VB.NET property get and cache

Question

If I use in VB.NET a property get of this shape:

Public ReadOnly Property Thing(i as long) as double(,)
    Get
        Return SomeCalculation(i )
    End Get
End Property

and the code calls many times the property get with the same i (after doing the same with another iscen and so on), will the result be cached until I use a new i or will it be recalculated each time?

Thanks!

It will be recalculated each time. There is no automatic provision of caching. — Steve, Dec 30 '16 at 14:01
A property is simply syntactic sugar and is implemented as one or a pair of methods. Method results do not get implicitly cached. I'd say that that should probably not even be a property but rather a method, in which case there would be no confusion. — jmcilhinney, Dec 30 '16 at 14:02
Thanks to both. Will you put it as an answer so the question is marked as answered? — Pierre, Dec 30 '16 at 14:04

score 1 · Accepted Answer · answered Dec 30 '16 at 14:13

1

No there is no automatic caching in VB.NET to store the result of repeated calculations. It is up to you to provide some kind of caching.

For example you can use a Dictionary

 Dim cache As Dictionary(Of Long, Double(,)) = New Dictionary(Of Long, Double(,))

Public ReadOnly Property Thing(i as long) as double(,)
    Get
       Dim result As Double(,)
       If Not cache.TryGetValue(i, result) Then
           result = SomeCalculation(i)
          cache.Add(i, result)
       End If
       Return result
   End Get
End Property

Of course,as any simple solution, there are some points to consider:

There is no invalidation rule (the cache remains active for the lifetime of the app)
It assumes that the calculation produces always the same result for the same input

answered Dec 30 '16 at 14:13

Steve

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Also consider using `Lazy`. – Cody Gray - on strike Dec 30 '16 at 14:16
@CodyGray interesting. I am not used to it. Could you provide an example? – Steve Dec 30 '16 at 14:18
1

Declare a private backing variable of type `Lazy`, and then create a public property that returns the `Value` member of the `Lazy` variable. See, for example: http://stackoverflow.com/a/2579378/366904 and http://stackoverflow.com/questions/5134786/cached-property-vs-lazyt – Cody Gray - on strike Dec 30 '16 at 14:21
Very nice answer. I have written another answer as an alternative, but it might have some syntax errors, as it was not tested. Can you take a look? – Lajos Arpad Dec 30 '16 at 15:00

score 1 · Answer 2 · answered Dec 30 '16 at 14:59

You could create a class for cached values

Public Class LongCaches(Of MyType)
     Protected MyDictionary Dictionary(Of Long, MyType) = New Dictionary(Of Long, MyType)
     Public Delegate Function MyFunction(Of Long) As MyType
     Protected MyDelegate As MyFunction
     Public Calculate As Function(ByVal input As Long) As MyType
         If Not MyDictionary.ContainsKey(input) Then
             MyDictionary(input) = MyFunction.Invoke(input)
         End If
         Return MyDictionary(input)
     End Function
     Public Sub New(ByVal myfunc As MyFunction)
         MyDelegate = myfunc
     End Sub
End Caches

You will need to use it like this:

Private _MyLongCacheProperty As LongCaches(Of Double(,))
Protected ReadOnly MyLongCacheProperty(i As Long) As LongCaches
Get
    If _MyLongCacheProperty Is Nothing Then
        _MyLongCacheProperty = New LongCaches(Of Double(,))(AddressOf SomeCalculation)
    End If
    Return _MyLongCacheProperty.Calculate(i)
End Get
End Property

Notice: This code is untested, if there are syntax errors, then please comment or edit rather than downvote.

VB.NET property get and cache

2 Answers2