Error when using WCF dataservice

Question

I'm following this guide and I am getting an error. Can anyone help me?

The code for my datamodel is below

 namespace Datalayer { public class DataModel { public DataModel() { using (btWholesaleDataContext db = new btWholesaleDataContext()) { //! requires auth var MACRequestList = from r in db.btRequests select new Models.BT.Request { ID = r.ID, Date = r.DateTime, StatusCode = 3, Status = r.Status }; MACRequests = MACRequestList.AsQueryable(); } } public IQueryable<Models.BT.Request> MACRequests { get; private set; } } }

The web service gives the error

Cannot access a disposed object.Object name: 'DataContext accessed after Dispose.'

When I access MACRequests

I have only posted the code I think is broken. If you want to see more just let me know.

CodingWithSpike · Accepted Answer · 2011-06-29 14:32:35Z

Your data context is being disposed at the end of your constructor, at the end of the using { } block. However when you use the IQueryable MACRequests property, it needs that underlying context, which has since been disposed.

One possible way to handle this is to make your class IDisposable and dispose the context that way:

public class DataModel : IDisposable { private btWholesaleDataContext wholesaleDataContext; public DataModel() { wholesaleDataContext = new btWholesaleDataContext(); //! requires auth var MACRequestList = ... ; MACRequests = MACRequestList.AsQueryable(); } public IQueryable<Models.BT.Request> MACRequests { get; private set; } public void Dispose() { if(wholesaleDataContext != null) wholesaleDataContext.Dispose(); } }

Then you have to make sure that DataModel is properly disposed by whatever uses it.

Another alternative is to make MACRequests the actual list of items instead of the IQueryable:

public class DataModel { public DataModel() { using (btWholesaleDataContext db = new btWholesaleDataContext()) { //! requires auth var MACRequestList = ... ; MACRequests = MACRequestList.ToList(); // ToList reads the records now, instead of later. } } public List<Models.BT.Request> MACRequests { get; private set; } }

Inherting from IDisposable is a very elegent solution. Thanks! Since im running a website that will be constantly running will the DataModel be created and disposed when the application starts and is closed or will it be created and disposed for each call?

LostInComputer · Accepted Answer · 2011-06-29 14:24:52Z

I think its because you are using an IQueryable<>. Its lazily queries the service. Use List<> instead so that it queries immediately

Or make "btWholesaleDataContext db" into a member variable

JamesG · Accepted Answer · 2011-06-29 14:24:51Z

Queries to MACRequests are deferred - once you're out of the using block and your DataContext is disposed you're not going to be able to make the query you want.

Jeff · Accepted Answer · 2011-06-29 14:28:36Z

You're creating the data context in a using block in the constructor of your DataModel...so by the time you access the MACRequests, the data context has been disposed.

Consider the following:

public class DataModel : IDisposable { btWholesaleDataContext db = new btWholesaleDataContext(); public void Dispose() { btWholesaleDataContext.Dipose(); } public IQueryable<Models.BT.Request> MACRequests { get { return from r in db.btRequests select new Models.BT.Request { ID = r.ID, Date = r.DateTime, StatusCode = 3, Status = r.Status }; } } }

Note that this usage will work:

using (var dm = new DataModel()) { dm.MACRequests.ToArray(); }

but this will fail for the same reason as the original:

IQueryable<Models.BT.Request> requests = null; using (var dm = new DataModel()) { requests = dm.MACRequests; } // this will fail because the context is disposed by the time we force enumeration of the query requests.ToArray();

...Alternatively, since WCF data services can't filter on projections, and thus all you can really do with the query

 from r in db.btRequests select new Models.BT.Request { ID = r.ID, Date = r.DateTime, StatusCode = 3, Status = r.Status };

is execute it...

just consider changing your original code to return an array or a list instead of leaving it as a queryable.

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