CODEWITHSUNDEEP
c#
Bob
Bob

Reputation: 7971

What happens when value types are created?

I'm developing a game using XNA and C# and was attempting to avoid calling new struct() type code each frame as I thought it would freak the GC out. "But wait," I said to myself, "struct is a value type. The GC shouldn't get called then, right?" Well, that's why I'm asking here.

I only have a very vague idea of what happens to value types. If I create a new struct within a function call, is the struct being created on the stack? Will it simply get pushed and popped and performance not take a hit? Further, would there be some memory limit or performance implications if, say, I need to create many instances in a single call?

Take, for instance, this code:

spriteBatch.Draw(tex, new Rectangle(x, y, width, height), Color.White);

Rectangle in this case is a struct. What happens when that new Rectangle is created? What are the implications of having to repeat that line many times (say, thousands of times)? Is this Rectangle created, a copy sent to the Draw method, and then discarded (meaning no memory getting eaten up the more Draw is called in that manner in the same function)?

P.S. I know this may be pre-mature optimization, but I'm mostly curious and wish to have a better understanding of what is happening.

Upvotes: 15

Views: 299

Answers (4)

Brian Rasmussen
Brian Rasmussen

Reputation: 116481

This is just to add to thecoops answer. For reference types the new operator allocates a new instance of the type on the heap and calls the specified constructor.

For a struct, the new operator initializes the fields according to the specified constructor. It is however possible to instantiate a struct without using new. In that case all the fields in the struct are uninitialized and cannot be used until they have been explicitly initialized.

For more info see the description on MSDN.

Upvotes: 1

Franci Penov
Franci Penov

Reputation: 76021

The value types are created on the stack if declared locally or on the heap if part of an object instance (as part of the object instance). In any case, struct instances are not collected by the GC, they are destroyed when their container goes out of scope.

The MSDN struct (C#) article has some more information about this.

Upvotes: 1

thecoop
thecoop

Reputation: 46158

When a new struct is created, it's contents are put straight into the location where you specify - if it's a method variable, it goes on the stack; if it's being assigned to a class variable, it goes inside the class instance being pointed to (on the heap).

When a struct variable is copied (or, in your case, passed to a function), the bytes making up the struct are all copied to the correct place on the stack or inside the class (if you're setting a field or property on an instance of a reference type).

Even though there may be copying of bytes, the JIT compiler will likely optimize all the unneccessary copies away so that it executes as fast as possible. Generally, it's not something you need to worry about - this is very much a micro-optimization :)

Does this answer your question?

Upvotes: 4

Cory Petosky
Cory Petosky

Reputation: 12656

While value types go on the stack, there's still performance implications to allocating and deallocating all that memory every frame -- especially on the Xbox 360. On a PC you'll likely not notice the difference, but on the 360 you probably will.

Upvotes: 1

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