Linq.Dynamic FirstOrDefault() nested in OrderBy

Question

I have the following Linq statement, which works totally fine:

query = query.OrderBy(m => m.MATERIAL_TXT.Where(mt => mt.LANG == "EN").FirstOrDefault().LTEXT);

Now I'm trying to make it dynamic by using the string based syntax from Linq.Dynamic:

query = query.OrderBy("MATERIAL_TXT.Where(LANG==\"EN\").FirstOrDefault().LTEXT");

But it throws the exception :

"No applicable aggregate method 'FirstOrDefault' exists"

It has to bedynamic so that it accepts other names instead of "MATERIAL_TXT".

What am I missing?

score 1 · Accepted Answer · answered Sep 20 '19 at 14:57

1

According to the documentation:

A subset of the Standard Query Operators is supported for objects that implement IEnumerable. Specifically, the following constructs are permitted, where seq is an IEnumerable instance, predicate is a boolean expression, and selector is an expression of any type:

seq.Where(predicate)

seq.Any()

seq.Any(predicate)

seq.All(predicate)

seq.Count()

seq.Count(predicate)

seq.Min(selector)

seq.Max(selector)

seq.Sum(selector)

seq.Average(selector)

FirstOrDefault isn't on the list, so it's reasonably safe to assume it isn't supported.

answered Sep 20 '19 at 14:57

Luaan

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Do you have any idea how i could achieve that functionality without using FirstOrDefault()? – dagon Sep 20 '19 at 15:18
@dagon It's not easy; this kind of operation is very hard to do in SQL (just look at what the original query you had looks like in SQL). In the end, that's why `FirstOrDefault` isn't supported - Any, Min, Count, Sum... all of those are aggregate functions supported by pretty much all databases and especially MS SQL, which is the main target. MS SQL *doesn't* have "first". You basically need to rewrite the whole query, rather than just supplying a custom order by expression. You'll probably need to do that manually - take inspiration from how EF does it in the original case. – Luaan Sep 20 '19 at 19:15
Ok, too bad. I'll accept your anwser, since what i want to do seems to be impossible. – dagon Sep 22 '19 at 14:40

masoud · Answer 2 · 2019-09-21T04:35:04.053

1

You can't use FirstOrDefault as string like that.

if you want to create dynamic orderBy try this :

Func<IQueryable<YourEntityType>, IOrderedQueryable<YourEntityType>> orderBy;

   orderBy = x => x.OrderBy(m => m.MATERIAL_TXT.Where(mt => mt.LANG == "EN").FirstOrDefault().LTEXT);

Then you can use it like this :

orderBy(query);

for example you can use it in another method :

public List<YourEntityType> YourMethodName(Func<IQueryable<YourEntityType>, IOrderedQueryable<YourEntityType>> orderBy,IQueryable<YourEntityType> query=null)
{
     query=query ?? GetYourEntityTypeList().AsQueryable();
     return orderBy(query).ToList();
}

I Hope it will be useful .

edited Sep 21 '19 at 04:35

answered Sep 21 '19 at 04:27

masoud

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"MATERIAL_TXT" needs to be dynamic, so that won't work. – dagon Sep 22 '19 at 14:37
1

@dagon This doesn't give you a completely dynamic approach, but you can write ten different versions in advance, and select between those at runtime. If the possible orderings you have to do is predictable, this is a great approach. – Luaan Sep 22 '19 at 15:31
In the end i ended up making an expression for each case individually, because i now all possibilities. Thanks for that. – dagon Oct 02 '19 at 09:14

Linq.Dynamic FirstOrDefault() nested in OrderBy

2 Answers2