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If you have ever worked with mechanical andengineering drawing or digital images, you are probably aware

of metadata. Metadata is information about the image, that's not part of the image itself. When an engineer

draws an image, metadata is often added, such as the following information: last updated, updated

by,data , place, and names. A photograph might include metadata such as image title, manufacturer, and

model.

In the .NET Framework library, the PropertyItem object is used as a placeholder for metadata. ThePropertyItem class provides four properties: Id, Len, Type, and Value. All of these properties have both read

and write access.

The Id property is a tag, which identifies the metadata item. Table 9.10 describes Id tag values.

The Value property is an array of values whose format is determined by the Type property. The Len property

represents the length of the array of values in bytes. The Type property represents the data type of values

stored in the array. Table 9.11 described the format of the Type property values.

TABLE 9.10: Id values

Hexadecimal

Value Description0x0320 Image title0x010F Equipment

manufacturer0x0110 Equipment

model0x9003 ExifDTOriginal0x829A EXIF exposure

time0x5090 Luminance

table0x5091 Chrominance

tableTABLE 9.11: Format of Type property values

Numeric Value Description1 A Byte object2 An array of Byte object encoded as ASCII3 A 16-bit integer4 A 32-bit integer5 An array of two Byte objects that represent a rational number6 Not used7

Undefined

8 Not used9 Slong10 SrationalAn Image object may contain more than one PropertyItem object. The PropertyItems property of the Image class

represents an array of PropertyItem objects corresponding to an image. The PropertyIdList property of the Image class

returns an array of property IDs stored in an image object. Listing 9.17 uses the PropertyItems property of the Image

class and reads all property items of an image.

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LISTING 9.17: Reading the metadata of a bitmap

PrivateSub Form1_Load(ByVal sender AsObject,ByVal e As System.EventArgs)

' Create an image from a file

Dim gAs Graphics = Me.CreateGraphics()

Dim curImageAs Image = Image.FromFile("C:/Documents and Settings/Manu/Desktop/16864z.Bmp")

Dim rect AsNew Rectangle(20, 20, 100, 100)g.DrawImage(curImage, rect)

' Create an array of PropertyItem objects and read items using PropertyItems

Dim propItemsAs PropertyItem() = curImage.PropertyItems' Create values of PropertyItem members

ForEach propItem As PropertyItem In propItems

Dim encoderAsNew System.Text.ASCIIEncoding()

Dim str AsString ="ID =" + propItem.Id.ToString("x")

str += ", Type =" + propItem.Type.ToString()

str += ", Length =" + propItem.Len.ToString()str += ", Value =" + encoder.GetString(propItem.Value)

MessageBox.Show(str)

Next' Dispose of object

g.Dispose()

EndSub

Figure 9.25 shows the output from Listing 9.17.

FIGURE 9.25: Reading the metadata of a bitmap