Re-Evaluate Value Type Options

[jamescourtney]

This is an increasingly common request, so this tracks re-evaluation of exposing structs as value types.

• Non Virtual
• No inheritance
• Can value-structs contain class-structs?
• Can class-structs contain value-structs?
• How do we deserialize value-structs? MemoryMarshal.Cast? Field-by-field?
• How do we expose fixed-length vectors in value structs? Similar to reference structs?

What won't work?

• Write through (Perhaps compiler can generate "Mutate" methods?)
• Non-Greedy deserialization

Is this better or worse than the object pooling approach?

[jamescourtney]

Alternatively, this could be a FlatSharp v6 feature and all FlatBuffer structs could be exposed as value types.

[jamescourtney]

This design is a little complicated. The canonical FlatBuffer library uses value structs for all operations, and that is OK because all objects are FlatBuffers, and hence have a backing buffer. Flatsharp takes the view that some objects may be backed by buffers, but others may not be. This is abstracted away through inheritance.

Value-type structs (value structs) prove to be an awkward addition to FlatSharp, because C# Structs are not inheritable (using an interface would induce boxing and defeat the point), so FlatSharp's normal codegen approaches fall short here.

However, structs have the distinguishing feature of being GC-friendly, and so there is some value in adding them. However, it's worth enumerating the constraints first.

Tentative design:

• FlatSharp will support value and reference type structs. Either is fine. Reference remains the default.
• Value structs will always be deserialized greedily.
• Value structs will have the same memory layout as indicated by the FBS schema. This implies a few things:
  • No extra metadata such as points to buffers.
  • Value structs may not contain reference structs.
  • Value structs will not implement IFlatBufferDeserializedObject or IFlatBufferAddressableStruct, as these require extra information.
• Reference structs may contain value structs.
• Value structs will be identified in metadata as fs_valueStruct.
• Value structs can be declared in C# with attributes, but will require Explicit struct layout:

        [FlatBufferStruct, StructLayout(LayoutKind.Explicit, Size = 16)]
        public struct SimpleValueStruct
        {
            [FieldOffset(0)]
            public long Long;

            [FieldOffset(8)]
            public byte Byte;

            [FieldOffset(12)]
            public uint Uint;
        }

• Value structs support fixed-size vectors via a similar mechanism to reference structs (with custom "vector" object).