i am trying to apply an expression filter dynamically and can't get it working. Would any of you guys know, how can apply the given expression filter inside the for each loop and then return the object of Type t when it matches?
Public Function FindByCondition( _
filter As Expressions.Expression(Of Func(Of T, Boolean))) As T Implements IRepository(Of T).FindByCondition
Dim metaData As New LinqMetaData
AutoMapper.Mapper.CreateMap(GetType(EntityType), GetEntityType)
Dim dataSource = TryCast(metaData.GetQueryableForEntity(CInt([Enum].Parse(GetType(EntityType), GetEntityType.Name))), IQueryable(Of EntityBase))
Dim q = (From p In dataSource _
Select p).ToList
Dim g = AutoMapper.Mapper.Map(Of IEnumerable(Of T))(q)
For Each k As T In g
k.Equals(filter)
Next
End Function
You need to compile the expression tree to a delegate, then call the delegate on each instance.
Dim compiled As Func(Of T, Boolean) = filter.Compile()
If compiled(k) Then
Or, more simply,
Return g.FirstOrDefault(compiled)
Or, much more simply,
Return AutoMapper.Map(Of T)(dataSource.FirstOrDefault(filter))
This will actually run the filter on the server (or whatever your IQueryable implementation does).
For all of the other cases, you should accept a Func(Of T, Boolean) rather than an expression tree, since you don't actually need the expression tree. Compile() is an expensive call.
Related
My desire is to run a given function by name through AddressOf with one input parameter, e.g. Function Foo(x as Integer) As Integer. The two inputs I need into the recursive function are the function name _name As String and an object of some type t _list As t (Integer, Double, List(Of Integer), etc). The goal is to process either an element or list of elements with the function name, as there are multiple times I need to process a list by a given function and I do not wish to replicate the list processing code in each location. The ways I've tried to call my best go at this type of function (below) that didn't crash completely resulted in this error:
Warning: List.Test operation failed. Overload resolution failed because no Public 'ProcessList' can be called with these arguments:
'Public Shared Function ProcessList(Of t)(_func As Func(Of Object,t), _list As System.Object) As IEnumerable(Of t)':
Type argument inference fails for argument matching parameter '_func'.
Iterator Function ProcessList(Of t)(_func As Func(Of Object, t), _list As Object) As IEnumerable(Of t)
If _list.GetType = GetType(List(Of t)) Then
Yield _list.SelectMany(Function(l) ProcessList(_func, l))
Else
Yield _func(_list)
End If
End Function
For reference, I found a snippet of Python code that effectively does what I need, but I'm a little rusty on translating in this direction (Python to VB.net), and I'm not as familiar with this type of programming in VB.net. The Python snippet is:
def ProcessList(_func, _list):
return map(lambda x: ProcessList(_func, x) if type(x)==list else _func(x), _list)
Any help as to how I need to call this function, or how to rework this function if my approach is flawed, would be greatly appreciated!
Update:
I re-examined how I was calling the function and a few other things based on #djv's info that my method is working. First, due to the nature of how I'm interfacing with these functions, I have to expose the above function with:
Public Shared Function Foo(ByVal _input As Object) As Object
Return Utilities.ProcessList(AddressOf Bar, _input)
End Function
I'm also now getting the error message:
Warning: List.Test operation failed.
Unable to cast object of type 'System.Int32' to type 'System.Collections.Generic.IList`1[System.Int32]'.
The issue at this point probably lies with the method in which I'm calling my ProcessList function, rather than the function itself as I thought. I'm interfacing with a GUI that is not happy with calling ProcessList on its own, so I need this intermediate "helper" function, which I am apparently not using correctly.
You will always get an IEnumerable(Of T) and T can either be a primitive (i.e. Integer) or list of primitive (i.e. List(Of Integer)). So when you try to call it with a List, you get a List(Of List(Of Integer)) for example.
We can see why by breaking ProcessList up into two methods. The difference between them is the type of the second argument which is either T or IEnumerable(Of T)
Sub Main()
Dim i As Integer = 1
Dim li As New List(Of Integer) From {1, 1, 1}
Dim ri As IEnumerable(Of Integer) = ProcessList(AddressOf foo, i).ToList()
Dim rli As IEnumerable(Of Integer) = ProcessList(AddressOf foo, li).ToList()
Dim d As Double = 1.0#
Dim ld As New List(Of Double) From {1.0#, 1.0#, 1.0#}
Dim rd As IEnumerable(Of Double) = ProcessList(AddressOf foo, d).ToList()
Dim rld As IEnumerable(Of Double) = ProcessList(AddressOf foo, ld).ToList()
Console.ReadLine()
End Sub
Function ProcessList(Of T)(f As Func(Of T, T), p As IEnumerable(Of T)) As IEnumerable(Of T)
Return p.Select(Function(i) ProcessList(f, i)).SelectMany(Function(i) i)
End Function
Iterator Function ProcessList(Of T)(f As Func(Of T, T), p As T) As IEnumerable(Of T)
Yield f(p)
End Function
Function foo(param As Integer) As Integer
Return param + 1
End Function
Function foo(param As Double) As Double
Return param + 1.0#
End Function
Previously, I could not even hit the line in your original code which did the SelectMany. Now, it is hit when the proper function is called. I also retooled that call to fit the new function signature.
The overloads are both called, based on the second argument passed them. However, you only need one foo method for each T (either a primitive or its IEnumerable).
I have a function which deserializes some custom serialization sent by an API.
I want to build a generic function so that the deserialized object is not of type Object but of the correct type.
The strings which contain the serialized object can be deserialized into one of the following types:
A String,
an IList(Of String),
an IDictionnary(Of String),
one of many SomeNameContainer classes, all derived from a
BaseContainer class,
an IList(Of SomeNameContainer), or
an IDictionnary(Of SomeNameContainer).
I would like to have a single Function Deserialize(Of T)(MyString as String) as T.
Inside this function, I tried to run some Select Case T: GetType(String):Etc tests in order to separate the different actions to run on MyString, depending on the expected object to create from the deserialization.
For example, deserializing into a SomeNameContainer is normally done via another generic function: Dim Deserialized as SomeNameContainer = GetFromContainer(SomeNameContainer)(MyString)
However, I get quickly limited, mainly because:
I cannot return a String type, because it is unable to cast it
into T.
String is a value type, whilst SomeNameContainer are classes. So it is not possible to add an (Of T As {New}) constraint. Which means I am unable to do something like Dim NameContainer as New T: If TypeOf NameContainer Is BaseContainer in order to apply the same operation to all the classes derived from BaseContainer.
One track I have found is to use CTypeDynamic(Of T)(obj as object), which casts at run-time. That might fix problem 1, but problem 2 is still on.
Function Deserialize(Of T)(MyString as String) as T
Select Case GetType(T)
Case GetType(String)
Return SomeFunction(String) '<- Only run-time casting allowed: Return CTypeDynamic(Of String)(SomeFunction(String))
Case GetType(IList(Of String)
Return SomeOtherFunction(String)
Case GetType(...)
'...
Case Else
Dim MyContainer as New T '<- Not Allowed to use New
if TypeOf MyContainer Is T then
Return GetFromContainer(Of T)(String)
else
'...
End If
End Select
End Function
I could decide to split each Type into a separate function. I would like to avoid so that I do not end up with 6 functions. That is because I also need to run some other operations on the string before it is deserialized. For the story, the strings come under various encoding/encryption formats. So if I have 4 formats, that is now 4x6=24 functions I would need to deal with.
I would love to have the luxury of encapsulating all the decoding/deserialization into a single function: Dim MyObject as Something = Deserialize(Of Something)(StringFromAPI, MyEncodingEnumOptions.Option42)
Many thanks in advance!
Performing a specific action depending on the type of a specific variable: that feels similar to Overloading, except that here instead of performing the action based on the type of the input variables, it should be base on the type of the output variables.
Unfortunately, it is not possible to overload the TypeName of a generic function. For example, Function MyFunction(Of T as New)(SomeParameter as String) as T and Function MyFunction(Of T as Structure)(SomeParameter as String) as T cannot coexist in the same namespace.
An alternative is to pass the expected output type as an input argument, so that regular overloading can be performed: Sub MyFunction(ByVal SomeParameter as String, ByRef OutputVar as SomeType). Each overload including a different SomeType TypeName.
The output of the "function" is stored into OutputVar, which is passed ByRef and retrieved after running the Sub:
Dim MyObject as Something = Deserialize(Of Something)(StringFromAPI, MyEncodingEnumOptions.Option42)
Becomes
Sub Deserialize(ByRef MyObject as String, ByVal MyString As String, ByVal EncodingOption As MyEncodingEnumOptions)
MyString = SomeDecoding(MyString, EncodingOption)
MyObject = SomeFunction(MyString)
End Sub
Sub Deserialize(ByRef MyObject as IList(Of String), ByVal MyString As String, ByVal EncodingOption As MyEncodingEnumOptions)
MyString = SomeDecoding(MyString, EncodingOption)
MyObject = SomeOtherFunction(MyString)
End Sub
'...
Dim MyObject as Something
Deserialize(MyObject, StringFromAPI, MyEncodingEnumOptions.Option42)
'Now MyObject has been filled with the relevant data.
An alternative is to use late binding / runtime object initilization, using Activator.CreateInstance(Of T). A typical switch over T would then look like:
Public Function GetDeserializedObject(Of T)(ByVal MyString As String) As T
Select Case GetType(T)
Case GetType(String)
Return CTypeDynamic(MyString, GetType(T)) '<-- Runtime Casting
Case Else
If Not MyString.IsDeserializable Then Throw New ArgumentException(String.Format("Unable to deserialize to a {0} object: The provided string is not valid.", GetType(T).ToString))
Select Case GetType(T)
Case GetType(IList(Of String))
Return CollectionString.ToStringList(MyString)
Case Else
Dim MyReturn As T = Activator.CreateInstance(Of T) '<-- Object instantiation to the type provided at Runtim
If TypeOf MyReturn Is BaseContainer Then '<-- Now we can use TypeOf ... Is ... which will return True for all Object derived from BaseContainer
Return Activator.CreateInstance(GetType(T), MyString)
ElseIf TypeOf MyReturn Is IList(Of BaseContainer) Then
Dim MyCollectionString As CollectionString = MyString
Return MyCollectionString.ExportToContainerList(MyReturn.GetType)
Else
Throw New ArgumentException(String.Format("Unable to deserialize to a {0} object: This type of object is not supported.", GetType(T).ToString))
End If
End Select
End Select
End Function
I've written the following function to check an added/modified record for overlapping time periods in the database:
<Extension> Function ClashRecords(Of T)(Query As IQueryable(Of T), Record As T, _
KeySelector As Expression(Of Func(Of T, Integer)),
FromSelector As Expression(Of Func(Of T, DateTime)),
TillSelector As Expression(Of Func(Of T, DateTime))) As IQueryable(Of T)
Dim key = KeySelector.Invoke(Record)
Dim fromDate = FromSelector.Invoke(Record)
Dim tillDate = TillSelector.Invoke(Record)
Dim criteriaExpr As Expression(Of Func(Of T, Boolean)) = Function(x) KeySelector.Invoke(x) = key And FromSelector.Invoke(x) <= tillDate And TillSelector.Invoke(x) >= fromDate
Return Query.AsExpandable.Where(criteriaExpr.Expand)
End Function
When calling the function as follows:
Dim de As New DataEntities()
Dim w=New Work With {.WorkerID=-1,.FromDate=New DateTime(2014,3,20,7,0,0),.TillDate=New DateTime(2014,3,20,8,30,0)}
Dim clashing = de.Works.ClashRecords(w,Function(x) x.ActivistID, Function(x) x.FromDate, Function(x) x.TillDate)
I get the following error:
InvalidCastException: Unable to cast object of type 'System.Linq.Expressions.FieldExpression' to type 'System.Linq.Expressions.LambdaExpression'.
I don't see that I am using any field expressions -- ActivistID, FromDate, TillDate are all properties.
I am using EF5 and generated POCO classes.
How can I resolve this?
Update
If I enter criteriaExpr.Expand in the Watch window, I don't get an exception. However, if I enter criteriaExpr.Expand.Compile I get the same exception.
See: How do closures work behind the scenes? (C#)
If a variable from the outer scope is accessed from an inner function, the compiler creates a hidden type to own the variables as fields. Your criteriaExpr accesses the outer function parameters, so it contains FieldExpressions.
I took a quick look at LinqKit's source code (which you seem to be using judging by the Expand and Compile), and it appears that it doesn't support Invoke on a FieldExpression.
When I write a Group By in query expression syntax, the compiler automatically picks Enumerable.GroupBy as my intended tareget method and I get an IEnumerable back instead of an IQueryable. That means a subsequent g.Sum call (inside of my Select) expects a Func(Of TSource, int) instead of an Expression(Of Func(Of TSource, int)). Is there a way to force the Group By to use Queryable.GroupBy instead and give me back an IQueryable?
Contrived Sample Code
Dim myQuery = From x In DataSource.Items
Group By x.Key Into g = Group
Select New With {
.Key = Key,
.RedItems = g.Sum(ItemsOfColor(Colors.Red)) '<== invalid because g.Sum expects a lambda
}
Private Function PurpleItems(color As Integer) As Expression(Of Func(Of Item, Integer))
Return Function(item) If(item.Color = color, 1, 0)
End Function
Why would I want to do this?
The compiler automatically converts between a lambda and an expression based on the target variable type (ie, both Dim f As Func(Of String, Integer) = Function(x) x.Length() and Dim e As Expression(Of Func(Of String, Integer)) = Function(x) x.Length() are valid) so there is no noticable difference in the code between an IEnumerable and IQueryable.
The problem is, LINQ to Entities (and I assume other db backed LINQ implementations) relies on expression trees to translate into SQL. That means the IEnumerable lambda version will not work against an IDbSet as I found in this old question.
The problem is that Queryable.GroupBy() returns IQueryable<IGrouping<TKey, TSource>>, where IGrouping<TKey, TSource> implemens IEnumerable<TSource>, but not IQueryable<TSource>.
And I believe your code wouldn't work anyway, because ItemsOfColor() wouldn't be actually called. Instead, the EF would get an expression that calls ItemsOfColor(). And since it doesn't know that method, it would throw an exception.
I am trying to use the "Except" method on a LINQ result set using a custom implementation if IEqualityComparer to exclude certain results based on the value of a single field from the result set.
So, in simplified form I have...
'' Get collection of published sites...
Dim List1 = (From i In db.Sites _
Where (i.StatusID = published) _
Select i.SiteID, _
i.SiteName)
'' Find those with a pending site, but exclue all those whose SiteID is in List1...
Dim insComparer = New insCompare
Dim List2 = (From i In db.Sites _
Where (i.StatusID = pending) _
Select i.SiteID, _
i.SiteName).Except(List1, insComparer)
My Comparer is as follows...
Public Class insCompare
Implements System.Collections.Generic.IEqualityComparer(Of Object)
Public Function Equals1(ByVal x As Object, ByVal y As Object) As Boolean Implements System.Collections.Generic.IEqualityComparer(Of Object).Equals
Return IIf(x.SiteID = y.SiteID, True, False)
End Function
Public Function GetHashCode1(ByVal x As Object) As Integer Implements System.Collections.Generic.IEqualityComparer(Of Object).GetHashCode
Return x.SiteID.ToString.ToLower.GetHashCode()
End Function
End Class
I get an invalid cast exception on the ".Except" line with the message "Unable to cast object of type '...insCompare' to type 'System.Collections.Generic.IEqualityComparer'"
Can anyone cast light on why this might be please.
Your problem here is that you implement IEqualityComparer(Of Object), but your lists are List(Of AT) where AT is an anonymous type, so you can't implement IEqualityComparer(Of AT).
I think your choices are:
Declare a class/struct to hold the SideID/SiteName, and select into an instance of that class, then implement IEqualityComparer(Of NewClass).
Use late-bound calls (ie. option explicit off, like it appears you are doing now), and put a .Cast(Of Object)() call on both lists before calling Except.
Use the following code.
from t in db.Sites
where
!
(from t0 in db.Sites2
select new {
t0.SomeID
}).Contains(new { t.SomeID })
select t
this is based in not in condition. I think this will help you. U are doing some complex thing.
It looks like it's asking that your comparer implement the non-generic interface IEqualityComparer, whereas yours implements IEqualityComparer (Of Object), which is a different interface.
It looks like you are using a database as the back end. You can't provide a custom comparer for this, as it can't be mapped to TSQL.
Have you tried Contains? i.e. where !List1.Contains(i.SiteID)?