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.NET Forum / Languages / C# / March 2008Lazy load of properties in DLinq
I tried query like var list=Db.Customers.ToList(); and noticed that all properties are retrieved immediately. Only few properties are actually used in each unit of work. Also properties contain large amout of data (images, attachments, text documentes). Retireving all properties over internet is slow. Also big properties take a lot of memory in users computer. How to force DLinq to retrieve those properties only which are are actually used? Andrus. > I tried query like > [quoted text clipped - 7 lines] > > Retireving all properties over internet is slow. Do you really have live *internet* access to the database? Why isn't this behind a web service or something similar? > Also big properties take a lot of memory in users computer. > > How to force DLinq to retrieve those properties only which are are actually > used? By specifying which ones you want: var list = Db.Select(customer => new { customer.Name, customer.Age }).ToList();  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 <snip> > > Also big properties take a lot of memory in users computer. > > [quoted text clipped - 5 lines] > var list = Db.Select(customer => > new { customer.Name, customer.Age }).ToList(); I should have expanded on this. I don't know if there's any way of retrieving one property at a time, only when you need it. It's possible that this isn't implemented in LINQ to SQL. (Personally I've never needed that for simple properties, only for associations.) I wouldn't be surprised to find it's available in the Entity Framework, mind you. It wouldn't be too hard to fake if you had to - create another class (e.g. CustomerShell) which is given the ID, and which lazily loads the properties as and when they're requested. The latency involved would be pretty hideous though, fetching a single property at a time. 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 Re the discussion of web-service; I was talking to Guy Smith-Ferrier about what ADO.NET Data Services (aka Astoria), which sounds like it will be a reasonable option for many "normal" LINQ implementations over a web-service, but note that it only (AFAIK) supports full entities, not bespoke projections. Still, something for consideration as it matures. Of course, I'm not sure that even a RESTful web-service could code very well with the dynamic type-creation model... ditto for any standard client proxy. Marc > Re the discussion of web-service; I was talking to Guy Smith-Ferrier about > what ADO.NET Data Services (aka Astoria), which sounds like it will be a > reasonable option for many "normal" LINQ implementations over a web-service, > but note that it only (AFAIK) supports full entities, not bespoke > projections. Last time I asked Guy about it, he wasn't aware of any attempt to use LINQ on the Astoria *client* - it was a possibility I was somewhat excited about. (LINQ on the server to actually fetch the data which would then be returned, sure - but that's old hat by now! ;) > Still, something for consideration as it matures. > > Of course, I'm not sure that even a RESTful web-service could code very well > with the dynamic type-creation model... ditto for any standard client proxy. There's an open source project which has similar goals: http://www.codeplex.com/glinq No idea how far it's got, but I should look into it at some point... 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 > Last time I asked Guy about it, he wasn't aware of any attempt to use > LINQ on the Astoria *client* Well, unless I had a brain-freeze, that is *precisely* what he was demoing in my office earlier this month ;-p You can get the PPT from his site (www.guysmithferrier.com); note slide 30: "LINQ to ADO.NET Data Services" - i.e. LINQ to Astoria *at the client*. I must admit, I haven't tried the CTP yet (the install is a little hairy), but I'm very keen to when I get a chance (and enough time to refresh a VM that I can trash). Marc > > Last time I asked Guy about it, he wasn't aware of any attempt to use > > LINQ on the Astoria *client* > > Well, unless I had a brain-freeze, that is *precisely* what he was demoing > in my office earlier this month ;-p Cool. It was a few months ago that I asked him, admittedly :) > You can get the PPT from his site (www.guysmithferrier.com); note slide 30: > "LINQ to ADO.NET Data Services" - i.e. LINQ to Astoria *at the client*. Fabulous. I'd love to think this was in response to my feedback, but I strongly suspect it was already in the works but not in the version he was demo-ing at the time. > I must admit, I haven't tried the CTP yet (the install is a little hairy), > but I'm very keen to when I get a chance (and enough time to refresh a VM > that I can trash). :) It is a great use of VMs... 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, >> How to force DLinq to retrieve those properties only which are are >> actually [quoted text clipped - 4 lines] > var list = Db.Select(customer => > new { customer.Name, customer.Age }).ToList(); Thank you. I need to create updatable customer list containing dynamic columns. I tried to use Dynamic Linq Library Select() function IQueryable<Customer> gqueryable = (IQueryable<Customer>)db.Customers.Select("new(CustomerID,City)"); and IQueryable<Customer> gqueryable = db.Customers.Select("new(CustomerID, City)").Cast<Customer>(); Both statements cause cast error at runtime. How to create updatable Customer instance list from dynamic Customer property list ? Andrus. > I need to create updatable customer list containing dynamic columns. I suspect you may find that tricky/impossible. I don't know for sure, but I doubt that you'll be able to load updatable entities partially. 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, >> I need to create updatable customer list containing dynamic columns. > > I suspect you may find that tricky/impossible. I don't know for sure, > but I doubt that you'll be able to load updatable entities partially. I tried Frans great suggestion in this thread to load and update using partial columns: Northwind db = CreateDB(); var cust = (from c in db.Customers select new Customer { CustomerID = c.CustomerID, City = c.City }).First(); cust.City = "Tallinn"; db.SubmitChanges(); This works OK. Why do you suspect that this is tricky or impossible ? Unresolved thing is to pass property list dynamically to this statement. Solutions: 1. Create expression tree or only select part programmatically 2. Modify MS Dynamic Linq library Select() to allow create customer class using syntax like Select( "new Customer ( " + columnlist+")" ); and allow to use generic IQueryable 3. Add Dynamic Select() to Marc Dynamic query extension library. 4. Use dynamic compilation. Which is the best solution ? Andrus. > >> I need to create updatable customer list containing dynamic columns. > > [quoted text clipped - 17 lines] > This works OK. > Why do you suspect that this is tricky or impossible ? I just didn't expect it to work if it had only loaded some of the columns. I'm glad for you that it does though. > Unresolved thing is to pass property list dynamically to this statement. > [quoted text clipped - 14 lines] > > Which is the best solution ? Not having done any of them, I'm not really in a position to recommend. 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 1 is sounds achievable via Expression.New with Expression.Bind, but won't be trivial; I'd start by compiling something similar and reverse- engineering it 2 is the same as 1? Just with writing a parser too... extra work 3 I don't undersand 4 You already know my thoughts on this, but it would be consistent with your other code, and probably less work than 1? I know I've said it before, but can I just say it again? You are fighting LINQ every step of the way... I fully expect that after / this/ issue, it won't be long before you're trying to out-maneuver LINQ in /another/ issue... Marc > This works OK. > Why do you suspect that this is tricky or impossible ? Jon's gut feeling was well-justified; this might work with DbLinq, but it doesn't work with LINQ-to-SQL; I haven't tried EF (I'm waiting for vNext on this CTP...). Additionally, and importantly, note that there is a major risk of data loss with this approach: if the back-end decides to set all columns, it will set most of them to blank/0/null etc. This is pretty-much guaranteed if you are using SPs to update data, but if you use dynamic (generated) CRUD it will /probably/ be OK. On your head be it, etc... I posted more details to the parallel chain: http://groups.google.co.uk/group/microsoft.public.dotnet.languages.csharp/msg/c9 f0cf167b355e20 Note that this also contains a "best guess" at option 1. It works with LINQ-to-objects, but I haven't tried it on DbLinq. In particular, note that my expression code isn't the problem: the following raises exactly the same error in LINQ-to-SQL: select new Customer { CustomerID = c.CustomerID, City = c.City } Annoyingly, I wrote the code using LINQ-to-objects while waiting for SQLExpress+management studio to download, only to find it fail at the first hurdle. Oh well. Let me know how the code behaves with DbLinq. Marc > I tried query like > [quoted text clipped - 11 lines] > How to force DLinq to retrieve those properties only which are are > actually used? var list = from c in db.Customers select new Customer { ID = c.ID, .. //rest of fields you want }; FB Signature------------------------------------------------------------------------ Lead developer of LLBLGen Pro, the productive O/R mapper for .NET LLBLGen Pro website: http://www.llblgen.com My .NET blog: http://weblogs.asp.net/fbouma Microsoft MVP (C#) ------------------------------------------------------------------------ Frans, >> How to force DLinq to retrieve those properties only which are are >> actually used? > > var list = from c in db.Customers > select new Customer { ID = c.ID, .. //rest of fields you want > }; Thank you. Excellent. List of properties required is only known at runtime in my application. How to build list of properties for this command at runtime ? I tried Dynamic Linq Library Select() function but this does not allow to create Customer type entity. Andrus. Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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