CODEWITHSUNDEEP
c#.netarrayscollectionsf#
Sam
Sam

Reputation: 2865

How can I make a Value Type collection in .NET?

The basic building block type of my application decomposes into a type (class or structure) which contains some standard value types (int, bool, etc..) and some array types of standard value types where there will be a small (but unknown) number of elements in the collection.

Given that I have many instances of the above building block, I would like to limit the memory usage of my basic type by using an array/collection as a Value Type instead of the standard Reference Type. Part of the problem is that my standard usage will be to have the arrays containing zero, one or two elements in them and the overhead of the array reference type in this scenario is prohibitive.

I have empirically observed and research has confirmed that the array wrapper itself introduces unwanted (by me, in this situation) overhead in each instance.

How do I make a collection a Value Type / Struct in .NET?

Side Note: it is interesting that Apple's Swift language has arrays as value types by default.

Pre-Emptive Comment

I am fully aware that the above is a non-standard way of using the .NET framework and is very bad practice etc...so it's not necessary to comment to that effect. I really just want to know how to achieve what I am asking.

Upvotes: 2

Views: 901

Answers (2)

V.B.
V.B.

Reputation: 6382

A mix of ideas from a DirectBuffer and a BufferPool could work.

If you use a buffer pool then fixing buffers in memory is not a big issue because buffers become effectively long-lived and do not affect GC compaction as much as if you were fixing every new byte[] without a pool.

The DirectBuffer uses flyweight pattern and adds very little overhead. You could read/write any blittable struct directly using pointers. Other than SBE, Flatbuffers and Cup'n Proto also use such approach as far as I understand. In the linked implementation you should change the delegate so that it returns a discarded byte[] to the pool.

Big advantage of such solution is zero-copy if you need to interop with native code or send data over network. Additionally, you could allocate a single buffer and work with offsets/lengths in ArraySegment-like fashion.

Update:

I have re-read the question and realized that it was specifically about collections as value types. However the main rationale seems to be memory pressure, so this answer could be an alternative solution for memory, even though DirectBuffer is a class.

Upvotes: 1

Thomas Boby
Thomas Boby

Reputation: 789

The fixed keyword referenced in the docs seems to be what you're looking for. It has the same constraints on types as structs do, but it does require unsafe. 

 internal unsafe struct MyBuffer
{
    public fixed char fixedBuffer[128];
}

If you wanted to also have a fixed array of your struct it would be more complicated. fixed only supports the base value types, so you'd have to drop into manual memory allocation.

Upvotes: 1

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