Program.cs

using System.Net.Http.Json;

public class CurrencyRepo : RepoBase, ICurrencyRepo
{
    private readonly HttpClient _http;
    private readonly string _baseUrl;

    public CurrencyRepo(
        HttpClient http,
        string baseUrl)
    {
        _http = http;
        _baseUrl = baseUrl;
    }

    public override async Task<HashSet<Currency>> GetCurrenciesAsync()
    {
        var currencies = await _http.GetFromJsonAsync<List<Currency>>(
            $"{_baseUrl}/currencies");

        return new HashSet<Currency>(currencies!);
    }
}

public record Currency();

public interface ICurrencyRepo
{
    Task<HashSet<Currency>> GetCurrenciesAsync();
}

public class RepoBase : ICurrencyRepo
{
    public virtual Task<HashSet<Currency>> GetCurrenciesAsync()
    {
        throw new NotImplementedException();
    }
}