C# structs and the heap question

[faustino Dina]

Hi,

I have a question about how C# handles struct. From the help, struct
variables are stored in the stack while class objects are stored in the
heap. Then what happens when I add a struct value to a colection?

struct MyStruct
{
public int X;
}

MyStruct s;

ArrayList list = new ArrayList();
list.Add(s);

In C/C++ store stack addresses on a global collection can lead to hard to
trace problems. But how C# works? In this case what is stored on the list
collection: struct, a reference to the stack or s is cloned into the heap
and that reference is added to the collection?

Thanks in advance

 

[Jon Skeet [C# MVP]]

I have a question about how C# handles struct. From the help, struct
variables are stored in the stack while class objects are stored in the
heap.

Unfortunately that's a rather simplistic approach.

See http://www.pobox.com/~skeet/csharp/memory.html

(For instance, a value type which is part of a reference type will
always be on the heap.)

Then what happens when I add a struct value to a colection?

struct MyStruct
{
public int X;
}

MyStruct s;

ArrayList list = new ArrayList();
list.Add(s);

This is actually a third case - here you're *boxing* the struct value.
This basically creates a strongly-typed object on the heap, containing
your value. To get it back as a value type, you need to unbox the
reference, which you do using casting syntax:

s = (MyStruct) list[0];

In C/C++ store stack addresses on a global collection can lead to hard to
trace problems. But how C# works? In this case what is stored on the list
collection: struct, a reference to the stack or s is cloned into the heap
and that reference is added to the collection?

Essentially the latter.

 

[Mickey Williams [C# MVP]]

Instances of structs are not always stored on the stack -- rather struct
variables always directly contain their value. In your example, the struct
instances are boxed and allocated on the heap.
See http://www.yoda.arachsys.com/csharp/memory.html