An issues-set-diff can be created through ExtensionMethodsIssue.CreateIssuesSetDiff or ComputeIssuesDiff(IAnalysisResult,ICompareContext).

When an IIssuesSet is referenced by an IIssuesSetDiff object, the IIssuesSet.Builder must not be used to add an remove rules and issues, the IIssuesSetDiffBuilder object must be used instead.

An issues-set-diff is not thread-safe. It is safe to call IIssuesSetDiff methods concurrently from several threads but it is not safe to call these methods and IIssuesSetDiffBuilder methods concurrently from several threads, this would provoke corrupted state and exceptions.

Reference

IIssuesSetDiffBuilder Members
NDepend.Issue Namespace