Dynamically composing LINQ expressions

Question

Is it possible to combine dynamically expressions:

from c in collection where c.Property == true select c

with expression

from result in results 
group result by result.Property 
into g 
select new g.Key

where 'results' should be the collection returned from first expression ?

I am going to use composed expression to fetch the data from db using NHibernate, so I would like the combined expression to be equal to if I would write it as

from c in collection 
where c.Property == true
group c by c.Property
into g
select new g.Key

The expressions are defined in the class:

public class MyClass : MyAbstractClass<User>
{
    public MyClass()
    {
        FirstExpression = users => from user in users where ... select user;              

        SecondExpression = results => from result in results
                       group result by result.Property into g
                       select g.Key

    }
}

Sergey Berezovskiy · Accepted Answer · 2013-01-11 09:30:05Z

1

var query = collection;

if (condition)
    query = query.Where(c => c.Property);

var result = query.GroupBy(c => c.Property).Select(g => g.Key);

answered Jan 11, 2013 at 9:30

Sergey Berezovskiy

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Comments

Daniel Hilgarth · Accepted Answer · 2013-01-11 09:34:33Z

0

Yes, that's possible. Simply assign the result of the first query to the results variable without enumerating it:

var results = from c in collection where c.Property == true select c;

from result in results 
group result by result.Property 
into g 
select g.Key;

BTW, Your second query is a simple distinct:

results.Distinct(r => r.Property);

edited Jan 11, 2013 at 9:34

answered Jan 11, 2013 at 9:29

Daniel Hilgarth

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7 Comments

petrov.alex Over a year ago

ok, but if both of the expressions are the properties of the class, eg FirstExpression and SecondExpression?

Daniel Hilgarth Over a year ago

@petrov.alex: Please try to rephrase that sentence, it doesn't make too much sence as it stands right now. Expressions are normally not properties of a class - expressions are written using the properties of a class.

petrov.alex Over a year ago

I have updated the question, please check if it makes sense now

Daniel Hilgarth Over a year ago

@petrov.alex: Not really. Please show how FirstExpression and SecondExpression are defined.

petrov.alex Over a year ago

has not really done this part, but was thinking to do it in this was: public Expression<Func<IEnumerable<TEntity>, IEnumerable>> FirstExpression { get; set; } public Expression<Func<IEnumerable<TEntity>, IEnumerable>> SecondExpresion { get; set; } possible that it all dont have sense, just started to dig into expressions

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