LINQ in the LightSwitch ...PreprocessQuery() method does not work with delegate.

Question

I have problems in designing an extended query in the ...PreprocessQuery() Method of a LightSwitch query.

It does not seem to be a LINQ problem itself, but ... there is no LightSwitch Forum ;)
1st of all, I tell you in a simplified way, what works:

partial void NeedHelp_PreprocessQuery(bool Admin, ref IQueryable<Device> query)
{
// Some code...
query = query.Where<Device>(d => d.IsAvailable());
}

The query does not show any compile error and does what it should do.

Now, when I try to put the logic into a method and use its delegate in the query, there's no compile error either, but I get an exception. Here is the code:

private bool Logic(Device TheDevice, bool Admin)
{
return Admin ? true : TheDevice.IsAvailable();
}
partial void NeedHelp_PreprocessQuery(bool Admin, ref IQueryable<Device> query)
{
// Some code...
Func<Device, bool, bool> LogicDelegate = new Func<Device, bool, bool>(this.Logic);
query = query.Where<Device>(d => LogicDelegate(d, Admin));
}

The Exception is in German, I try to translate the essential:

The expression is not supported. Expression: Invoke(value(LightSwitchApplication.ApplicationDataService+<>c_DisplayClass4).LogicDelegate, d, value(LightSwitchApplication.ApplicationDataService+<>c_DisplayClass4).Admin) Message of inner exception: The type "ApplicationData.Implementation.Device" cannot be used for a parameter of type "LightSwitchApplication.Device".

Since I am only using "Device", I do not understand this confusion between ApplicationData.Implementation.Device and LightSwitchApplication.Device! What am I doing wrong? Or is this kind of call simply not possible in LightSwitch?

Satria Sudewo · Answer

I can also imagine that there are strange things going on in the background or during runtime. It feels like. And I happen to already have posted the question in the very forum you have linked thanks ;)

Vulpes · Answer

I tried to model the situation (partial methods and all) in 'ordinary' code which can sometimes give one an insight into a problem such as this. However even though I had two Device classes in different namespaces I just couldn't get it to compile unless I resolved the ambiguity by using fully qualified names. The code then ran fine. As your code does compile I suspect that one of the Device classes is being generated dynamically by the runtime. I doubt whether using the Cast extension method will help as it's not clear what if any relationship there is between the two Device classes. As this is a new product it is of course likely that there are bugs or limitations in it. I just looked at MSDN and there is a LightSwitch forum over there ( http://social.msdn.microsoft.com/Forums/en-US/lightswitch/threads ). I'd try reposting the question on there in case no one else has an answer on this site.

Satria Sudewo · Answer

Hi. Thanks for your quick reply. In the IDE so during development the namespace/class ApplicationData.Implementation.Device is unknown / not available. And using LightSwitchApplication.Device explicitly doesn't help which I had already tried of course. The definition of the class Device is not done by the developer but by LightSwitch somewhere in the code when you create the table Device in the Data designer and usually you use Device in every relevant module. From what is exposed to the developer this all is very confusing because for you you only created ONE Device entity... There's also the method .cast () of every IQueryable or IEnumerable. Can this be of any use? Currently I have not been able to put it somewhere it made sense or helped me out :)

Vulpes · Answer

I'll start by saying that I know next to nothing about LightSwitch so I can only talk about the C# aspects. It's looks like there's a conflict in your latest code between classes with the same name (Device) but in different namespaces which the compiler is unable to resolve. I'd try being explicit about which you mean - in other words replace the type name 'Device' with the fully qualified type name of the Device class you want to use.