public interface IBackendService {

    // when set, it will always be sent with
    // every request
    [Header("access-key")]
    AccessKey { get; set; }

    [Get("/location/{ip}")]
    Task<IPInfo> GetLocationInfoAsync([Path("ip")] string ip);

    [Post("/location/{ip}")]
    Task<IPInfo> SaveLocationInfoAsync([Path("ip")] string ip, [Body] IPInfo info);
        
    [Get("/voice/{id}.mp3")]
    Task<byte[]> GetByteArrayAsync([Query("id")] string id);
    
    // Response Object with Header
    [Get("/projects")]
    Task<GitLabResponse<GitLabProject>> GetProjectsAsync();


    // Retrieve http response for detailed response.
    // HttpResponseMessage is not disposed, it is responsibility of caller
    // to dispose the message (which will close open network streams)
    // This will not throw an error message if there was HTTP Error.
    [Get("/video/{id}.mp4")]
    Task<HttpResponseMessage> GetRawResponseAsync([Query("id")] string id);

    // Multi Part Form for uploads...
    [Post("/upload")]
    Task<HttpResponseMessage> UploadFile(
        // other form element items
        [Multipart("name")] string attachmentName,

        // it can accept stream
        [MultipartFile("file1")]  Stream fileStream,

        // it can accept HttpContent which may contain content type
        [MultipartFile("file2")]  HttpContent someOtherContent
        );

}

public class GitLabResponse<T>: ApiResponse<T[]> {

   // set by RetroClient when response is received
   [Header("x-total-pages")]
   public int TotalPages {get;set;}

}

    var client = RetroClient.Create<IBackendService, BaseService>( new Uri("base url...") , httpClient);

    // get request
    RequestBuilder.Get("/location/{ip}")
        .Path("ip", ipAddress)
        .GetResponseAsync<IPInfo>(httpClient);

    // post request
    RequestBuilder.Post("/location/{ip}")
        .Path("ip", ipAddress)
        .Body(info)
        .GetResponseAsync<IPInfo>(httpClient);

    // get raw response
    RequestBuilder.Post("/location/{ip}")
        .Path("ip", ipAddress)
        .Body(info)
        .GetResponseAsync(httpClient); // no generics

    // Multipart Upload
    RequestBuilder.Post("/upload")
        .Multipart("name", attachmentName)
        .MultipartFile("file1", fileStream, fileName: "photo.jpg")
        .GetResponseAsync(httpClient);

    // Get Api Response with Headers...
    RequestBuilder.Post("/api/v4/projects/{id}")
        .Path("id", projectId)
        .GetResponseAsync<GitLabResponse<Project>>(httpClient);

public class GitLabResponse<T>: ApiResponse<T[]> {

    // set by RetroClient when response is received
    [Header("x-total-pages")]
    public int TotalPages {get;set;}

}