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BitmapMetadata.GetQuery Method

.NET Framework 4

Provides access to a metadata query reader that can extract metadata from a bitmap image file.

Namespace:  System.Windows.Media.Imaging
Assembly:  PresentationCore (in PresentationCore.dll)
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public Object GetQuery(
	string query
)

Parameters

query
Type: System.String
Identifies the string that is being queried in the current BitmapMetadata object.

Return Value

Type: System.Object
The metadata at the specified query location.
Exception Condition
ArgumentNullException

query is null.

Metadata that is associated with an image is data that describes the image but that is not necessary for display of the image. Each supported bitmap image format handles metadata differently, but the facility for reading and writing metadata is the same.

Windows Presentation Foundation (WPF) supports the following image metadata schemas: Exchangeable image file (Exif), tEXt (PNG Textual Data), image file directory (IFD), International Press Telecommunications Council (IPTC), and Extensible Metadata Platform (XMP).

The following code example demonstrates how to use the SetQuery method to write metadata to a Portable Network Graphics (PNG) file.

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Stream pngStream = new System.IO.FileStream("smiley.png", FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite);
PngBitmapDecoder pngDecoder = new PngBitmapDecoder(pngStream, BitmapCreateOptions.PreservePixelFormat, BitmapCacheOption.Default);
BitmapFrame pngFrame = pngDecoder.Frames[0];
InPlaceBitmapMetadataWriter pngInplace = pngFrame.CreateInPlaceBitmapMetadataWriter();
if (pngInplace.TrySave() == true)
{ pngInplace.SetQuery("/Text/Description", "Have a nice day."); }
pngStream.Close();

After the metadata is written, the GetQuery method is used to read that data and emit it as a text string.

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// Add the metadata of the bitmap image to the text block.
TextBlock myTextBlock = new TextBlock();
myTextBlock.Text = "The Description metadata of this image is: " + pngInplace.GetQuery("/Text/Description").ToString();

.NET Framework

Supported in: 4, 3.5, 3.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.
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Is it just me, or is this the dumbest thing ever?
In order to read the metadata, you have to open the file for writing. Then, call SetQuery to set a value that you can then read back using GetQuery, but only immediately after calling SetQuery. The value isn't persisted to the file, and if the file is opened read only, the CreateInPlaceBitmapMetadataWriter (a writer that you need for reading?) will fail because the file isn't writeable, obviously.

Where is the corresponding CreateInplaceMetadataReader method? If I want to open an image from a URI and then read the metadata properties, why do I need to have write access to the file? I can get the metadata directly from the BitmapDecoder Metadata property, so why does the GetQuery method even exist? I know that there are forums for questions such as these, but really, these are more rhetorical in nature.

The example code is just the same copy from the SetQuery content, so it really doesn't help in creating the, "Oh, I get it now." level of understanding.

Look, Ma, no write-access:

            Uri imageUri = new Uri("smiley.png", UriKind.Relative);
            PngBitmapDecoder pngDecoder = new PngBitmapDecoder( imageUri,
                                                                                                        BitmapCreateOptions.PreservePixelFormat,
                                                                                                        BitmapCacheOption.Default);
            Comments.Text = "The Comments metadata of this image is: " + pngDecoder.Metadata.Comment;


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