[Analyzer Proposal]: Recommend using JsonElement.Parse

[stephentoub]

#108930 represents an approved API for .NET 10 that adds JsonElement.Parse methods. Before its existence, developers would write things like:

JsonElement element = JsonDocument.Parse("json").RootElement;

which ends up renting an array from the ArrayPool with the JsonDocument but then never returning it because the JsonDocument isn't disposed.

We should add an analyzer/fixer that flags common patterns folks have used for getting a JsonElement, as called out in #108930, and recommend switching to use the new JsonElement.Parse methods.

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[bartonjs]

Video

Looks good as proposed

Category: Performance
Severity: Info

[gregsdennis]

which ends up renting an array from the ArrayPool with the JsonDocument

Where will the memory live if there's not a JsonDocument?

For static strings like the example you've shown, the memory is basically guaranteed to exist for the lifetime of the application.

However, if I create a string dynamically and pass that to JsonElement.Parse(), what happens when the string is deallocated? Or does JsonElement hold a reference to the string (I don't think it does currently, which is what JsonDocument is for)?

Are we going to also support JsonElement.Parse(ReadOnlySpan<char>)?

[stephentoub]

Where will the memory live if there's not a JsonDocument?

Still in an array on the heap... but not allocated from the ArrayPool.