How does Array.GetLength(dimension) in C# actually work?

Question

When using Array.GetLength(dimension) in C#, does the size of the array actually get calculated each time it is called, or is the size cached/stored and that value just gets accessed?

What I really want to know is if setting a local variable to the length of the dimension of an array would add any efficiency if used inside a big loop or if I can just call array.GetLength() over and over w/o any speed penalty.

Answer 1

It is most certainly a bad idea to start caching/optimizing by yourself here.

When dealing with arrays, you have to follow a standard path that the (JIT) optimizer can recognize. If you do, not only will the Length property be cached but more important the index bounds-check can be done just once before the loop.

When the optimizer loses your trail you will pay the penalty of a per-access bounds-check.

This is why jagged arrays (int[][]) are faster than multi-dim (int[,]). The optimization for int[,] is simply missing. Up to Fx2 anyway, I didn't check the status of this in Fx4 yet.

If you want to research this further, the caching you propose is usually called 'hoisting' the Length property.