Span<T> reinterpret cast

[PhilPJL]

I notice that Stephen Toub says that Span<T> supports reinterpret cast in this article.

Spans provide a multitude of benefits beyond those already mentioned. For example, spans support the notion of reinterpret casts, meaning you can cast a Span<byte> to be a Span<int> (where the 0th index into the Span<int> maps to the first four bytes of the Span<byte>). That way if you read a buffer of bytes, you can pass it off to methods that operate on grouped bytes as ints safely and efficiently.

Is that true and if so can I have an example? The compiler objects if I try something simple like:

Span<byte> s = stackalloc byte[20];
Span<int> t = (Span<int>)s;

[PhilPJL]

Ok found it. Span<int> t = MemoryMarshal.Cast<byte, int>(s);

[jkotas]

Note that MemoryMarshal.Cast is not supported in all situations. From https://learn.microsoft.com/en-us/dotnet/api/system.runtime.interopservices.memorymarshal.cast: This method is supported only on platforms that support misaligned memory access or when the memory block is aligned by other means.

For example, if you use MemoryMarshal.Cast<byte, double> and the buffer is not 8-byte aligned, you are going to see DataMisalignedException on Arm processors.

[tannergooding]

It might be worth us exposing some Unsafe.SupportsMisalignedData property or similar so that devs can better write "fast" code for the common case.

[PhilPJL]

Thanks for pointing that out. Luckily I'm on x64 only.

[peterdettman]

If MemoryMarshal.Cast is used with target type TTo == typeof(byte), is that guaranteed to meet the clause "the memory block is aligned by other means"?

[stephentoub]

Yes