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.NET Forum / Languages / C# / March 2008How to resolve constant object property reference in DLinq expression
The following code fails: CompaniesDetails c2 = (from c in db.CompaniesDetails where c.CompanyID == comp.ID select c).Single(); "comp" is an instance of class Company: public class Company { public int ID; { get; set; } DLinq provider does not know what comp is in runtime. So compiler must replace comp.ID with value when creating expression. Any idea how to fix this ? Andrus. > The following code fails: > [quoted text clipped - 13 lines] > DLinq provider does not know what comp is in runtime. > So compiler must replace comp.ID with value when creating expression. No, the *compiler* doesn't have to do that at all. The *compiler* will just be calling db.CompaniesDetails.Where(c => c.CompanyID == comp.ID) So, it's up to Where to understand the expression tree. If the DLinq provider doesn't understand it, that sounds like a bug in DLinq to me.  SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet World class .NET training in the UK: http://iterativetraining.co.uk > No, the *compiler* doesn't have to do that at all. The *compiler* will > just be calling [quoted text clipped - 3 lines] > So, it's up to Where to understand the expression tree. If the DLinq > provider doesn't understand it, that sounds like a bug in DLinq to me. Provider gets string "comp.ID". Lets assume that comp is local object name. Provider does not have access to method local name table to resolve name "comp". As far as I know local variable names like "comp" are not stored in assembly and Linq does not pass such table to provider. So provider cannot resolve the name "comp" in any way. Andrus. > > No, the *compiler* doesn't have to do that at all. The *compiler* will > > just be calling [quoted text clipped - 5 lines] > > Provider gets string "comp.ID". No, the provider will get an expression tree, not a string, unless something has already converted it into a string - which it certainly shouldn't have done. Put it this way - the compiler works exactly the same way with LINQ to SQL, so do you believe *that* handles it? This is certainly an issue with DbLINQ, *not* with the C# compiler. SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet World class .NET training in the UK: http://iterativetraining.co.uk Jon, > Put it this way - the compiler works exactly the same way with LINQ to > SQL, so do you believe *that* handles it? This is certainly an issue > with DbLINQ, *not* with the C# compiler. So you can confirm, that Linq-SQL can evaluate this expression, right ? If So I must study passed expression tree a bit more. Where to find some Linq provider sample code which resolves local object references line comp.ID to values? Andrus. > > Put it this way - the compiler works exactly the same way with LINQ to > > SQL, so do you believe *that* handles it? This is certainly an issue > > with DbLINQ, *not* with the C# compiler. > > So you can confirm, that Linq-SQL can evaluate this expression, right ? It's as easy for you to try as it is for me - easier, as you know the real context of your code. However, I've certainly done similar things without any problem. However, I can certainly confirm that the expression tree *won't* just contain "comp.ID" as a string. > If So I must study passed expression tree a bit more. > Where to find some Linq provider sample code which resolves local object > references line comp.ID to values? I think you'll find it simple when you look at the actual expression tree. SignatureJon Skeet - <skeet@pobox.com> http://www.pobox.com/~skeet Blog: http://www.msmvps.com/jon.skeet World class .NET training in the UK: http://iterativetraining.co.uk
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