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i have a two list
List<Sent> SentList; List<Messages> MsgList; both have the same property called MsgID;
MsgList SentList MsgID Content MsgID Content Stauts 1 aaa 1 aaa 0 2 bbb 3 ccc 0 3 ccc 4 ddd 5 eee i want to compare the MsgID in Msglist with the sentlist and need items which are not in the sent list using linq
Result MsgID Content 2 bbb 4 ddd 5 eee 
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in conclusion no, it won't return null since null can't say sequence contains no elements it will always say object reference not set to an instance of an object ;) Save this answer. Show activity on this post.
While the LINQ methods always return a new collection, they don't create a new set of objects: Both the input collection (customers, in my example) and the output collection (validCustomers, in my previous example) are just sets of pointers to the same objects.
By using Intersect , we can check which elements in source list are also contained in compare list. var source = new List<string>() { "a", "b", "c" }; var compare = new List<string>() { "b", "c", "d" }; var result = source. Intersect(compare);
You could do something like:
HashSet<int> sentIDs = new HashSet<int>(SentList.Select(s => s.MsgID)); var results = MsgList.Where(m => !sentIDs.Contains(m.MsgID)); This will return all messages in MsgList which don't have a matching ID in SentList.
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The naive approach:
MsgList.Where(x => !SentList.Any(y => y.MsgID == x.MsgID)) Be aware this will take up to m*n operations as it compares every MsgID in SentList to each in MsgList ("up to" because it will short-circuit when it does happen to match).
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