I want to query count of selected products with a Combobox in VB.NET, Linq and Entity Framework 6. This query generates error (cmbProducts is a Combobox):
Dim Count = (From Product In db.Products
Where Product.Type = cmbProducts.SelectedValue
).Count
And this is the error:
LINQ to Entities does not recognize the method 'System.Object CompareObjectEqual(System.Object, System.Object, Boolean)' method, and this method cannot be translated into a store expression.
But when I run this query with db.Products.local, it executes without any errors:
Dim Count = (From Product In db.Products.local
Where Product.Type = cmbProducts.SelectedValue
).Count
You really should turn Option Strict On in the project properties and also in the IDE options, so it will be On by default for all future projects. If you do that then that code won't even compile. That would force you to do as you should have and cast the SelectedValue, which is type Object, as the actual type of the underlying object, which is presumably String or Integer. You can use DirectCast or else CInt, CStr or the like to perform the cast, e.g.
Where Product.Type = CInt(cmbProducts.SelectedValue)
Ideally, you should be assigning the result of that cast to a variable and using that in your LINQ query. It's important to remember that, while LINQ syntax is always the same, each LINQ provider has a different implementation. LINQ to Entities converts your query to SQL that it can execute against the database and that means that some things that are supported by LINQ in general, and will thus compile, will actually fail at run-time against LINQ to Entities. It's generally a good idea to keep anything remotely exotic out of your EF queries. You'd probably be OK in this case but it's a good habit to get into as it can help avoid subtle issues.
Make sure they are of the same type.
I think Product.Type is a string but cmbProducts.SelectedValue is an int, try to use cmbProducts.SelectedItem.Text
When comparing locally .Net will be able to compare them by SQL Server may not.
Related
Edited to answer my own question. This appears to be a LINQ/VB bug.
A simple LINQ query returning an anomymous type will sometimes change the field names specified in the query so as to capitalize them. Perhaps passing the result of the query as a parameter to a method call:
someThing.someMethod(From someStuff In stuffList _
Select text = someStuff.Name(), _
value = someStuff.Id
)
where someMethod has signature
Public Sub someMethod(ByVal list As IEnumerable(Of Object))
If you step into the execution of someMethod, and then examine the value of list in quickwatch, you may or see the field names as "text"&"value" or "Text"&"Value".
LINQ queries should not be changing the field names as specified in the query, so the correct behavior is fieldnames "text"&"value". Yet production builds of our application have the incorrect capitalization behavior (which can be determined indirectly), and debug builds have shown it both happening both ways at different times and/or for different developers' machines.
I've looked high & low for some feature of LINQ which controls this behavior, but now am virtually certain it is a bug. (msdn forum thread, MS Connect bug page)
This is likely to only cause a problem if you are using reflection, such as type.getfield() such as in
listItem = list.ElementAt(index)
itemTextField = listItem.GetType().GetField("text")
itemText = CType(itemTextField.GetValue(listItem),String)
If this happens to you, the workaround is to use overload of GetField with bindingflags to make it case-insensitive:
itemTextField = listItem.GetType().GetField("text", BindingFlags.IgnoreCase)
It must be pretty rare to encounter this bug, but maybe the next person will spend less time scratching their head if they find this info here.
=========original post===========
Getting different behavior in my debug build environment than in my coworkers' and our production envirnonment, relating to LINQ and reflection...
While running debug build of legacy code, the following code
Dim objectType As Type = obj.GetType()
Dim field As FieldInfo = objectType.GetField(name)
Dim prop As PropertyInfo = objectType.GetProperty(name)
results in Nothing for field & prop.
The value of obj is passed down from above and is the result of a LINQ query (it is a single element of the list generated by the query):
From bpt In CustomBProcessTypes Select text = bpt.Name(), value = bpt.Id
The value of name is also passed from above and is "Text" (note capitalization).
I can examine obj in the debugger and confirm that the fieldnames of the object created by the LINQ query are 'text' and 'value' (note lack of capitalization) which is what I would expect.
So failure to find the field by the capitalized name makes sense. However, our production builds and my coworkers builds do not have this problem.
Because calls to type.getfield(string) are expressly cas-sensitive, the only thing I can think of at this point is there must be some configuration of LINQ relating to auto-capitalization of column/fieldnames, and my environment is not set up the same as the others.
Using visual studio 2012. I don't know much of anything about LINQ, per se.
Anyone have any idea what could be going on here?
(NOTE: if I can get an opportunity, I'll have a coworker step through the relevant code and see if in their environment the object created by the linq query ends up with capitalized field names)
EDIT: I verified with a coworker in his debug build: his LINQ query creates a list of objects with field names "Text" and "Value", but on in my environment the LINQ query ends up with field names "text" and "value". The code is the same, but there must be something about how LINQ is configured in my environment which fails to auto-capitalize those field names, but which happens on their machines and in our production environment.
I suppose it is possible that some compiler settings are resulting in different capitalization. Normally this would make no difference because VB.NET is a case-insensitive language so obj.Text and obj.text both work just as well. But to use case insensitivity in reflection lookups, you need to specify it by including BindingFlags.IgnoreCase in the second parameter of GetField or GetProperty:
Dim field As FieldInfo = objectType.GetField(name,
BindingFlags.Public Or BindingFlags.Instance Or BindingFlags.IgnoreCase)
I'm confused as to where name is coming from, though. Some other code is getting the field name from reflection on the query? I didn't see where this was explained in your question.
I have answered my own question (insofar as is possible). Boils down to a bug in LINQ/vb.net.
Fully explained at top of original post (edited in). Hope this saves someone time in the future.
I have a Linq query that yields anonymous types. However, now I want to work with the parameters of this anonymous type and it does not seem to work.
For Each obj As Object in Query
Dim row As DataRow = obj.parameter
...
Next obj
Now the compiler throws an error on the expression obj.parameter: "Option Strict On disallows late binding". If I understand it right, the compiler doesnt know the parameters of the anonymous type. I tried Option Infer On (and removed As Object), based on Google results, but it didnt help. Which seems to make sense, because it always seems to be a widening conversion to me.
Is there anyway to fix this, or should I just create a custom type?
The code that declares the anonymous type (i.e. the Select part of your LINQ query) must be in the same method as the the code that uses it and the Query variable's declaration must have an inferred type. You cannot access the properties of an anonymous type after it has been cast to an Object since there is no named type to which you can cast it.
So make sure that your LINQ query (or, at least, the part that Selects into a new anonymous type) is in the same method. E.g.
Dim Query = From prod In products
Select prod.Name, prod.Price
For Each obj in Query
Dim name = obj.Name
...
Next obj
We have several projects in VB.Net, using .Net Framework 4 and Linq to Entities for many of our SQL queries. Moving to EF is a new shift for us (been using it for about 4-6 months) and has the backing of upper management because we can code so much faster. We still use a lot of stored procs, but we even execute those through Linq to Entities as well.
I'm hoping to clear some confusion up and I can't find a direct answer that makes sense. We have some queries where we want records where a specific field has a NULL value. These are simple select queries, no aggregates or left joins, etc. Microsoft recommends the query look something like this MSDN Link:
dim query = from a in MyContext.MyTables
Where a.MyField = Nothing
Select a
I have several projects where I do exactly this and it works great, no warnings in the IDE. Recently a new project was created by another developer and when he did his null check like above, we all get this warning in the IDE:
Warning 1 This expression will always evaluate to Nothing (due to null propagation from the equals operator). To check if the value is null consider using 'Is Nothing'.
Comparing the projects, option explicit and option strict are on for each one. If we ignore the warning, we get the exact record set we are looking for when the app runs. The warning goes away if I change the = sign to IS. But why did this warning appear in one project and not others? It's confusing when even on MSDN there are examples using the equals operator.
Generated column should be a Nullable(Of T)
So you can check if that field has value or not like this:
dim query = from a in MyContext.MyTables
Where Not a.MyField.HasValue
Select a
I believe what you're seeing here is that MyField is a Nullable(Of T) type. Likely a primitive Integer, Single, etc ...
The reason you're seeing this warning is because the compiler promotes the normal equality operator for the primitive type to the Nullable(Of T) version. It essentially executes the following
Dim myField As Integer? = a.MyField
Dim other As Integer? = Nothing
If myField = other Then
...
End If
The issue though is that when Integer? has the value Nothing it won't compare equal to anything. Hence the above Where clause will always return False. The compiler is attempting to warn you about this problematic corner of Nullable(Of T) and push you to a Is Nothing check which will determine if a.MyField has a non-null value.
This blog article has a very detailed explanation of why this warning is being generated and all of the mechanics behind it. The article is written for C# but the basic premise is applicable to VB.Net as well.
http://blogs.msdn.com/b/abhinaba/archive/2005/12/14/503533.aspx
At least in LINQ to objects you can use this instead:
Nullable(Of Integer).Equals(a, b)
This works fine with both, either or none of the two values being Nothing.
I've a simple class like this:
Public Class CalculationParameter{
public Long TariffId{get;set;}
}
In a workflow activity, I've an Assign like this:
(From tariffDetail In db.Context.TariffDetails
Where tariffDetial.TariffId = calculationParameter.TariffId).FirstOrDefault()
Dto is passed to Activity as an Input Argument.
It raise following error and I'm wondering how to assign Id. Any Idea?
LINQ to Entities does not recognize the method 'Int64
GetValue[Int64](System.Activities.LocationReference)' method, and this
method cannot be translated into a store expression.
How can I assign the calculationParameter.TariffId to tariffDetial.TariffId?!
UPDATE:
Screen shot attached shows that how I'm trying to assign calculationParameter.TariffId to tariffDetail.TariffId (car.Id = Dto.Id) and the query result should assign to CurrentTrafficDetail object.
Here's your problem. I don't know if there is a solution to it.
As you said in a (now deleted, unfortunately necessitating that I answer) comment, the exception you're getting is
LINQ to Entities does not recognize the method Int64 GetValue[Int64](System.Activities.LocationReference) method, and this method cannot be translated into a store expression.
in your Linq query, calculationParameter is a Variable defined on the workflow. That Variable is actually an instance that extends the type System.Activities.LocationReference and NOT CalculationParameter.
Normally, when the workflow executes, the LocationReference holds all the information it needs to find the value which is assigned to it. That value isn't retrieved until the last possible moment. At runtime, the process of retrieval (getting the executing context, getting the value, converting it to the expected type) is managed by the workflow.
However, when you introduce Linq into the mix, we have the issue you are experiencing. As you may or may not know, your expression gets compiled into the extension method version of the same.
(From tariffDetail In db.Context.TariffDetails
Where tariffDetial.TariffId = calculationParameter.TariffId)
.FirstOrDefault()
is compiled to
db.Context.TariffDetails
.Where(x => x.TariffId = calculationParameter.TariffId)
.FirstOrDefault();
When this executes, L2E doesn't actually execute this code. It gets interpreted and converted into a SQL query which is executed against the database.
As the interpreter isn't omniscient, there are a well defined set of limitations on what methods you can use in a L2S query.
Unfortunately for you, getting the current value of a LocationReference is not one of them.
TL:DR You cannot do this.
As for workarounds, the only thing I think you can do is create a series of extension methods on your data context type or add methods to your CalculationParameter class that you can call from within the Expression Editor. You can create your Linq to Entities queries within these methods, as all types will already have been dereferenced by the workflow runtime, which means you won't have to worry about the L2E interpreter choking on LocationReferences.
*Edit: A workaround can be found here (thanks to Slauma who mentioned this in a comment on the question)
I'm trying to write this as LINQ,
Original code:
For Each CurrentForm As Form In MyForms
AddLink(CurrentForm.GetLink())
Next
I'm a LINQ beginner, so far I'm not quite sure where to use and where not to. If in this case LINQ will do more harm then help, feel free to flame me.
Edit : You can assume that there is an overload for AddLink() which takes IEnumerable
Unless there is an overload of AddLink which takes a collection, LINQ won't avoid the loop.
Is there is such an overload then something like:
AddLinks(MyForms.Select(f => f.GetLink())
would do it.
How the above expression works (briefly):
LINQ is about expressions, taking some object (for LINQ to Objects used here, always a collection)
Select extension method takes a collection and a function and returns a collection. The function is passed each element of the input collection. And then Select returns the collection made up of all the function return values.
I have used a lambda expression to create an anonymous function that takes one argument called f (its type will be determined by the compiler) and returns the value of the expression (now corrected).
AddLinks is an assumed variant of your AddLink which takes a collection of links.
There is a lot going on, this is one of the advantages of LINQ, it is a compact way of expressing data manipulation without the usual overheads of explicit loops and temporary variables.
No flames here, but LINQ won't really help here. If LINQ had a ForEach method (as has been discussed in a previous question, as well as elsewhere) then you could use that - but it's not built into LINQ, and in this case there doesn't really seem to be much use for it.
Of course, it depends exactly what AddLink does - if it adds a link to a list, and you could instead use (say) List.AddRange, then you could use LINQ. But this code seems pretty simple and readable already, so I wouldn't worry in this case.