Eliza really hits a sweet spot of 'often quite good conversation' from very simple code. There's a C# version here: https://github.com/sirkris/ELIZA.NET

Main class that does processing is interesting to look at:

https://github.com/sirkris/ELIZA.NET/blob/master/src/ELIZA.N...

Main script file data structure:

https://raw.githubusercontent.com/sirkris/ELIZA.NET/master/s...

There's a few other nuts and bolts but thats basically it