If you have a fixture whose methods take or return a class that is not one of the standard Slim types, you can add a simple custom type converter very easily, using the generic converter fitSharp.Machine.Engine.Converter.

using fitSharp.Machine.Engine;

public class MyCustomConverter: Converter<CustomClass> {

    protected override CustomClass Parse(string input) {
       return new CustomClass ... created from input string ...
    }

    protected override string Compose(CustomClass input) {
        return ... string representation of custom class ...
    }
}

Then include your CustomConverter in the Suite Configuration File :

<?xml version="1.0" encoding="utf-8" ?>
<config>
  <ApplicationUnderTest>
    <addAssembly>mybuild\myassembly.dll</addAssembly>
    <addNamespace>My.Namespace</addNamespace>
  </ApplicationUnderTest>
  
  <fitSharp.Slim.Service.Service>
    <addOperator>MyCustomConverter</addOperator>
  </fitSharp.Slim.Service.Service>
</config>