查询没有返回值的列表

Question

IEnumerable<ATable> stuff = 
        _aTableRepository.Entity.ByTitle(TITLE).ByDocTypeCode(Id).ToList();

The above returns a list of items

decimal? xyz = stuff.Where(x => x.CLAUSE == "test").FirstOrDefault().Id ?? 0;

Now I want to query the list and get the id, which is a decimal type. However, there are no clauses that equal "test" and this line bombs out. It seems like if nothing is found, that the nullable decimal would get set to 0.

Answer 1

The problem with your query, is that FirstOrDefault will return default entity value if there is no matching entities. I believe entity is a reference type in your application, so you have null as default value. Trying to get Id of null gives you NullReferenceException .

Project sequence of entities to sequence of ids and select first one or use default value:

decimal? xyz = stuff.Where(x => x.CLAUSE == "test")
                    .Select(x => x.Id).FirstOrDefault() ?? 0;

You can also use DefaultIfEmpty to provide default value:

decimal? xyz = stuff.Where(x => x.CLAUSE == "test")
                    .Select(x => x.Id)
                    .DefaultIfEmpty(0).First();

And old school approach - simply get first matching entity and check if its not null:

var testEntity = stuff.FirstOrDefault(x => x.CLAUSE == "test");
decimal? xyz = testEntity == null ? 0 : testEntity.Id;