RestSharp 106.2.2发送带文件和JSON请求缺失boundary问题求助

我正在构建一个跨.NET Framework和.NET Standard的封装库，通过SendEmail函数向Azure Function的HTTP端点发送JSON载荷及多个文件。但使用RestSharp 106.2.2发送请求时，请求未携带boundary信息，导致Azure Function初始化MultipartMemoryStreamProvider失败。

控制台测试应用 #

private static void Main(string[] args) { 
    var emailService = new EmailService(); 
    var email = new MicroserviceMailMessage(
        "from@test.com", 
        "to@test.com", 
        "Test Subject from Framework App", 
        "Test Body from Framework App", 
        true, 
        DateTime.Now
    ); 
    email.Attachments.Add(new Attachment(@"c:\Temp\soccor5.jpg")); 
    emailService.SendEmail(email); 
}

封装库代码 #

public void SendEmail(MicroserviceMailMessage email) { 
    var client = new RestClient("http://localhost:7071/api"); 
    var request = new RestRequest("SendEmail", Method.POST); 
    var transformedEmail = TransformEmail(email); 
    request.AlwaysMultipartFormData = true; 
    request.AddHeader("Content-Type", $"multipart/form-data"); 
    foreach (var attachment in email.Attachments) { 
        var inline = attachment.ContentDisposition.Inline ? "inline." : ""; 
        request.Files.Add(new FileParameter { 
            Name = $"attachments.{inline}{attachment.Name}", 
            Writer = (s) => { 
                attachment.ContentStream.CopyTo(s); 
                attachment.ContentStream.Dispose(); 
            }, 
            ContentType = attachment.ContentType.ToString(), 
            ContentLength = attachment.ContentStream.Length, 
            FileName = attachment.Name 
        }); 
    } 
    request.AddParameter("email", $"{JsonConvert.SerializeObject(transformedEmail)}", ParameterType.RequestBody); 
    IRestResponse response = client.Execute(request); 
}

Azure接收函数 #

[FunctionName("EmailReceiver")] 
public static async Task<HttpResponseMessage> Run(
    [HttpTrigger(AuthorizationLevel.Function, "post", Route = null)]HttpRequestMessage req, 
    ...
) { 
    ... 
    var provider = new MultipartMemoryStreamProvider(); 
    await req.Content.ReadAsMultipartAsync(provider); // Fails on this line 
    ... 
}

对比RestSharp 105.2.3（或Postman）发送的请求，其包含正确的boundary信息，而106.2.2的请求缺失该信息，报错：

Invalid 'HttpContent' instance provided. It does not have a 'multipart' content-type header with a 'boundary' parameter.\r\nParameter name: content

我尝试过降级到RestSharp 105.2.3，该版本在.NET Framework下可正常工作，但我需要封装库兼容.NET Framework和.NET Standard，因此该方案无法彻底解决问题。

解决方案 #

问题的核心在于RestSharp 106+版本的行为变化：如果你手动设置Content-Type为multipart/form-data，会完全覆盖库自动生成的包含boundary的头信息。以下是具体的修复步骤：

1. 移除手动添加的Content-Type头
   删除这行代码，让RestSharp自动生成带boundary的Content-Type：

   // 删掉这行
   request.AddHeader("Content-Type", $"multipart/form-data");
   

2. 修正JSON参数的添加方式
   你当前使用ParameterType.RequestBody添加JSON参数，这会和multipart表单结构冲突。应该将其作为表单字段添加：

   // 替换原来的AddParameter(RequestBody)
   request.AddParameter(
       "email", 
       JsonConvert.SerializeObject(transformedEmail), 
       ParameterType.GetOrPost
   );
   

3. 优化文件参数添加（可选）
   可以使用RestSharp内置的AddFile方法简化文件添加逻辑，避免手动创建FileParameter：

   foreach (var attachment in email.Attachments) {
       var inline = attachment.ContentDisposition.Inline ? "inline." : "";
       request.AddFile(
           name: $"attachments.{inline}{attachment.Name}",
           writer: s => {
               attachment.ContentStream.CopyTo(s);
               attachment.ContentStream.Dispose();
           },
           fileName: attachment.Name,
           contentLength: attachment.ContentStream.Length,
           contentType: attachment.ContentType.ToString()
       );
   }
   

原理说明 #

在RestSharp 105.x版本中，即使手动设置了multipart/form-data的Content-Type，库仍然会自动追加boundary参数。但106+版本为了更灵活地支持自定义头，改变了这个行为——手动设置的Content-Type会完全替代自动生成的头，导致缺失boundary。而当你移除手动设置的头后，RestSharp会在检测到multipart请求（设置了AlwaysMultipartFormData = true且包含文件/表单参数）时，自动生成包含随机boundary的正确Content-Type头。

内容的提问来源于stack exchange，提问作者tokyo0709