Quite recently, I have to admit, I found that MSBuild has been enhanced (in Version .NET 3.5) with so called ItemDefinitionGroup elements. According to MSDN
“The ItemDefinitionGroup element lets you define a set of Item Definitions, which are metadata values that are applied to all items in the project, by default.”
This makes one use of ItemDefinitionGroup quite apparent – declare default values for items. At a higher level you might want to think about ItemDefinitionGroup as the definition (type) for concrete items (instance). So that ItemDefinitionGroup instances serve as a template for your concrete ItemGroup instances. Although this is not really true and not enforced by MSBuild runtime, it might help to conceptualize it this way. The reason why it is not true is because you may extend ItemGroup instances according to your liking, which is clearly strictly the case with instances of an object. Another simplification might be that there is no coupling of data and behavior. Although, you might find your way around this via dynamic generation and execution of code from meta-data item properties.
For example, In a recent project I customized the build process for SSIS projects by defining an ItemDefinitionGroup SSISProject :
<Name>A Default Value</Name>
Instead of specifying explicitly whether or not to exclude every SSIS project in the build I, now have the chance to use the global DoBuild meta-data item to define a central global policy which can (and is) overridden by individual projects:
This declaration of SSISProject would set the DoBuild meta-data property for MySSISProject to true but leave the Name meta-data property untouched. Consequently, you may use ItemDefinitionGroup instances to not only avoid verboseness and redundancy but also to associate items with rich meta-data, such as documentation, and process triggering information. For example, the DoBuild property can be evaluated in targets and tasks to conditionally decide whether or not to include a particular SSIS project into the build. A description property could be used to automatically generate build process or solution documentation (think of html files automatically generated using an MSBuild task).
Lately, I have made use of ItemDefinitionGroup quite a bit. Although, admittedly mostly to avoid otherwise introduced verboseness and redundancy, i.e. defining item default values. In case you have to share further usage scenarios, I would be most interested in hearing them and possibly add them to this post.