微信三方平台开发上传base64格式图片至临时素材

  public string UploadImgByB64(string b64)
        {
            //access_token 需要自己获取
            string access_token = getToken();
            string url = $"https://api.weixin.qq.com/cgi-bin/media/upload?access_token={access_token}&type=image";
            byte[] data = Convert.FromBase64String(b64);
            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="{Guid.NewGuid()}.png"" + "");
            contentByte.Headers.Remove("Content-Type");
            contentByte.Headers.TryAddWithoutValidation("Content-Type", "image/png");
            try
            {
                var result2 = client.PostAsync(url, content);
                if (result2.Result.StatusCode != HttpStatusCode.OK)
                    throw new Exception(result2.Result.Content.ReadAsStringAsync().Result);
                string jsonstr = result2.Result.Content.ReadAsStringAsync().Result;
                var dic = Newtonsoft.Json.Linq.JObject.Parse(jsonstr);
                return dic["media_id"].ToString();
            }
            catch (Exception ex)
            {
                return null;
            }
        }