Converting large global statically allocated arrays to Collections

Question

I am working on retrofitting an application written in VB.NET. It is a simple program that uses 0.5GB (!) of RAM, because of a large amount (30+) of globally defined arrays, like:

Public Temp(1000000) As Double
Public ThisIsAnotherVariable(5000, 10) As String 
Public ThisIsAVeryLargeArray(64, 50000) As Double

Most of the time, these large "buffers" are barely used, so I would like to convert them to use something from Collections.Generic. Is there any semi-transparent way to convert these? Or a bit of trickery to get the CLR to only allocate the used segments?

There's no reason to use 500MB of RAM when the arrays are barely used. I would like to reduce RAM footprint. — Tyler Lewis, May 23 '17 at 13:07

Heinzi · Accepted Answer · 2017-05-23T14:04:00.470

If these are "sparse" arrays, i.e., if almost all array entries are entries, the simplest solution might be to replace them with dictionaries:

Public Temp(1000000) As Double                     ' Old
Public Temp As New Dictionary(Of Int32, Double)()  ' New

Assignment would be source-code compatible:

Temp(10) = 2.0    ' Works for arrays and dictionaries

Reading would be source-code compatible, if the value is present:

Dim x = Temp(3)   ' Works for arrays and dictionary, if Temp(3) has been assigned

However, accessing values which have not been assigned will yield a KeyNotFoundException. If you need that, you'd have to use a "default value dictionary" instead of a regular dictionary. Unfortunately, no such dictionary is built-in in the BCL, but there are others who have already looked at that problem and shared their implementation. If you want to use a plain .NET dictionary, you will need to replace every read access with a method call.

This is an excellent and clever solution. Thanks for the advice, I will report back if anything goes haywire. — Tyler Lewis, May 23 '17 at 13:18

Converting large global statically allocated arrays to Collections

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