HttpClient 是微软在.NET Framework 4.5 引入的一个HTTP客户端库，命名空间为 System.Net.Http，目的是简化开发者对Http的调用。

下面演示使用HttpClient 实现多文件同时上传

  class Program
    {
        private const string Url = "https://localhost:44312/api/Upload";
        private static List<string> FilePath = new List<string>
        {
            @"C:\Users\xxx\Desktop\test\large (10) - 副本 - 副本.jpg",
            @"C:\Users\xxx\Desktop\test\large (11) - 副本 - 副本.jpg"
        };
        static async Task Main(string[] args)
        {
            Console.WriteLine("开始上传?");
            Console.ReadKey();
            HttpClientHandler handler = new HttpClientHandler();
            ProgressMessageHandler progressMessageHandler = new ProgressMessageHandler(handler);
            progressMessageHandler.HttpSendProgress += ProgressMessageHandler_HttpSendProgress;
            HttpClient httpClient = new HttpClient(progressMessageHandler);
            using var multipartFormData = new MultipartFormDataContent();
            foreach (var file in FilePath)
            {
                multipartFormData.Add(new ByteArrayContent(await File.ReadAllBytesAsync(file)), "files", file);
            }
            var response = await httpClient.PostAsync(Url, multipartFormData);
            if (response.IsSuccessStatusCode)
            {
                var data = await response.Content.ReadAsStringAsync();
                Console.WriteLine(data);
            }
        }

        private static void ProgressMessageHandler_HttpSendProgress(object sender, HttpProgressEventArgs e)
        {
            Console.WriteLine(e.ProgressPercentage+"%");
        }
    }

ProgressMessageHandler 是为了向调用者报告上传或者下载的进度。
注：要上传多文件multipartFormData添加的HttpContent的Name要为files，ASP.NET Core API 后端才能正常解析出IFormFIle。