public class PropertyTester extends org.eclipse.core.expressions.PropertyTester{

	@Override
	public boolean test(Object receiver, String property, Object[] args,
			Object expectedValue) {
		return ((IDeleteable)receiver).isDeletable();
	}
}

<extension
         point="org.eclipse.ui.handlers">
      <handler
            class="test.DeleteHandler"
            commandId="Delete">
         <activeWhen>
            <with
                  variable="activePart">
               <test
                     forcePluginActivation="true"
                     property="ProperyTester">
               </test>
            </with>
         </activeWhen>
      </handler>
   </extension>

	@Override
	public void init(IEditorSite site, IEditorInput input)
			throws PartInitException {
		final IEvaluationService evaluationService = (IEvaluationService) getSite()
		.getWorkbenchWindow().getWorkbench().getService(
		IEvaluationService.class);
		deleteSourceProvider = new DeleteSourceProvider();		evaluationService.addSourceProvider(deleteSourceProvider);

	}

	@Override
	public void dispose() {
		super.dispose();
		final IEvaluationService evaluationService = (IEvaluationService) getSite()
		.getWorkbenchWindow().getWorkbench().getService(
		IEvaluationService.class);
		evaluationService.removeSourceProvider(deleteSourceProvider);
		deleteSourceProvider.dispose();
	}