private static Expression <Func <TResult> > getValueExpression(
IReadScalar <INotifyCollectionChanged> sourceScalar)
{
Expression <Func <TSourceItem> > summarizingExpression = () => sourceScalar.Summarizing <TSourceItem>().Value;
Expression <Func <int> > countExpression = () => sourceScalar.Value != null ? ((IList)sourceScalar.Value).Count : 0;
return(getExpression(summarizingExpression, countExpression));
}