如何使用缺少元素的Linq进行排序？

Question

Usually when using Linq I would usually filter out empty/null records using Where or similar. However, I need to order a List on multiple criteria and retain all items in the list but in order.

The following works only when .Dimension1.Count > 0 for all items in the list

var orderedList = mList.Elements
                       .OrderBy(x => x.Property1)
                       .ThenBy(x => x.Property2)
                       .ThenBy(x => x.Property3.Dimension1[0].Value2)
                       .ToList();

If any of the elements have Dimension1.Count == 0 then I get error:

'Index was out of range. Must be non-negative and less than the size of the collection.'

Which is expected as the array is not dimensioned.

Is there a way to make this work when the list contains items which have .Dimension1.Count = 0 ?

Note that Dimension1[0].Value2 is type double.

Answer 1

You can do something like this:

var orderedList = mList.Elements.OrderBy(x => x.Property1)
.ThenBy(x => x.Property2)
.ThenBy(x => x.Property3.Dimension1.Count == 0
    ? -1
    : x.Property3.Dimension1[0].Value2)
.ToList();

I am assuming here that Value2 is an integer. If it is a string for example, you can use null instead of -1 .

The idea is that you use a special value when Count is 0.

Answer 2

You don't mention whether you're using, for example, Entity Framework. It's doubtful this would convert to a valid SQL statement (it might..), but this may be worth a try in your case.

.ThenBy(x => x.Property3.Dimension1[0].Count > 0 ? x.Property3.Dimension1[0].Value2 : -1)

I'm assuming Value2 is always > 0 , so that any records missing values should be assigned -1 and be pushed further down the list. If that's not the case, you may need to change -1 to something more appropriate to your situation.

Answer 3

You can just provide a default value for the second ThenBy .

.ThenBy(x => x.Property3.Dimension1.Any() 
             ? x.Property3.Dimension1[0].Value2 
             : // Some default value for the type of Value2.
               // Go high or low depending on the type to put 
               // them at the top or bottom of the list
        );

Answer 4

There is a dedicated LINQ method for that, DefaultIfEmpty , which

Returns the elements of the specified sequence or the specified value in a singleton collection if the sequence is empty.

var orderedList = mList.Elements
                       .OrderBy(x => x.Property1)
                       .ThenBy(x => x.Property2)
                       .ThenBy(x => x.Property3.Dimension1.DefaultIfEmpty(someDefaultValue).ElementAt(0).Value2)
                       .ToList();

Answer 5

如果你想确定Dimensions1数组不会是null异常：

.Then(x => (x.Property3.Dimensions1 ?? new object[0]).Count > 0 ? x.Property3.Dimensions1[0].Value2 : -1)