int startIndex = 5;
int endIndex = 60;
TextDocumentSnapshot tdsp = this.mySyntaxEditor.ActiveDocumentView.CurrentSnapshot;
TextLocation stl = tdsp.LocationFromOffset(startIndex);
TextLocation etl = tdsp.LocationFromOffset(endIndex);
SnapshotPoint ssp = SnapshotPoint.FromTextLocation(stl, tdsp);
SnapshotPoint esp = SnapshotPoint.FromTextLocation(etl, tdsp);
this.mySyntaxEditor.ActiveDocumentView.SelectionManager.SelectionStart = ssp;
this.mySyntaxEditor.ActiveDocumentView.SelectionManager.SelectionEnd = esp;