Confusion about StringBuilder memory allocation

[samibinsami]

Type of issue

Other (describe below)

Description

Referring to the article: https://learn.microsoft.com/en-us/dotnet/fundamentals/runtime-libraries/system-text-stringbuilder

In first section it says, A StringBuilder object maintains a buffer to accommodate expansions to the string. New data is appended to the buffer if room is available; otherwise, a new, larger buffer is allocated, data from the original buffer is copied to the new buffer, and the new data is then appended to the new buffer.

From above explanation, I can say that StringBuilder object's data resides in a single block of memory(or in other words called buffer because old and new data resides in a new single buffer).

But later on in the article, in section(Iterate StringBuilder characters), it says that, Performance is severely impacted because each character access walks the entire linked list of chunks to find the correct buffer to index into.

Now this text says that, StringBuilder is chunky, meaning the objects data resides in scattered chunked in memory connected via references, hence called linked list of chunks.

So I see there are 2 different explanation of how StringBuilder memory is declared.

Page URL

https://learn.microsoft.com/en-us/dotnet/fundamentals/runtime-libraries/system-text-stringbuilder#search-the-text-in-a-stringbuilder-object

Content source URL

https://github.com/dotnet/docs/blob/main/docs/fundamentals/runtime-libraries/system-text-stringbuilder.md

Document Version Independent Id

7d655701-437d-26bf-18d5-cb2d56e3754c

Article author

@gewarren

Metadata

• ID: cb0f5618-bcb2-07fa-c480-14d6fee0c3fb
• Service: dotnet-fundamentals

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Associated WorkItem - 308214

[gewarren]

Thanks @samibinsami, I agree that is confusing. I think the "chunked" description is more accurate based on https://github.com/dotnet/runtime/blob/9a31a5b47aa7484bec71ba28aa59712c7898b902/src/libraries/System.Private.CoreLib/src/System/Text/StringBuilder.cs#L26-L28. @stephentoub Can you confirm, and I will update this doc?

[stephentoub]

Yes, it's (mostly) an implementation detail, but StringBuilder is implemented today as a linked list of buffers. New buffers are added rather than existing ones being replaced.

[samibinsami]

Thanks @stephentoub for resolving my confusion.

I assume the next step would be to update the document?

[samibinsami]

@stephentoub any updates to my previous comment would be helpful.

[gewarren]

any updates to my previous comment would be helpful.

Yes, the doc will be updated.