It's in the
System namespace, so provided that you're using "implicit usings" then you shouldn't need to include anything.
Are you using .NET 7/8? Currently it looks like the is only supported in those versions:
I have found System.Numerics.BigInteger and that works great in my C# and VB.net code;
But i see no reference or option to include the UInt128 structure and class; is it in a reference type that I need to include? It is not listed as an option under System.Numerics