{
_Date = value;
ViewState["Date"] = value;
}
get
{
if (_Date.Ticks == 0 && ViewState["Date"] != null)
{
_Date = (DateTime)ViewState["Date"];
}
return _Date;
}
}
... which would make it possible for me to "set and forget" the date from the page containing that user control. For example:
protected void Page_Load(object sender, EventArgs e)
{
if (!Page.IsPostBack)
{
AddTrackedTime1.Date = DateTime.Today;
}
}
So the first time the page loads, it sets the user control's date. You could have events set up to change the date on that user control if you want. Each time the date gets set, it will add a binary value to the ViewState, which will persist in the form of a hidden field on the webpage that is being viewed. When the time comes to do something with that date value, it will automatically be loaded out of the ViewState. Because the viewstate is persisted in a hidden form field, it will continue to work properly even if the user clicks "back" several times in the browser and then clicks on a control from their browsing history. Also, unlike the Session, the ViewState is specific to the control instance, so you could have several of these controls on the same page and they wouldn't step on each others' toes.

The only potential downside is that you increase the amount of data sent to and from the browser, which could slow transactions somewhat.

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