GetPokémonEvolutions.cs

using System;
using System.Collections.Generic;
using System.Threading.Tasks;
using PokeApiNet;

namespace GetPokémonData
{
    class Program
    {
        static async Task<(PokemonSpecies, PokemonSpecies)> GetEvolutionPair(PokeApiClient client, EvolutionChain chain)
        {
            var first = await client.GetResourceAsync<PokemonSpecies>(chain.Chain.Species.Name);
            var next = chain.Chain;
            while(next.EvolvesTo.Count != 0)
                next = next.EvolvesTo[0];
            var last = await client.GetResourceAsync<PokemonSpecies>(next.Species.Name);
            return (first, last);
        }
        static string GetJapaneseName(PokemonSpecies species)
        {
            foreach(var name in species.Names)
            {
                if(name.Language.Name.StartsWith("ja"))
                    return name.Name;
            }
            throw new ArgumentException(); // Didn't find anything, so let's crash!
                                           // We probably got enough Pokémon by now anyway.
        }
        static async Task Main(string[] args)
        {
            PokeApiClient client = new();
            List<EvolutionChain> chains = new();
            for(int i = 1; i <= 10000; i++)
            {
                try { chains.Add(await client.GetResourceAsync<EvolutionChain>(i)); }
                catch { break; } // We failed getting one, so let's just stop here.
                Console.WriteLine($"Got chain {i}.");
            }
            foreach(var chain in chains)
            {
                var (baby, evolved) = await GetEvolutionPair(client, chain);
                try { Console.WriteLine($"{GetJapaneseName(baby)},{GetJapaneseName(evolved)}"); }
                catch { break; } // No Japanese name? We're probably at the end of usable data then.
            }
        }
    }
}