Composing LINQ Expressions

Question

I've read a bunch of articles on putting LINQ expressions together, but they only seem to cover and's and or's. I have this solution implemented:

http://blogs.msdn.com/b/meek/archive/2008/05/02/linq-to-entities-combining-predicates.aspx

How do I write this code so that it works?

Expression<Func<Crop, bool>> chooseOranges = crop => crop.IsOrange;
Expression<Func<Farm, bool>> selectFarm = farm => farm.HasCows;

selectFarm = selectFarm.And(farm => farm.Crops.Any(chooseOranges);

It compiles, but at runtime it throws a .NET 1025 error. If I replaced chooseOranges with the literal text it would work, but chooseOranges is built piece by piece with business logic. It seems like the code I linked above relies on the various Expression.Foo() functions, but I couldn't find one that would help.

Chris Pitman · Accepted Answer · 2012-05-08T23:04:07.587

2

Use LinqKit's ability to evaluate nested expressions and return a single expression usable by any provider that would support the same expression written out:

Expression<Func<Crop, bool>> chooseOranges = crop => crop.IsOrange;
Expression<Func<Farm, bool>> selectFarm = farm => farm.HasCows;    
Expression<Func<Farm, bool>> combined = (farm) => selectFarm.Invoke(farm) && farm.Crops.Any(inner => chooseOranges.Invoke(inner));
combined = combined.Expand();

edited May 08 '12 at 23:04

answered May 03 '12 at 21:45

Chris Pitman

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Usable by *any* provider who also supports the same expression written explicity. – usr May 03 '12 at 21:59
The last line throws a StackOverflowException when I try to run it. Is that a problem on my end? – MrEff May 08 '12 at 21:41
@MrEff Try this edit, the `selectFarm` local may have been captured – Chris Pitman May 08 '12 at 23:04
It seems that defining a new variable is enough to make it work. Thanks. – MrEff May 09 '12 at 14:42

score 1 · Answer 2 · edited May 23 '17 at 11:48

I would write it like this:

Expression<Func<Crop, bool>> chooseOranges = crop => crop.IsOrange;  
Expression<Func<Farm, bool>> selectFarm = farm => farm.HasCows;  

IQueryable farmQuery = GetQuery();
farmQuery = farmQuery.Where(selectFarm);
farmQuery = farmQuery.Where(farm => farm.Crops.Any(chooseOranges);

Each row must meet both criteria to be in the result, so that's And.

For Or, I did that over there.

Composing LINQ Expressions

2 Answers2