[DllImportGenerator] Initialize by-value out array to default #61431
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jkoritzinsky
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Nov 10, 2021
AaronRobinsonMSFT
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Nov 10, 2021
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AaronRobinsonMSFT
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Do we want to mentioned the discovered behavior around implicit out with the built-in in the compat doc?
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| In the source-generated marshalling, arrays will be allocated on the stack (instead of through [`AllocCoTaskMem`](https://docs.microsoft.com/dotnet/api/system.runtime.interopservices.marshal.alloccotaskmem)) if they are passed by value or by read-only reference if they contain at most 256 bytes of data. The built-in system does not support this optimization for arrays. | ||
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| In the built-in system, marshalling a `char` array by value with `CharSet.Unicode` would default to also marshalling data out. In the source-generated marshalling, the `char` array must be marked with the `[Out]` attribute for data to be marshalled out by value. |
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This is due to the fact that a char array with CharSet.Unicode used to be pinnacle as char is blittable with CharSet.Unicode
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Yeah, I figured that was it once I saw the behaviour, but it was not something I realized immediately 😕
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This was referenced Nov 29, 2021
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When marshalling an array out by value, we were leaving the native storage uninitialized. This clears it out, so that we don't end up with whatever garbage was there in the array (matches built-in system behaviour).
This is the reason behind the failures in #61389.
@AaronRobinsonMSFT @jkoritzinsky