State in .NET

Print QR Code in .NET State

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The page is very different, as shown in Figure 7-8.

The TextBox still shows the current value. As mentioned in the note above, simple controls such as TextBox ignore the EnableViewState property and always preserve view state. However, the Label and GridView controls do respect that property. Since they are only populated the first time the page is loaded, they no longer display the current data after the page is posted back to the server. The Label reverts to its default Text property, and the GridView is not even rendered to the browser if there is no data bound to it. In addition to preserving the values of controls, View state is very handy for something else; you can programmatically stash your own stuff in a data structure known as the state bag, using the ViewState keyword. The state bag stores data as attribute/ value pairs in a dictionary. The attribute is a string, which is the name of the object stored as the value. The types of objects that can be stored efficiently are the primitive data types, including integers, strings, bytes, and Booleans. You can also store arrays of primitive types. Complex types, such as DataSets and custom objects can be stored as well, but performance will suffer. The next example demonstrates stashing values in the state bag and then retrieving them. In this example, a counter keeps track of the number of times a button on a page has been clicked. As long as the page is current, the count will be correct. If you move to another page and then return to this page, the count will start over. Create a new web site called StateBag. On the Default page, drag a Label, called lblCounter, and two buttons. Set the Text property of one button to Increment Counter, and the Text property of the other button to Navigate.

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You ve seen variables in previous chapters, and you ve seen collections, like the collection of ListItems in a RadioButtonList. An array is simply another type of collection, in which you can store a bunch of objects in a single variable, provided they re all of the same type. You declare an array similar to the way you would create a variable:

This code creates an array called myArray, which can hold integers. Specifically, the (5) states that the array can hold six integers. Why not five as the code suggests Because just as with controls, array indexes begin at zero. To access the third integer in the array, you d just use this syntax:

myArray(2)

Use curly braces if you want to initialize the array when you create it:

Dim myArray( ) As Integer = {42, 36, 128, 53, 7, 85}

Notice you don t have to specify the length of the array; the compiler will automatically set it to a length of six. Arrays are particularly useful with ForEach loops, like this:

That little loop increments each element in the array by 1, and you don t need to know what values are in the array, or how many there are. One drawback to arrays is that if you want to retrieve a value from an array, you either have to know the index of the value you want, or else loop through the array until you find it. The dictionary is a specific kind of array (also known as a Hashtable) that solves this problem by associating each value with a key, instead of an index. For example, you could have a dictionary of U.S. states, using their abbreviations as the key values. Then, to retrieve a state s name, you d just have to know the abbreviation (the key). The following code snippet creates and partially populates a Hashtable to hold the states:

Dim States as New Hashtable( ) States.Add("CA", "California") States.Add("MA", "Massachusetts") States.Add("PA", "Pennsylvania")

To retrieve the name of the dictionary entry with the key value of MA, you would use the following code:

Dim strStateName as string = States("MA").ToString( )

In the case of the state bag, the attribute names are the keys, and the values of those attributes are stored as the value part of each dictionary pair.