Tim Mackey's Weblog

Background

In my content management system, i allow the user to define their own 'objects' (e.g. Staff Member) and then i provide templated data entry forms to let them populate instances of these objects.  It's aimed at non-techies so i have my own datatypes called 'Text' which maps to System.String, 'Number' maps to System.Double etc.  I also have a few custom data types called 'File' and 'Image' to allow the user to add files or images to an instance of the object. 

Problem

This business of doing column-mapping was ok as long as my data types had obvious .Net equivalents, but 'Image' doesn't in my case. i'm only storing a reference to the image, but in my application, it's not to be treated just as a System.String.  When the user is creating a new object with an 'Image' field in it, i want to display a file upload instead of a textbox, and when i go to display the object on the site, I want to display a html IMG tag with the SRC set to the value of the image field. 

Solution

The dataset is serialised into an xml file with the schema embedded. i needed to find some way of encoding my own custom data type information into the dataset that would persist into the xml file.  I looked through the VS intellisense and found the 'ExtendedProperties' data column property.  This property allows you to plug in any number of key/value pairs of information to each column.  This was exactly what i needed, so i added in a pair with something like "MyDataType=Image" for each column.  This persisted nicely into the xml file as follows:

<xs:complexType>
  <xs:sequence>
     <xs:element name="Photo" msprop:MyDataType="Image" type="xs:string" minOccurs="0" />

Note that the official type of the field is "xs:string", because it contains a path to the image. but now it also has the custom data type tagged on to the column definition. in this respect, i'm glad to see that MS have provided a very elegant and flexible framework.