我可以得到一个跨度 <byte> 引用非字节类型的结构成员字段？

Question

Say I have a struct like the following:

struct MyStruct {
  Guid g;
}

Is it possible to get a Span<byte> that references the bytes of the struct?

struct MyStruct {
  Guid g;

  public void Foo() {
    Span<byte> bytes = ???
  }
}

Such that bytes would be a Span<byte> of length 16 that would allow reading and writing the individual bytes of the Guid field directly.

I can do something similar with unsafe code, but it seems that this should now be possible with safe code via span, but I can't figure out how to produce the span.

Edit: Clarify that I want a Span that points to the actual storage location of the Guid. Meaning new Span<byte>(g.ToByteArray)) is not what I'm looking for. That will allocate a new array, copy the bytes to the array, and create a Span referencing the newly allocated array. Modifying bytes via such a span will not modify the Guid.

Answer 1

You can force the Guid to an array

struct MyStruct {
   public Guid g;
}

...

var s = new MyStruct();
var span = new Span<byte>(s.g.ToByteArray());
span[2] = 4;

Note : The above will not modify the original struct. However, as you pointed out, you can do this with unsafe

var s = new MyStruct();
var span = new Span<byte>(&s , Marshal.SizeOf(s));

// woah it just become mutable 
span[2] = 4;

Answer 2

Try this in .Net Core 2.1 or above:

            Span<MyStruct> valSpan = MemoryMarshal.CreateSpan(ref mystruct, 1);
            Span<byte> span = MemoryMarshal.AsBytes(valSpan);

But use with caution, you should not contain any pointer or reference type in your struct, since GC can move reference type. Actually, it will do runtime check and throw an exception if you do that.

Answer 3

Look at the constructors of Span class, the only ways to create a Span is providing a managed array or a pointer.

But Guid is made up with 1 int, 2 shorts and 8 bytes, not a byte array.

So it's impossible (at least not now).