最烦做微信公众平台的东西。。文档说得不清不楚，又没示例代码，只能自己 慢慢搜索，弄了一晚上，基本弄出来了，把本地的图片上传到微信的临时素材那里，返回媒体ID，用于其他操作，代码如下 ：（自己导入相应的类System.Net.Http，JSON解析用的LitJson）
/// <summary>
/// 上传临时素材
/// 返回media_id
/// </summary>
/// <param name="userid"></param>
/// <returns></returns>
public string UploadLinShiSuCai(int userid) {
string imgpath = HttpContext.Current.Server.MapPath($"/upload/erweima/{userid}_2.png");
string appid = WxPayConfig.APPID;
string secret = WxPayConfig.APPSECRET;

        //1. 获取AccessToken（有效期7200秒，开发者必须在自己的服务全局缓存access_token）
        string url1 = $"https://api.weixin.qq.com/cgi-bin/token?grant_type=client_credential&appid={appid}&secret={secret}";
        string result = HttpService.Get(url1);
        JsonData jd = JsonMapper.ToObject(result);
        string access_token = (string)jd["access_token"];

        string url2 = $"https://api.weixin.qq.com/cgi-bin/media/upload?access_token={access_token}&type=image";

        //图片转为流
        Image img = new Bitmap(imgpath);
        MemoryStream stream = new MemoryStream();
        img.Save(stream, ImageFormat.Png);
        BinaryReader br = new BinaryReader(stream);
        byte[] data = stream.ToArray();
        stream.Close();



        var boundary = "fbce142e-4e8e-4bf3-826d-cc3cf506cccc";
        var client = new HttpClient();
        client.DefaultRequestHeaders.Add("User-Agent", "KnowledgeCenter");
        client.DefaultRequestHeaders.Remove("Expect");
        client.DefaultRequestHeaders.Remove("Connection");
        client.DefaultRequestHeaders.ExpectContinue = false;
        client.DefaultRequestHeaders.ConnectionClose = true;
        var content = new MultipartFormDataContent(boundary);
        content.Headers.Remove("Content-Type");
        content.Headers.TryAddWithoutValidation("Content-Type", "multipart/form-data; boundary=" + boundary);
        var contentByte = new ByteArrayContent(data);
        content.Add(contentByte);
        contentByte.Headers.Remove("Content-Disposition");
        contentByte.Headers.TryAddWithoutValidation("Content-Disposition", $"form-data; name=\"media\";filename=\"{userid}_2.png\"" + "");
        contentByte.Headers.Remove("Content-Type");
        contentByte.Headers.TryAddWithoutValidation("Content-Type", "image/png");
        try
        {
            var result2 = client.PostAsync(url2, content);
            if (result2.Result.StatusCode != HttpStatusCode.OK)
                throw new Exception(result2.Result.Content.ReadAsStringAsync().Result);
            string jsonstr = result2.Result.Content.ReadAsStringAsync().Result;
            JsonData jd2 = JsonMapper.ToObject(jsonstr);
            result = (string)jd2["media_id"];
            return result;
        }
        catch (Exception ex)
        {
            throw new Exception(ex.Message + ex.InnerException.Message);
        } 

    }