How to run a target specifically when an error occurs?

MSBuild has a special tag for this -- it's the only tag you can put under <Target> that doesn't represent a task. It's called OnError.

Quick Example

Let's illustrate this with a simple example where we want to run the SendMail target if and only if an error occurs in the Build target:

		 <Project xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
		     <Target Name="Build">
	

<Error Text="This simulates a build error. This task will return false from Execute()"/>
<OnError ExecuteTargets="SendFailureMail"/>

		     </Target>
		     <Target Name="SendFailureMail">
	

<Message Text="Sending failure mail"/>

		     </Target>
		 </Project>
	

Rules

• No tasks can follow an <OnError> tag in a target.
• You can have more than one <OnError> tag in a target: they'll be executed in order.
• Conditions are allowed.
• OnError tags form a stack: nested OnError tags will unwind outwards, and errors in targets called by OnError will execute any OnError tags in those targets. (I could explain this better couldn't I?)
• If a task error is masked with ContinueOnError="true", OnError will not be triggered.
• Once an error has occurred, the build cannot be prevented from aborting: but as it aborts, it will execute your OnError targets.