C# Pinvoke IntPtr in structs and memory

Question

If i have understood right, when using structs with non blitable values, the struct data from unmanaged memory is copied into the managed memory (basically having the same struct twice).

Also if im not wrong, IntPtr variables, are stored in managed memory, but the data they point to is in the unmanaged memory.

Now lets say i have a delegate method, which is being called by a c++ function and receives a struct as ref, and a handler would be something like:

private void _handler(ref MyStruct p){}

Now the api says I should not keep the reference to the struct as the memory might be recycled and used for further calls, but since the unmanaged struct is copied into managed memory, and after I assign it to a local variable it is copied again (because it is a struct!) i shouldnt have any issues at all if the unmanaged memory gets freed or re written. However, if the struct contains an IntPtr variable, i might save a copy of the pointer but not of the data it points, so if i try to access the IntPtr data from my saved struct, i might get some memory fault, is this correct?

One last question is, doing changes to the struct in managed memory, will also affect the unmanaged memory struct, since it is passed by ref, and it implicitly means IN/OUT, so after the event handler call ends, changes in managed memory will be made in the unmanaged memory as well?

Answer 1

Your understanding is largely correct. Taking away the details of blittabiltiy, the root question is one of ownership of the memory being passed to your callback. Typically, the contract between the caller of the callback and the callback itself is that the memory is owned by the caller and will be valid only for the duration of the callback itself. (Other contracts are possible as well - for instance, the caller could transfer ownership of the memory to the callback, in which case the callback is responsible for freeing the memory when it is done).

In the documentation you reference, the API is telling you not to keep a reference to the memory, which means that you are very likely in the standard case where the caller owns the structure. What this means is that both the contents of the structure and everything pointed to by that structure should be valid during the call to your callback but cannot be relied upon after your callback completes.

Copying a structure to a local wouldn't be problematic because the native memory should not be freed until after your callback completes, so keeping as many copies as you need during the execution of the callback should be fine. What would be problematic is saving a copy of the structure to a location, such as a static field, which will be alive after your callback returns.

Since the structure is implicitly in/out, then modifications you make to it will be reflected on the native side of the call as well. This is true regardless of if the structure contained only blittable data. If the structure contained non-blittable data then the CLR would marshal your changes to the structure back out to the native copy when your code completes.

Note that just because you can modify the native structure does not mean that the API contract expects you to do so. Frequently API signatures contain pointers to structures in C++ in order to avoid copying the entire structure when making a call, not to enable modification of the structure.