刚搞过一个工具，所以没怎么整理，凑合用吧。

public class WeiXinReport
{
string AccessToken { get; set; }

const string GetAccessTokenFormat = "https://qyapi.weixin.qq.com/cgi-bin/gettoken?corpid={0}&corpsecret={1}";

const string UploadTempFileFormat = "https://qyapi.weixin.qq.com/cgi-bin/media/upload?access_token={0}&type={1}";

const string SendMessageFormat = "https://qyapi.weixin.qq.com/cgi-bin/message/send?access_token={0}";

const int AccessTokenExpiresRange = -10;

void GetAccessToken()
{
AccessToken = AppConfigOperation.GetAppConfig("AccessToken");
string Expires = AppConfigOperation.GetAppConfig("AccessTokenExpires");

DateTime AccessTokenExpires = DateTime.Now.AddDays(-1);
if (!string.IsNullOrEmpty(Expires))
{
try {
AccessTokenExpires = Convert.ToDateTime(Expires);
}
catch{

}
}
//如果AccessToken为空，或者过期时间已经到了
if (string.IsNullOrEmpty(AccessToken) || AccessTokenExpires < DateTime.Now)
{
using (WebClient webClient = new WebClient())
{
string CorpId = AppConfigOperation.GetAppConfig("CorpId");
string Secret = AppConfigOperation.GetAppConfig("Secret");
Byte[] bytes = webClient.DownloadData(string.Format(GetAccessTokenFormat, CorpId, Secret));
string result = Encoding.GetEncoding("utf-8").GetString(bytes);

string[] temp = result.Split(',');
string[] tp = temp[0].Split(':');
AccessToken = tp[1].ToString().Replace('"', ' ').Trim().ToString();
AppConfigOperation.UpdateAppConfig("AccessToken", AccessToken);
//时间设置稍微靠前一些
AppConfigOperation.UpdateAppConfig("AccessTokenExpires", DateTime.Now.AddSeconds(AccessTokenExpiresRange).ToString("yyyy-MM-dd HH:mm:ss"));
}
}
}

UploadJsonResult UploadTempFile(string filename, byte[] files)
{
using (WebClient webClient = new WebClient())
{
string CorpId = AppConfigOperation.GetAppConfig("CorpId");
string Secret = AppConfigOperation.GetAppConfig("Secret");

string url = string.Format(UploadTempFileFormat, AccessToken, "image");
HttpWebRequest request = WebRequest.Create(url) as HttpWebRequest;
CookieContainer cookieContainer = new CookieContainer();
request.CookieContainer = cookieContainer;
request.AllowAutoRedirect = true;
request.Method = "POST";
string boundary = DateTime.Now.Ticks.ToString("X"); // 随机分隔线
request.ContentType = "multipart/form-data;charset=utf-8;boundary=" + boundary;
byte[] itemBoundaryBytes = Encoding.UTF8.GetBytes("\r\n--" + boundary + "\r\n");
byte[] endBoundaryBytes = Encoding.UTF8.GetBytes("\r\n--" + boundary + "--\r\n");

//请求头部信息
StringBuilder sbHeader = new StringBuilder(string.Format("Content-Disposition:form-data;name=\"file\";filename=\"{0}\";filelength=\"{1}\"\r\nContent-Type:application/octet-stream\r\n\r\n", filename, files.Length));
byte[] postHeaderBytes = Encoding.UTF8.GetBytes(sbHeader.ToString());

//FileStream fs = new FileStream(file, FileMode.Open, FileAccess.Read);
//byte[] bArr = new byte[fs.Length];
//fs.Read(bArr, 0, bArr.Length);
//fs.Close();

Stream postStream = request.GetRequestStream();
postStream.Write(itemBoundaryBytes, 0, itemBoundaryBytes.Length);
postStream.Write(postHeaderBytes, 0, postHeaderBytes.Length);
postStream.Write(files, 0, files.Length);
postStream.Write(endBoundaryBytes, 0, endBoundaryBytes.Length);
postStream.Close();

//发送请求并获取相应回应数据
HttpWebResponse response = request.GetResponse() as HttpWebResponse;
//直到request.GetResponse()程序才开始向目标网页发送Post请求
Stream instream = response.GetResponseStream();
StreamReader sr = new StreamReader(instream, Encoding.UTF8);
//返回结果网页（html）代码
string content = sr.ReadToEnd();
UploadJsonResult result = JsonConvert.DeserializeObject<UploadJsonResult>(content);
return result;
}
}

/// <summary>
/// {
/// "touser": "UserID1|UserID2|UserID3",
/// "toparty": " PartyID1 | PartyID2 ",
/// "totag": " TagID1 | TagID2 ",
/// "msgtype": "image",
/// "agentid": 1,
/// "image": {
/// "media_id": "MEDIA_ID"
/// },
/// "safe":"0"
///}
/// </summary>
public void SendImage(string filename, byte[] files, SettingItem item) {
GetAccessToken();
UploadJsonResult ujresult = UploadTempFile(filename, files);

using (WebClient webClient = new WebClient())
{
CorpSendImage csi = new CorpSendImage();
csi.toparty = item.WxPartyIds;
csi.msgtype = UploadMediaFileType.image.ToString();
csi.agentid = "2";
csi.SetMediaId(ujresult.media_id);
csi.safe = "0";

var data = JsonConvert.SerializeObject(csi);
Byte[] bytes = webClient.UploadData(string.Format(SendMessageFormat, AccessToken), System.Text.Encoding.UTF8.GetBytes ( data ));
string result = Encoding.UTF8.GetString(bytes);
//{
// "errcode": 0,
// "errmsg": "ok",
// "invaliduser": "UserID1",
// "invalidparty":"PartyID1",
// "invalidtag":"TagID1"
//}
string[] temp = result.Split(',');
string[] tp = temp[1].Split(':');
string errmsg = tp[1];
}
}
}

public class CorpSendImage : CorpSendBase {
public MediaInfo image {get;set;}
public class MediaInfo {
public string media_id { get; set; }
}

public CorpSendImage() {
image = new MediaInfo();
}

public void SetMediaId(string media_id) {
image.media_id = media_id;
}
}

/// <summary>
/// 上传多媒体文件的返回结果
/// </summary>
public class UploadJsonResult
{
/// <summary>
/// 媒体文件类型，分别有图片（image）、语音（voice）、视频（video）和缩略图（thumb，主要用于视频与音乐格式的缩略图）
/// </summary>
public UploadMediaFileType type { get; set; }

/// <summary>
/// 媒体文件上传后，获取时的唯一标识
/// </summary>
public string media_id { get; set; }

/// <summary>
/// 媒体文件上传时间戳
/// </summary>
public long created_at { get; set; }
}

///// 图片（image）: 1M，支持JPG格式
/// 语音（voice）：2M，播放长度不超过60s，支持AMR\MP3格式
/// 视频（video）：10MB，支持MP4格式
/// 缩略图（thumb）：64KB，支持JPG格式。
public enum UploadMediaFileType {
image,
voice,
video,
thumb
}

/// <summary>
/// 企业号发送消息的基础消息内容
/// </summary>
public class CorpSendBase
{
/// <summary>
/// UserID列表（消息接收者，多个接收者用‘|’分隔）。特殊情况：指定为@all，则向关注该企业应用的全部成员发送
/// </summary>
public string touser { get; set; }

/// <summary>
/// PartyID列表，多个接受者用‘|’分隔。当touser为@all时忽略本参数
/// </summary>
public string toparty { get; set; }

/// <summary>
/// TagID列表，多个接受者用‘|’分隔。当touser为@all时忽略本参数
/// </summary>
public string totag { get; set; }

/// <summary>
/// 消息类型
/// </summary>
public string msgtype { get; set; }

/// <summary>
/// 企业应用的id，整型。可在应用的设置页面查看
/// </summary>
public string agentid { get; set; }

/// <summary>
/// 表示是否是保密消息，0表示否，1表示是，默认0
/// </summary>
[JsonProperty(NullValueHandling = NullValueHandling.Ignore)]
public string safe { get; set; }

}