OpenAPI to
C#.

// Client/PaymentsClient.cs
using System.Net.Http;
using System.Text.Json;
using System.Threading;
using Example.Payments.Models;
using System;
using System.Collections.Generic;
using System.Threading.Tasks;

namespace Example.Payments.Client;

public class PaymentsClient : BaseHttpClient
{
    public PaymentsClient(
        HttpClient httpClient,
        string baseUrl,
        JsonSerializerOptions? jsonOptions)
        : base(httpClient, baseUrl, jsonOptions) { }

    public async Task<Payment> GetPaymentAsync(
        string id,
        string? expand = null,
        CancellationToken cancellationToken = default)
    {
        var _queryParts = new List<string>();
        if (expand != null) _queryParts.Add($"{Uri.EscapeDataString("expand")}={Uri.EscapeDataString((expand?.ToString()) ?? "")}");
        var _qs = _queryParts.Count > 0 ? "?" + string.Join("&", _queryParts) : "";
        return await ExecuteAsync<Payment>(
            HttpMethod.Get,
            $"/payments/{id}{_qs}",
            cancellationToken);
    }

    public async Task<Payment> CreatePaymentAsync(
        CreatePaymentRequest requestBody,
        CancellationToken cancellationToken = default)
    {
        return await ExecuteAsync<CreatePaymentRequest, Payment>(
            HttpMethod.Post,
            $"/payments",
            requestBody,
            cancellationToken);
    }
}

// Client/BaseHttpClient.cs (shared, condensed)
public abstract class BaseHttpClient
{
    protected readonly HttpClient _httpClient;
    protected readonly string _baseUrl;
    protected readonly JsonSerializerOptions? _jsonOptions;

    public BaseHttpClient(
        HttpClient httpClient,
        string baseUrl,
        JsonSerializerOptions? jsonOptions = null)
    {
        _httpClient = httpClient;
        _baseUrl = baseUrl.TrimEnd('/');
        _jsonOptions = jsonOptions;
    }

    protected async Task<TResponse> ExecuteAsync<TResponse>(
        HttpMethod method, string path, CancellationToken cancellationToken)
    {
        var requestUrl = $"{_baseUrl}{path}";
        using var request = new HttpRequestMessage(method, requestUrl);
        using var response = await _httpClient
            .SendAsync(request, cancellationToken)
            .ConfigureAwait(false);
        response.EnsureSuccessStatusCode();
        var content = await response.Content
            .ReadAsStringAsync()
            .ConfigureAwait(false);
        return JsonSerializer.Deserialize<TResponse>(content, _jsonOptions)!;
    }

    // Sibling overloads handle request body / no response /
    // request body + no response. All use the same SendAsync +
    // EnsureSuccessStatusCode + Deserialize pipeline.
}