Stack and heap in c sharp

Question

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Why are structs stored on the stack while classes get stored on the heap(.NET)?

Can anyone tell me that how the allocation of memory is done that which object is to be stored in stack and which to be in heap portion of the memory?

Answer 1

In the Microsoft implementation of the C# compiler and CLR, value types are stored on the stack when the value is a temporary value, local variable or formal parameter, that is neither a closed-over outer variable of an anonymous method nor in an iterator block.

Of course, why store stuff on the stack if you don't need to? Some local variables of value type never get on the stack at all; they stay in registers for their entire lifetimes.

Other values of value types are stored on the heap - boxed value types, value-typed fields on a reference type, and so on.

Value types can of course be stored on neither the stack, nor registers, nor the managed heap; they could be stored in unmanaged memory using some completely other memory manager not under control of the CLR.

(And of course note that using "the" in "the stack" is subtly misleading; there can be many stacks in a process. There need not be just one.)

All this is an implementation detail and subject to change without notice.

Also, obviously stuff allocated with the stack alloc declaration is allocated on the stack.

For more information on this topic see my articles on it:

http://blogs.msdn.com/b/ericlippert/archive/2009/04/27/the-stack-is-an-implementation-detail.aspx

http://blogs.msdn.com/b/ericlippert/archive/2009/05/04/the-stack-is-an-implementation-detail-part-two.aspx

Why do you care? The runtime manages all these details for you so that you don't have to worry about it. Are you just curious, or is this leading to some larger question?

Answer 2

3 rules of thumb:

1. Objects are stored on the heap. These include instances of reference-types and boxed value-types.
2. Local variables and parameters are stored on the stack. For local value-types, this means that the value itself is stored on the stack. For local reference-types, only the reference will be on the stack (Edit: Exceptions noted by Eric Lippert - value-type locals closed over outer variables, iterator-block value-types).
3. Fields are stored where the containing instance is located. For example, a value-type field of a class will be stored on the heap. The reference-part of a reference-type field of a struct declared as a local stored on the stack will also be on the stack.