microsoft/FASTER · error · Exception

Cannot create new SpanByteAndMemory using serialized…

Error message

Cannot create new SpanByteAndMemory using serialized SpanByte

What it means

Exception thrown by the SpanByteAndMemory(SpanByte) constructor when the given SpanByte is marked Serialized. A serialized SpanByte wraps externally-owned storage (its payload lives behind a length-prefixed pointer), so it cannot be wrapped into a SpanByteAndMemory whose memory-state transitions assume an in-memory, unserialized payload.

Solutions

Materialize the serialized SpanByte into a copy (e.g. SpanByteAndMemory.FromMemorySpan or copy payload to a new buffer) before wrapping
Use the pointer/length constructor with deserialized bytes instead of the SpanByte overload
Check spanByte.Serialized before constructing and handle the serialized case separately

Example fix

// before
var sb = input.Serialized ? input : input; // input is a serialized record SpanByte
var output = new SpanByteAndMemory(sb); // throws
// after
var mem = MemoryPool<byte>.Rent(input.Length);
input.AsSpan().CopyTo(mem.Span);
var output = SpanByteAndMemory.FromMemorySpan(mem); // or use deserialized SpanByte

Defensive patterns

Strategy: validation

Validate before calling

SpanByteAndMemory MakeOutput(SpanByte sb) => sb.Serialized ? CopyToNewMemory(sb) : new SpanByteAndMemory(sb);

Type guard

bool IsWrappable(SpanByte sb) => !sb.Serialized;

Try / catch

try { return new SpanByteAndMemory(spanByte); }
catch (Exception ex) when (ex.Message.Contains("serialized SpanByte"))
{ return SpanByteAndMemory.FromMemorySpan(CopyPayload(spanByte)); }

Prevention

Never wrap SpanBytes read directly from serialized log records
Copy payloads out of records before constructing outputs
Assert !spanByte.Serialized in debug builds before construction

When it happens

Trigger: Passing a SpanByte obtained from a serialized record/log (spanByte.Serialized == true) into the SpanByteAndMemory(SpanByte) constructor — e.g. reading a value from the log and directly wrapping it for an output collector.

Common situations: Read-modify-write sessions reusing a record's SpanByte as an output; copying values out of tail/log records into IVariableLengthStruct output buffers; upgrading FASTER versions where serialization flags differ.

Understand the failure class

Background: "Invalid state transition" errors: "status must be X, actually Y", "already rejected/charging/uninstalled", "cannot ... while running" — what they mean when a library rejects your call — this error's family across 31 libraries.

Related errors

AI-assisted analysis of microsoft/FASTER@321d872eab (2026-09-15). Data as JSON: /api/errors/ef94e4cd355f77a9. Report an issue: GitHub.

Appendix: source

Thrown at cs/src/core/VarLen/SpanByteAndMemory.cs:30

    public unsafe struct SpanByteAndMemory : IHeapConvertible
    {
        /// <summary>
        /// Stack output as SpanByte
        /// </summary>
        public SpanByte SpanByte;

        /// <summary>
        /// Heap output as IMemoryOwner
        /// </summary>
        public IMemoryOwner<byte> Memory;

        /// <summary>
        /// Constructor using given SpanByte
        /// </summary>
        /// <param name="spanByte"></param>
        public SpanByteAndMemory(SpanByte spanByte)
        {
            if (spanByte.Serialized) throw new Exception("Cannot create new SpanByteAndMemory using serialized SpanByte");
            SpanByte = spanByte;
            Memory = default;
        }

        /// <summary>
        /// Constructor using SpanByte at given (fixed) pointer, of given length
        /// </summary>
        public SpanByteAndMemory(void* pointer, int length)
        {
            SpanByte = new SpanByte(length, (IntPtr)pointer);
            Memory = default;
        }

        /// <summary>
        /// Get length
        /// </summary>
        public int Length
        {

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