Extension Method for Entity Framework 4

Question

I use an extension method for entity Framwork entity:

    <Extension()>
    Public Function IsExcluded(ByVal item As AnomalyProduct) As Boolean

        Dim result As Boolean

        If (item.WholesalerProductCode IsNot Nothing) AndAlso (item.WholesalerProductCode = _excludedChar) Then
            result = True
        Else
            result = False
        End If

        Return result

    End Function

I want to get a list of entity depending of the Extension Method result:

    Private Function HasPacksExcluded(ByVal contextId As Guid) As Boolean

        Dim result As Boolean
        Dim context As ContextManager.ContextData
        Dim repo As ILancelotLabEntities

        context = _context.GetContext(contextId)
        repo = context.RepositoryContext

        result = repo.AnomalyProducts.ToList.Where(Function(p) p.IsExcluded).Any


        Return result

    End Function

But by this way i need to load all AnomalyProducts from Database. It take a long time just to get finally a boolean.

I think Expression Tree can help me, but i am not able to do that.

some help will be appreciated.

Why don't you do `repo.AnomalyProducts.Where(Function(p) p.WholesalerProductCode = _excludedChar).Any()`? — Gert Arnold, Jan 29 '13 at 19:21
@Arnold Because exclusion critera may change in the future, and exclusion test "IsExcluded" is call on many point in the program. So when critera will change i don't want read all of my code to track it. — Exatex, Jan 29 '13 at 22:38

score 2 · Answer 1 · answered Jan 29 '13 at 19:27

2

You can only do that with data on memory. When you put some linq inside the Where (or any other) clause, entity framework will translate that to T-SQL.

This can't be translated to any T-SQL:

repo.AnomalyProducts.Where(Function(p) p.IsExcluded).Any

This can (because you're getting all data in memory:

repo.AnomalyProducts.ToList.Where(Function(p) p.IsExcluded).Any

To minimize effort, you could create a Expression (that is what the Where clause expects) and use that. This will only minimize the amount of places you'll have to copy and paste your code.

Dim exp As Expression(Of Func(Of AnomalyProduct, Boolean)) = Function(a) Not String.IsNullOrEmpty(a.WholesalerProductCode) AndAlso a.WholesalerProductCode <> _excludedChar
repo.AnomalyProducts.Where(exp).Any

I havent tested it, but it should work just fine.

answered Jan 29 '13 at 19:27

Luís Deschamps Rudge

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Should you want to, you can also let your `IsExcluded` method return the above expression. – jessehouwing Jan 29 '13 at 19:32
@jessehouwing How would you use that? `.Where(SomeClass.IsExcluded())`? That doesn't feel right. – svick Jan 29 '13 at 20:57
When it's in the same class you can use `.Where(x => IsExcluded(x))` or probably even `.Where(IsExcluded)`. You could also create a class with a proper name such as `ProductFunctions` and use the `ProductFunctions.IsExcluded` as you've proposed. – jessehouwing Jan 29 '13 at 22:54
The important thing is return the expression, and not the result itself. – Luís Deschamps Rudge Jan 30 '13 at 04:35

Extension Method for Entity Framework 4

1 Answers1