Use 'Write' instead of 'WriteInt32' for union type keys #2212
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Union type keys set with are usually small positive integers, likely to fit in a single byte. As late as version 2.5.198, the library would encode [Union(0, typeof(Foo))] as two bytes 0x92 0x00 (one for "two-element array", and one for the 0 integer key), followed by the serialization of type Foo. In version 3.1.4, [Union(0, typeof(Foo))] is instead encoded as six bytes 0x92 0xD2 0x00 0x00 0x00 0x00 (one for "two-element array", one for "32-bit integer", and the four bytes of the 0 integer key). Although the two representations are compatible, for code bases that use many small polymorphic objects, the migration from 2.x to 3.x would lead to a significant increase in the size of the serialized data. This behavior is caused by the generated code for the interface using `WriteInt32` (which always encodes the integer as five bytes), and is fixed by instead using `Write` (which uses the smallest possible number of bytes for its argument).
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@dotnet-policy-service agree company="Lokad"
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AArnott
approved these changes
Jul 18, 2025
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Union type keys are usually small positive integers, likely to fit in a single byte. As late as version 2.5.198, the library would encode [Union(0, typeof(Foo))] as two bytes 0x92 0x00 (one for "two-element array", and one for the 0 integer key), followed by the serialization of type Foo.
In version 3.1.4, [Union(0, typeof(Foo))] is instead encoded as six bytes 0x92 0xD2 0x00 0x00 0x00 0x00 (one for "two-element array", one for "32-bit integer", and the four bytes of the 0 integer key). Although the two representations are compatible, for code bases that use many small polymorphic objects, the migration from 2.x to 3.x would lead to a significant increase in the size of the serialized data.
This behavior is caused by the generated code for the interface using
WriteInt32(which always encodes the integer as five bytes), and is fixed by instead usingWrite(which uses the smallest possible number of bytes for its argument).