UnmanagedMemoryWriter.cs

class UnmanagedMemoryWriter: Stream
{
    private static T NotImplemented<T>()
    {
        throw new NotImplementedException();
    }

    private UnmanagedMemoryWriter()
    {
    }

    [ThreadStatic]
    private static UnmanagedMemoryWriter threadLocalWriter;

    public static UnmanagedMemoryWriter FromPtr(IntPtr ptr)
    {
        if (threadLocalWriter == null)
        {
            threadLocalWriter = new UnmanagedMemoryWriter();
        }

        threadLocalWriter.ptr = ptr;
        threadLocalWriter.offset = 0;
        return threadLocalWriter;
    }

    private IntPtr ptr;
    private int offset = 0;

    protected override void Dispose(bool disposing)
    {
        this.ptr = IntPtr.Zero;
        this.offset = 0;
    }
    
    public override void Write(byte[] buffer, int offset, int count)
    {
        Marshal.Copy(buffer, offset, ptr + this.offset, count);
        this.offset += count;
    }
    
    public override bool CanWrite => true;
    public override bool CanRead => false;

    public override bool CanSeek => NotImplemented<bool>();
    public override long Length => NotImplemented<long>();
    public override long Position
    {
        get { return NotImplemented<long>(); }
        set { NotImplemented<object>(); }
    }
    public override void Flush() => NotImplemented<object>();
    public override int Read(byte[] buffer, int offset, int count) => NotImplemented<int>();
    public override long Seek(long offset, SeekOrigin origin) => NotImplemented<long>();
    public override void SetLength(long value) => NotImplemented<object>();
}