Azure AD OAuth2访问令牌请求错误-400错误请求

我的WPF桌面应用程序（C#）正试图通过Microsoft Graph API读取用户的Outlook电子邮件。我被困在身份验证过程中；我已经收到了一个身份验证代码，现在我正试图从Azure获取访问令牌，但在发送访问令牌:请求时，一直收到HTTP 400错误代码

/**** Auth Code Retrieval ****/
string authCodeUrl = "https://login.microsoftonline.com/common/oauth2/authorize";
authCodeUrl += "?client_id" = clientId;
authCodeUrl += "&redirect_uri=" + redirectUri;
authCodeUrl += "&response_type=code";
authCodeUrl += "&resource=https%3A%2F%2Fgraph.microsoft.com%2F";
Process.start(authUrl); // User logs in, we get the auth code after login
string code = "......"; // Hidden for this post
/**** Access Token Retrieval ****/
string tokenUrl = "https://login.microsoftonline.com/common/oauth2/token"
string content = "grant_type=authorization_code";
content += "&client_id=" + clientId;
content += "&resource=https%3A%2F%2Fgraph.microsoft.com%2F";
content += "&code=" + code;
content += "&redirect_uri=" + redirectUri;
WebRequest request = WebRequest.Create(tokenUrl);
request.ContentType = "application/x-www-form-urlencoded";
byte[] data = Encoding.UTF8.GetBytes(content);
request.ContentLength = data.Length;
request.Method = "POST";
try 
{
  using (Stream stream = request.GetRequestStream())
  {
    stream.Write(data, 0, data.Length);
  }
  WebResponse response = request.GetResponse(); // This throws exception
}
catch (Exception error) // This catches the exception
{
  Console.WriteLine(error.Message); // Outputs 400, bad request
}

以上是用于检索身份验证代码的代码，然后是检索访问令牌的尝试。我们没有client_secret，因为机密仅用于Web应用程序，而这是一个本地桌面WPF应用程序。我读到这不是问题。我在网上学习了很多教程和官方文档，主要是官方的Graph授权文档，但我仍然不知道自己做错了什么。如有任何帮助，我们将不胜感激，谢谢。