A new BigRational class - in short:

BigRat.cs (single file solution, just copy and try)

• No stack-machine, no nested classes.
• No static members, no shared buffers, no cache.
• No bad tricks, no hacks.
• Conforms to the .NET Framework Design Guidelines.
• Performance 30-50% less compared to stack-machine (only - currently).
• In assembly size ~28 KB.
• Tested for .NET 6, 7, 8 alpha, (ARM without INumber interfaces)

New approach:

• Using processor stack stackalloc instead of a stack-machine.
  (This due to the observation that the performance breaks down to an unusable level long before stackalloc becomes critical.)
• Normalization only on user request.
  (This due to the observation that normalization often reduces the size less but cost performance)
• Exceptions instead of NaN or Infinity. (Makes not much sense to emulate floating-point behavior)

This as a first reference version. Major improvements/optimizations are possible. Of course, the stackallocs will be secured.