要在ASP.Net Core Web API中上传文件，需要在Controller中提供上传方法，一般为Post类型。这时候客户端一般使用MultipartFormDataContent作为上传内容，可以设置token以及一些和文件相关的参数。在ControllerBase中，可以通过HttpContext.Request.Form.TryGetValue方法获取参数内容，在HttpContext.Request.Form.Files中获取上传的文件列表。

1、Controller代码

using System;
using System.Collections.Generic;
using System.IO;
using System.Text;
using System.Threading.Tasks;
using CommonUploadService.Models;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Extensions.Logging;

namespace CommonUploadService.Controllers
{
    /// <summary>
    /// 文件上传控制器
    /// </summary>
    [ApiController]
    [Route("api/v1/[controller]/[action]")]
    [ApiExplorerSettings(GroupName = nameof(ApiVersion.V1))]
    public class FileUploadController : ControllerBase
    {
        private readonly ILogger<DefectUploadController> _logger;
        private readonly string _absolutePath;

        private static readonly string UploadFilePath = @"UploadFile";

        public FileUploadController(ILogger<DefectUploadController> logger,
            IWebHostEnvironment webHostEnvironment)
        {
            _logger = logger;

            var webRootPath = webHostEnvironment.WebRootPath;
            _absolutePath = Path.Combine(webRootPath, UploadFilePath);
            if (!Directory.Exists(_absolutePath))
            {
                Directory.CreateDirectory(_absolutePath);
            }
        }

        //http://localhost:5000/api/v1/FileUpload/Upload
        [HttpPost]
        public async Task<ActionResult<string>> Upload()
        {
            HttpContext.Request.Form.TryGetValue("GroupId", out var groupId);
            _logger.LogDebug($@"Receive file form group: {groupId}.");

            if (string.IsNullOrWhiteSpace(groupId))
            {
                _logger.LogError(@"GroupId is null or white space.");
                return @"Error: GroupId is null or white space.";
            }

            var groupPath = Path.Combine(_absolutePath, groupId);
            if (!Directory.Exists(groupPath))
            {
                try
                {
                    Directory.CreateDirectory(groupPath);
                }
                catch (Exception e)
                {
                    var message = $@"Create Group Directory failed: {e.Message}.";
                    _logger.LogError(message);
                    return $@"Error: {message}.";
                }
            }

            var fileNames = new List<string>();
            var files = HttpContext.Request.Form.Files;
            foreach (var file in files)
            {
                var fileName = file.FileName;
                _logger.LogDebug($@"Receive file: {fileName}.");

                try
                {
                    var filePath = Path.Combine(groupPath, fileName);
                    using (var stream = new FileStream(filePath, FileMode.Create))
                    {
                        await file.CopyToAsync(stream);
                    }

                    fileNames.Add(filePath);
                }
                catch (Exception e)
                {
                    _logger.LogError($@"Copy file [{fileName}] failed: {e.Message}.");
                }
            }

            var sb = new StringBuilder();
            foreach (var fileName in fileNames)
            {
                sb.Append($@"{fileName}, ");
            }

            var filesString = sb.ToString().TrimEnd().TrimEnd(',');
            _logger.LogInformation($@"收到文件：groupId = {groupId}, fileNames = {filesString}.");

            return fileNames.Count > 0 ? @"OK" : @"Error";
        }
    }
}

2、客户端(Winform/WPF)上传方法

public bool UploadFile(string groupId, List<string> filePathList)
{
    using (var httpClient = new HttpClient())
    {
        var content = new MultipartFormDataContent
        {
            {new StringContent(groupId), "\"GroupId\""}
        };

        foreach (var filePath in filePathList)
        {
            var fileInfo = new FileInfo(filePath);
            if (!fileInfo.Exists)
            {
                LogHelper.Warn($@"File not exists: {filePath}.");
                return false;
            }

            content.Add(new ByteArrayContent(
                    File.ReadAllBytes(filePath)),
                "\"file\"", //该参数 form 中的name属性值,和 服务器端 必须对应起来
                $"\"{fileInfo.Name}\"");
        }

        var response = httpClient.PostAsync(_serverUrl, content).Result;
        var ret = response.Content.ReadAsStringAsync().Result;
        LogHelper.Info($@"Upload group file [{groupId}] result: {ret}.");
        return ret.ToUpper() == @"OK";
    }
}