How to LINQ query with variable where and select?

Question

I've got a LINQ query which looks like this:

var query = from produkt in Entity.ProduktCollection.produktCollection
            let p = produkt as Entity.Produkt

            from version in p.version
            let v = version as Entity.Version

            from customer in v.customerCollection
            let c = customer as Entity.Customer

            from fehler in v.fehlerCollection
            let f = fehler as Entity.Fehler

            where f.id == GetGuid();
            select p;

but what I really need is a way to make the f plus its property as well as the p variable, so I might change them to every other possible combination, for example:

where c.name == "frank"
select f;

or

where p.id == GetGuid2()
select c;

Is there any way to achieve this? As far as I know there is no way to insert a switch-case block inside the query between the from/let-part and the where/select-part.

MarcinJuraszek · Accepted Answer · 2013-12-11 17:09:57Z

6

You can create your query in multiple statements because LINQ queries execution is deferred. It applies nicely to different Where statements situation:

var querySource = from produkt in Entity.ProduktCollection.produktCollection
                  let p = produkt as Entity.Produkt
                  from version in p.version
                  let v = version as Entity.Version
                  from customer in v.customerCollection
                  let c = customer as Entity.Customer
                  from fehler in v.fehlerCollection
                  let f = fehler as Entity.Fehler
                  select new { p, v, c, f };

if(/* first condition */)
{
    querySource = querySource.Where(x => x.f.id == GetGuid());
}
else if(/* second condition */)
{
    querySource = querySource.Where(x => x.p.id = 34);
}

var query = querySource.Select(x => x.p);

You can make the Select part conditional as well, but because returned IEnumerable<T> will differ in T part, you won't be able to assign them all to the same variable.

answered Dec 11, 2013 at 17:09

MarcinJuraszek

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1 Comment

Raymond Osterbrink Over a year ago

This is great, i hoped something like this might work! The only problem which is left: now i've got all different types inside my query. I tried to specify with "query = query.Where(x=>x.GetType() == T);" but this istn't working, is there a way to make this possible?

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