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sList.Add(14, “item 5”) sList.Add(18, “item 1”) sList.Add(12, “item 7”) sList.Add(19, “item 0”) sList.Add(13, “item 6”) While SLEnum.MoveNext

Console.WriteLine(“Key = “ & SLEnum.Key.Tostring & “, Value= “ & _ SLEnum.Value.ToString & “, Cos(key) = “ & _ Math.Cos(CType(SLEnum.Key, Double)))

End While

If you execute these statements, they will generate the following output:

Key = 16, Value= item 3, Cos(key) = -0.9576594803233847

Key = 10, Value= item 9, Cos(key) = -0.8390715290764524

Key = 15, Value= item 4, Cos(key) = -0.7596879128588213

Key = 17, Value= item 2, Cos(key) = -2.7516333805159693E-01

Key = 11, Value= item 8, Cos(key) = 4.4256979880507854E-03

Key = 14, Value= item 5, Cos(key) = 0.1367372182078336

Key = 18, Value= item 1, Cos(key) = 0.6603167082440802

Key = 12, Value= item 7, Cos(key) = 0.8438539587324921

Key = 13, Value= item 6, Cos(key) = 0.9074467814501962

Key = 19, Value= item 0, Cos(key) = 0.9887046181866692

Once you have declared a custom comparer, you can use it with the BinarySearch method as well. The binary search algorithm uses consecutive comparisons to locate an item in a sorted collection. If the custom comparer is in place, it simply calls your Compare() function to perform the comparisons and locate the item in the collection. The form of the BinarySearch method that uses a custom comparer to locate an item is

BinarySearch(object, comparer())

where object is the item you’re looking for and comparer is the name of the custom comparer. For every collection that uses a custom comparer for its Sort method, you must call this form of the BinarySearch method.

In Chapter 8 we created a class to represent shapes and added a method to calculate the area of each shape. You can create an ArrayList of Shape objects and write your own comparer to sort the elements of the ArrayList according to the area of each shape.

The Serialization Class

You have seen how the various collections of VB.NET store items, how to access their elements, and even how to sort and search the collections. The last piece of information you need before you can use collections in your applications to store large sets of data is how to store collections to disk files. In the last section of this chapter, you’ll learn how to do this so you can reuse a collection at a later time. What good is it to create a long collection, if your application can’t retrieve from a disk file in another session?

None of the collections exposes a Save or a similarly named method. Fortunately, there’s a mechanism that can store arbitrary objects to disk: the Serialization class. This class exposes the Serialize

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522 Chapter 11 STORING DATA IN COLLECTIONS

method, which saves an object to disk. The Deserialize method does the opposite: it reads a file created by the Serialize method and re-creates the original object. The Serialization class is a complicated one, but in this book I will discuss its Serialize and Deserialize methods, which you can use to persist your collection (and custom data types) to disk. When it comes to saving objects to disk (or even exchange them with other applications), the proper term is persist, which basically means to make an entity like an object, or variable, available to the application between sessions.

Serializing Individual Objects

To serialize an object, you must call the Serialize method of the System.Runtime.Serialization

.Formatters.Binary object. First, declare an object of this type with a statement like the following:

Dim BFormatter As New BinaryFormatter()

To avoid fully qualifying the BinaryFormatter class, import the class into your project with the following statement:

Imports System.Runtime.Serialization.Formatters.BinaryFormatter

The BinaryFormatter class persists objects in binary format. You can also persist objects in text format, using the SoapFormatter. The SoapFormatter persists the objects in XML format, which is quite verbose and the corresponding files are considerably lengthier. To use the SoapFormatter object, you must add a reference to the following .NET component through the Add Reference dialog box:

System.Runtime.Serialization.Formatters.Soap

Notice that this isn’t a class you can import; you must add a reference to the class. After that, you can declare a SoapFormatter variable with the following statement:

Dim formatter As Soap.SoapFormatter

If you’re wondering where the name of this class comes from, SOAP is an acronym for Simple Object Access Protocol. This is a protocol for accessing objects over HTTP—in other words, it’s a protocol that allows the encoding of objects in text format. The SOAP protocol was designed to enable distributed computing over the Internet, and it’s used with Web services. So, if SOAP can be used to access objects over the Internet, why not use it to persist objects in text format?

The methods of the BinaryFormatter and SoapFormatter are equivalent, so I will use the BinaryFormatter in the examples of this section. At the end of the section, I will show the code behind the menu of the WordFrequencies project, which persists the HashTable with the words and their frequencies to both binary and text format.

The syntax of the Serialize method is

BFormatter.Serialize(stream, object)

where stream is a variable that represents a stream and object is the object you want to serialize. Since we want to persist our objects to disk files, the stream argument represents a stream to a binary file.

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THE SERIALIZER CLASS 523

The File object and its methods are discussed in detail in Chapter 13; here, I will only explain briefly the statements we’ll use to store data to a disk file and read it back. The following statements create such a Stream object:

Dim saveFile As FileStream

saveFile = File.Create(“C:\SHAPES.BIN”)

The saveFile variable represents the stream to a specific file on the disk, and the Create method of the same variable creates a stream to this file.

After you have set up the Stream and BinaryFormatter objects, you can call the Serialize method to serialize any object. To serialize a Rectangle object, for example, use the following statements:

Dim R As New Rectangle(0, 0 , 100, 100)

BFormatter.Serialize(saveFile, R)

The event handler in Listing 11.20 persists two Rectangle objects to the Shapes.bin file in the root folder. The file’s extension can be anything. Since the file is binary, I’ve used the BIN extension:

Listing 11.20: Serializing Distinct Objects

Protected Sub Button1_Click(ByVal sender As Object, ByVal e As System.EventArgs) Dim R1 As New Rectangle()

R1.X = 1

R1.Y = 1 R1.Size.Width = 10 R1.Size.Height = 20

Dim R2 As New Rectangle() R2.X = 10

R2.Y = 10 R2.Size.Width = 100 R2.Size.Height = 200

Dim saveFile As FileStream

saveFile = File.Create(“C:\SHAPES.BIN”) Dim formatter As BinaryFormatter formatter = New BinaryFormatter() formatter.Serialize(saveFile, R1) formatter.Serialize(saveFile, R2) saveFile.Close()

End Sub

Notice that the Serialize method serializes a single object at a time. To save the two rectangles, the code calls the Serialize method once for each rectangle. To serialize multiple objects with a single statement, you must create a collection, append all the objects to the collection, and then serialize the collection itself, as explained in the following section.

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524 Chapter 11 STORING DATA IN COLLECTIONS

Serializing a Collection

Serializing a collection is quite similar to serializing any single object. The second argument to the Serialize method is the object you want to serialize, and this object can be anything, including a collection. To demonstrate the serialization of an ArrayList, we’ll modify the previous code a little, so that instead of persisting individual items, it will persist an entire collection. Declare the two Rectangle objects as before, but this time append them to an ArrayList collection. Then add a color value to the collection, as shown in Listing 11.21, which serializes an ArrayList collection to the file

C:\ShapesColors.bin.

Listing 11.21: Serializing a Collection

Private Sub Button2_Click(ByVal sender As System.Object, _

ByVal e As System.EventArgs) Handles Button2.Click Dim R1 As New Rectangle()

R1.X = 1

R1.Y = 1 R1.Width = 10 R1.Height = 20

Dim R2 As New Rectangle() R2.X = 10

R2.Y = 10 R2.Width = 100 R2.Height = 200

Dim shapes As New ArrayList() shapes.Add(R1) shapes.Add(R2) shapes.Add(Color.Chartreuse)

shapes.Add(Color.DarkKhaki.GetBrightness)

shapes.Add(Color.DarkKhaki.GetHue)

shapes.Add(Color.DarkKhaki.GetSaturation) Dim saveFile As FileStream

saveFile = File.OpenWrite(“C:\ShapesColors.bin”) saveFile.Seek(0, SeekOrigin.End)

Dim formatter As BinaryFormatter = New BinaryFormatter() formatter.Serialize(saveFile, shapes)

saveFile.Close()

MsgBox(“ArrayList serialized successfully”) End Sub

The last three Add methods add the components of another color to the collection. Instead of adding the color as is, we’re adding three color components, from which we can reconstruct the color Color.DarkKhaki. Then we proceed to save the entire collection to a file using the same statements as before. The difference is that we don’t call the Serialize method for each object. We call it once and pass the entire ArrayList as argument.

If you open the ShapesColors.bin file with a text editor (start Notepad and drop the file on its window), you will see that most of the file contains binary information—most of the characters are

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