Linq将外部列表索引添加到内部元素

var result = outer.SelectMany((inner, index) => inner.Select(item => Tuple.Create(item, index)));

我想出来了。。。

var input = new []{ new []{'a', 'b', 'c'}, new []{'d', 'e', 'f'}};

var rez = input
    .Select((list, listIdx) => new {list, listIdx})
    .SelectMany(listAndIdx => listAndIdx.list
        .Select(elem => new {elem, listAndIdx.listIdx}));