4年后可能关注的人：

To whom it may concern after 4 years:

因为您只对第一个缺失的密钥感兴趣，解决方案是：

Because you are only interested in the first missing key, the solution is:

int[] keys = { 1, 2, 3, 5, 7 };

var arrayWithFirstMissingKey = keys
    .Select((key, index) => new { Key = key, Index = index })
    .Where(tuple => (tuple.Index + 1 < keys.Length) && (keys[tuple.Index + 1] > tuple.Key +1))
    .Select(tuple => tuple.Key +1)
    .Take(1)
    .ToArray();

第一个 选择创建（Key，Index）元组，因为Enumerables不支持索引， 

The 1st  Select creates (Key, Index) tuples, because Enumerables don't support indexes, 

Where子句检测到与其后继有间隙的第一个密钥。

The Where clause detects the first key which has a gap to it's successor.

第二个选择确定缺口的第一个缺失密钥。

The 2nd select determines the first missing key of the gap.

在找到第一个缺失密钥后，Take子句可以防止不必要的开销。

The Take clause prevents unnecessary overhead after the first missing key is found.