Stack vs. Heap in .NET [closed]

In your actual programming experience, how did this knowledge of STACK and HEAP actually rescue you in real life? Any story from the trenches? Or is this concept good for filling up programming books and good for theory?

13.10.2009 16:15:22
This is more of a discussion topic than a question with a real answer; consider switching this to a wiki.
Ron Warholic 13.10.2009 16:20:15
It does seem more of a discussion given the answers posted so far. This question can be answered either "Concept not useful", or "Concept useful and here's an example of how". "Concept useful" with no specific example doesn't actually answer the question.
sylvanaar 13.10.2009 17:05:15
see this post talks about the stack and heap for .net…
DRobertE 9.10.2012 18:03:27

To me it is the difference between being a "developer/programmer" and a "craftsman". Anyone can learn to write code and see how things just "magically happen" for you not knowing why/how. To really be valuable at what you do, I think there is a great importance to find out as much as you can about the Framework you're using. Remember it's not just a language, it's a Framework that you leverage to create the best application to your abilities.

I've analyzed many memory dumps over the years and found it extremely helpful knowing the internals and differences between the two. Most of these have been OutOfMemory conditions and unstable applications. This knowledge is absolutely necessary to use WinDbg when looking at dumps. When investigating a memory dump, knowing how memory is allocated between the kernel/user-mode process and the CLR can at least tell you where to begin your analysis.

For example, let's take an OOM case: The allocated memory you see in the Heap Sizes, Working Set, Private Memory, Shared Memory, Virtual Memory, Committed Memory, Handles, and Threads can be a big indicator of where to start.

There about 8 different heaps that the CLR uses:

  1. Loader Heap: contains CLR structures and the type system
  2. High Frequency Heap: statics, MethodTables, FieldDescs, interface map
  3. Low Frequency Heap: EEClass, ClassLoader and lookup tables
  4. Stub Heap: stubs for CAS, COM wrappers, P/Invoke
  5. Large Object Heap: memory allocations that require more than 85k bytes
  6. GC Heap: user allocated heap memory private to the app
  7. JIT Code Heap: memory allocated by mscoreee (Execution Engine) and the JIT compiler for managed code
  8. Process/Base Heap: interop/unmanaged allocations, native memory, etc

Finding what heap has high allocations can tell me if I have memory fragmentation, managed memory leaks, interop/unmanaged leaks, etc.

Knowing that you have 1MB (on x86)/ 4MB (on x64) of stack space allocated for each thread that your app uses reminds me that if I have 100 threads you will have an additional 100MB of virtual memory usage.

I had a client that had Citrix servers crashing with OutOfMemory problems, being unstable, slow responsiveness when their app was running on it in multiple sessions. After looking at the dump (I didn't have access to the server), I saw that there were over 700 threads being used by that instance of the app! Knowing the thread stack allocation, allowed me to correlate the OOMs were caused by the high thread usage.

In short, because of what I do for my "role", it is invaluable knowledge to have. Of course even if you're not debugging memory dumps it never hurts either!

8.11.2016 21:08:04
I'd just like to add for completeness: While each thread does have a default stack of 1 MiB, it doesn't mean the memory is commited (it isn't, at least not on 64-bit Windows 7 with .NET framework 4). The defaults I've seen are more like 4 kiB commited by default. So it does take 1 MiB of virtual address space, but not 1 MiB of physical memory (be it a page file, other memory mapped file or actual RAM). On a 64-bit system, you can have thousands of stacks and still only take just a few megs of "real" memory. In fact, I've seen the managed heap allocate TiBs, while only committing a few megs.
Luaan 13.02.2014 17:15:53
Thank you note the need to clarify - I've edited my response to explicitly call out Virtual Memory instead of Committed.
Dave Black 17.02.2014 23:16:34

Personally, this is one of the very few technical questions that I ask every person I'm going to hire.

I feel that it is critical to understanding how to use the .NET framework (and most other languages). I never hire somebody who doesn't have a clear understanding of memory usage on the stack vs. the heap.

Without understanding this, it's almost impossible to understand the garbage collector, understand .NET performance characteristics, and many other critical development issues.

13.10.2009 16:19:20
I agree with you, but you really didn't provide a good example of what to know about the Stack and Heap. I am interested in learning something new :)
Stan R. 13.10.2009 16:22:00
I agree with leppie, the distinction between a reference and value type is very important, but whether they end up on the stack or heap... you haven't convinced me why it would matter so much.
Meta-Knight 13.10.2009 16:26:13
Well, I usually ask in general terms, and try to get the candidate to explain the difference to me. This has become one of my benchmarks on the level of understanding - I feel that somebody who knows how the memory allocation in .NET works will at least be willing and able to learn just about anything else required. I think you need to understand 1)The stack, in general terms, 2)The heap, in general terms, 3)How reference types work, 4)How value types work, 5)Argument passing using ref/out, and how it differs from by value, especially with reference types (not stack/heap, but semi-related)
Reed Copsey 13.10.2009 16:26:18
One dissenting voice is Eric Lippert of course, who thinks the distinction between reference and value types is much more than stack versus heap (which he describes as an implementation detail).…
MarkJ 13.10.2009 16:49:26
@kcbeard It really depends a lot on what you're doing day to day. I work in technical computing, so memory management and issues relating to it are a near daily concern. For typical LOB apps, you may never care. However, I still consider this important knowledge to understand overall, especially since it also plays a large role when dealing with interop code, etc. Granted, native code makes it more important to understand, as managed code hides a lot of this complexity, but eventually, the lines blur a bit...
Reed Copsey 19.01.2012 17:58:45

The distinction in .NET between the semantics of reference types and value types, is a much more important concept to grasp.

Personally, I have never bothered thinking about the stack or heap in all my years of coding (just CLR based).

13.10.2009 16:21:02
mmm - It's difficult to understand reference type vs value type semantics (especially the why behind them) without understanding the stack and heap.
Reed Copsey 13.10.2009 16:28:12
Maybe a better question would be: "Explain why value::reference != stack::heap". :)
Craig Stuntz 13.10.2009 16:50:17
@Reed, I disagree entirely. Using Heap vs. Stack to explain reference vs. value type semantics generally results in confusion and disinformation.
tnyfst 3.11.2010 20:45:32
@ReedCopsey Reference vs value type has nothing to do with stack and heap. While it's a notion that's repeated even by MSDN in some cases, it's simply false. For example, value types can be stored on the heap (pinned handles, boxing, ...), on the stack, in registers (I've heard so many people say "C# is slow because it's stack based" it's not even funny anymore), the large object heap even. In any case, it's an implementation detail, and if the .NET runtime decides, it can do whatever it wants with the values - that's its job. Stack / heap is about scope (now), not about data types.
Luaan 13.02.2014 17:20:53
Excellent articles on the matter discussed above (if you haven't read them already):… and…
Dave Black 6.05.2015 17:56:05

I don't think it matters if you're just building average business applications, which I think most .NET programmers are.

The books I've seen just mention stack and heap in passing as if memorizing this fact is something of monumental importance.

13.10.2009 16:35:35

It certainly is helpful to understand the distinction when one is building compilers.

Here are a few articles I've written about how various issues in memory management impact the design and implementation of the C# language and the CLR:

13.10.2009 16:44:58

The important distinction is between reference types and value types. It's not true that "value types go on the stack, reference types go on the heap". Jon Skeet has written about this and so has Eric Lippert.

13.10.2009 16:54:45

We had a Claim Entity (business Object) which contained data for an entire claim. One of the requirements of the application was to create an audit trail of every single value changed by the user. In order to this without hitting the database twice we would maintain Original Claim Entity in the form and a Working Claim Entity. The Working Claim Entity would get updated when the user clicked Save and we would then compare the Original Claim Entity properties with corresponding Working Claim Entity properties to determine what changed. One day we noticed hey our compare method is never finding a difference. This is where my understanding of the Stack and Heap saved my rear end (specifically value types vs reference types). Because we needed to maintain to copies of the same object in memory the developer simply created two objects

Dim originalClaim As ClaimBE
Dim workingClaim As ClaimBE

then called the business layer method to return the claim object and assigned the same claimBE to both variables

originalClaim = BLL.GetClaim()
workingClaim = originalClaim

hence two reference types pointing to the same value type. Nightmare averted.

3.11.2010 20:37:55
Your story has nothing to do stack vs. heap; it is only value vs. reference. As others have stated, value types have the distinction of being able to be stored on the stack, but that is where the relationship ends. Whether originalClaim and workingClaim were stored on the stack or heap is irrelevant to your story.
Gabe 7.11.2010 06:26:34
@Gabe Even funnier, reference types can also go on stack. It does not happen often (one example is the stackalloc keyword in unsafe context), but that might change in future versions of .NET if it's ever deemed worth the trouble - this would be a huge help when doing allocation heavy operations that only use the allocated objects inside a given scope - no need to put that on the heap for the GC to collect later, just use stack, push, pop, done, no GC needed.
Luaan 13.02.2014 17:26:31