The document discusses the challenges and implementation of string handling in C#, focusing on the inefficiencies of managed strings and the benefits of using custom string implementations with unmanaged memory. It highlights performance issues related to garbage collection, memory allocations, and scenarios where custom strings can be advantageous, such as serialization and large-scale text processing. Additionally, it provides implementation details and optimization techniques for managing string lifetimes and memory issues.

1. Strings, C# and Unamanaged
Memory
Hibernating Rhinos
Michael Yarichuk

2. https://www.imgflip.net

3. Strings in C#
• Immutable and reference type
• Allocated on the managed heap
New allocation
New allocation
New allocation
One line of code  three new allocations

4. (Invisible) cost of Garbage Collector
Massive
usage of
strings
Lots of
allocations
Lots of GC
cycles

5. https://www.imgflip.net

6. Custom strings
implementation
http://www.memegen.com/

7. First, a little example…
• Import into RavenDB
• Total size of data ~4GB
• 4.5 millions of documents

8. Memory allocations
Custom Strings .Net Strings

9. Time spent in GC
Custom Strings .Net Strings

10. Time spent in GC
Custom Strings .Net Strings

11. So, when are such strings can be useful?
• Serialization
• Large scale text processing
• Мemory mapped files + json/xml
• Custom network protocols

12. The idea
Unmanaged
memory
byte* Ptr
Int Size = 20kb
String 1
20kb
String 2
46kb
byte* Ptr
Int Size = 46kb

13. The definition
Pointer arithmetics!

14. Why byte and not char?
EFACABD6B7D79CEFAD8BD79D20D7A2EFAD8BD79CD6B7D79D
UTF8 Encoding
48656C6C6F20776F726C64
Length != Size
‫ם‬ ַ‫ל‬ ‫וֹ‬ ‫ע‬ ‫ם‬ ‫וֹ‬ ‫ל‬ַ‫שׂ‬
H e l l o w o r l d

15. Implementation detail example

16. Implementation detail example

17. More differences…
VS
P/Invoke to memcmp

18. System.String interop
Encoding Type

19. Optimizations…
• Centralized Factory for managing lifetime of strings
• Can be used with Flyweight, Object Pool and caching

20. Some useful techniques for
implementation

21. Memory issues
• Buffer overlaps
• Using memory block after release
• Leaks

22. Buffer overlaps – Electric Fence
NOACCESS NOACCESS
READWRITE

23. Memory Leaks – Reference Counting
Reference Count = 1 Reference Count = 2
UString UString

24. Memory Leaks/Usage after release – Finalizer
Using unmanaged memory doesn’t mean we can’t use
.Net mechanisms!

25. There is no silver bullet…
https://openclipart.org/

26. Questions?
michael.yarichuk@hibernatingrhinos.com
https://openclipart.org/ 26
https://github.com/ravendb/ravendb/tree/v4.0/src/Sparrow