Reading Metadata

Some image files contain metadata that you can read to determine features of the image. For example, a digital photograph might contain metadata that you can read to determine the make and model of the camera used to capture the image. With GDI+, you can read existing metadata, and you can also write new metadata to image files.

GDI+ stores an individual piece of metadata in a PropertyItem object. You can read the PropertyItems property of an Image object to retrieve all the metadata from a file. The PropertyItems property returns an array of PropertyItem objects.

A PropertyItem object has the following four properties:

Id

A tag that identifies the metadata item. Some values that can be assigned to Id are listed in the following table.

Value

An array of values The format of the values is determined by the Type property.

Len

The length (in bytes) of the array of values pointed to by the Value property.

Type

The data type of the values in the array pointed to by the Value property. The formats indicated by the Type property values are given in the following table.

The following example reads and displays the seven pieces of metadata in the file FakePhoto.jpg.

'Create an Image object. 
Dim image = New Bitmap("FakePhoto.jpg")
      
'Get the PropertyItems property from image.
Dim propItems As PropertyItem() = image.PropertyItems
      
'Set up the display.
Dim font As New Font("Arial", 12)
Dim blackBrush As New SolidBrush(Color.Black)
Dim X As Integer = 0
Dim Y As Integer = 0
      
'For each PropertyItem in the array, display the id, type, and length.
Dim count As Integer = 0
Dim propItem As PropertyItem
For Each propItem In  propItems
   e.Graphics.DrawString( _
      "Property Item " + count.ToString(), _
      font, _
      blackBrush, _
      X, Y)
         
   Y += font.Height
         
   e.Graphics.DrawString( _
      "   iD: 0x" + propItem.Id.ToString("x"), _
      font, _
      blackBrush, _
      X, Y)
         
   Y += font.Height
         
   e.Graphics.DrawString( _
      "   type: " + propItem.Type.ToString(), _
      font, _
      blackBrush, _
      X, Y)
         
   Y += font.Height
         
   e.Graphics.DrawString( _
      "   length: " + propItem.Len.ToString() + " bytes", _
      font, _
      blackBrush, _
      X, Y)
         
   Y += font.Height
         
   count += 1
Next propItem
[C#]
//Create an Image object. 
Image image = new Bitmap("FakePhoto.jpg");

//Get the PropertyItems property from image.
PropertyItem[] propItems = image.PropertyItems; 

//Set up the display.
Font font = new Font("Arial", 12);
SolidBrush blackBrush = new SolidBrush(Color.Black);
int X = 0;
int Y = 0;

//For each PropertyItem in the array, display the id, type, and length.
int count = 0;
foreach (PropertyItem propItem in propItems)
{
   e.Graphics.DrawString(
   "Property Item " + count.ToString(),
   font,
   blackBrush,
   X, Y);
   
   Y += font.Height;

   e.Graphics.DrawString(
      "   iD: 0x" + propItem.Id.ToString("x"),
      font,
      blackBrush,
      X, Y);

   Y += font.Height;

   e.Graphics.DrawString(
      "   type: " + propItem.Type.ToString(),
      font,
      blackBrush,
      X, Y);

   Y += font.Height;

   e.Graphics.DrawString(
      "   length: " + propItem.Len.ToString() + " bytes",
      font,
      blackBrush,
      X, Y);

   Y += font.Height;

   count++;
}

The preceding code produces output similar to the following:

Property Item 0
  id: 0x320
  type: 2
  length: 16 bytes

Property Item 1
  id: 0x10f
  type: 2
  length: 17 bytes

Property Item 2
  id: 0x110
  type: 2
  length: 7 bytes

Property Item 3
  id: 0x9003
  type: 2
  length: 20 bytes

Property Item 4
  id: 0x829a
  type: 5
  length: 8 bytes

Property Item 5
  id: 0x5090
  type: 3
  length: 128 bytes

Property Item 6
  id: 0x5091
  type: 3
  length: 128 bytes

The second (index 1) property item in the list has Id 0x010F (equipment manufacturer) and Type 2 (ASCII-encoded byte array). The following code, which is a continuation of the preceding example, displays the value of that property item:

'Convert the value of the second property to a string, and display it.
Dim encoding As New System.Text.ASCIIEncoding()
Dim manufacturer As String = encoding.GetString(propItems(1).Value)
      
e.Graphics.DrawString( _
   "The equipment make is " + manufacturer + ".", _
   font, _
   blackBrush, _
   X, Y)
[C#]
//Convert the value of the second property to a string, and display it.
System.Text.ASCIIEncoding encoding = new System.Text.ASCIIEncoding();
string manufacturer = encoding.GetString(propItems[1].Value);

e.Graphics.DrawString(
   "The equipment make is " + manufacturer + ".",
   font,
   blackBrush,
   X, Y);

The preceding code produces output similar to the following:

The equipment make is Northwind Camera.