cats.cs

    public class Demo
    {
        public static void DemoIt()
        {
            var cat = new GenericBuilder<TypeFalse, TypeFalse>()
                  .SetAge(11)
                  .SetName("Totoro")
                  .Build();
        }
    }

    public interface ITypeBool { }

    public class TypeTrue : ITypeBool { }
    public class TypeFalse : ITypeBool { }

    public class Cat
    {
        public readonly string Name;
        public readonly int Age;

        public Cat(string name, int age)
        {
            Name = name;
            Age = age;
        }
    }

    public class GenericBuilder<TName, TAge>
    {
        public string Name { get; private set; }
        public int Age { get; private set; }

        public GenericBuilder()
        {
        }

        public GenericBuilder<TypeTrue, TAge> SetName(string name)
        {
            return new GenericBuilder<TypeTrue, TAge> { Name = name, Age = Age };
        }

        public GenericBuilder<TName, TypeTrue> SetAge(int age)
        {
            return new GenericBuilder<TName, TypeTrue> { Name = Name, Age = age };
        }
    }

    public static class Ext
    {
        public static Cat Build<TA, TB>(this GenericBuilder<TA, TB> builder)
            where TA : TypeTrue
            where TB : TypeTrue
        {
            return new Cat(builder.Name, builder.Age);
        }
    }