C# .NET Core 3.1 Web API Post parameter is Null

Question

I am trying to make a post request from WPF to Web API using the following code but the request parameter is always null.

Request Model

public class Document
{
   public string FileName { get; set; }
   public byte[] Buffer { get; set; }
}


public class Request
{
   public string Uploader { get; set; }
   public List<Document> Documents { get; set; }
}

WPF Client

var obj = new Request()
{
    Uploader = "John Doe",
    Documents = new List<Document>
    {
        new Document()
        {
            FileName ="I Love Coding.pdf",
            Buffer = System.IO.File.ReadAllBytes(@"C:\Users\john.doe\Downloads\I Love Coding.pdf.pdf")
        }
    }
};

using (var http = new HttpClient())
{
    var encodedJson = JsonConvert.SerializeObject(obj);
    var conent = new StringContent(encodedJson, Encoding.UTF8, "application/json");
    HttpResponseMessage response = await http.PostAsync("https://my-app.com/api/upload", conent);
    response.EnsureSuccessStatusCode();
}

Web API

[Route("")]
public class AppController : ControllerBase
{
    [HttpPost]
    [Route("api/upload")]
    public async Task<IActionResult> UploadDocumentsAsync([FromBody] Request request)
    {
        // request is always null when app is running in production
        // https://my-app.com/api/upload

        //request is not null when running on https://localhost:8080/api/upload
    }
}

Please what am I missing in the above implementation? The request parameter is not null on localhost but always null in production.

Answer 1

Please what am I missing in the above implementation? The request parameter is not null on localhost but always null in production.

Well, not sure how are getting data on local server becuse, you are sending MultipartFormData means your POCO object and file buffer. As you may know we can send json object in FromBody but not the files as json. Thus, I am not sure how it working in local and getting null data is logical in IIS Or Azure.

what am I missing in the above implementation?

As explained above, for sending both POCO object and Files as byte or steam we need to use FromForm and beside that, we need to bind our request object as MultipartFormDataContent to resolve your null data on your UploadDocumentsAsync API action.

Required Change For Solution:

WPF:

In your WPF http request please update your request code snippet as following:

var obj = new Request()
            {
                Uploader = "John Doe",
                Documents = new List<Document>
                    {
                        new Document()
                        {
                            FileName ="I Love Coding.pdf",
                            Buffer = System.IO.File.ReadAllBytes(@"YourFilePath")
                        }
                    }
            };


            var httpClient = new HttpClient
            {
              BaseAddress = new("https://YourServerURL")
            };

           
            var formContent = new MultipartFormDataContent();
            
            formContent.Add(new StringContent(obj.Uploader), "Uploader");

            formContent.Add(new StringContent(obj.Documents[0].FileName), "Documents[0].FileName");
            formContent.Add(new StreamContent(new MemoryStream(obj.Documents[0].Buffer)), "Documents[0].Buffer", obj.Documents[0].FileName);



            var response = await httpClient.PostAsync("/api/upload", formContent);

            if (response.IsSuccessStatusCode)
            {
                var responseFromAzureIIS = await response.Content.ReadAsStringAsync();
            }

Note: Class in WPF side would remain same as before. No changes required.

Asp.net Core Web API:

In asp.net core web API side you should use [FromForm] instead of [FromBody] So your controller Action would as following:

    [Route("")]
    public class AppController : ControllerBase
    {
        [HttpPost]
        [Route("api/upload")]
        public async Task<IActionResult> UploadDocumentsAsync([FromForm] Request file)
        {
            if (file.Documents[0].Buffer == null)
            {
                return Ok("Null File");
            }
           
            return Ok("File Received");
        }
    }

Note: For remote debugging I have checked the logs and for double check I have used a simple conditionals whether file.Documents[0].Buffer == null. I have tested both in local, IIS and Azure and working accordingly.

Update POCO Class in API Project:

For buffer you have used byte for your WPF project but for Web API project update that to IFormFile instead of byte. It should be as following:

public class Document
    {
        public string FileName { get; set; }
        public IFormFile Buffer { get; set; }
    }


    public class Request
    {
        public string Uploader { get; set; }
        public List<Document> Documents { get; set; }
    }

Output:

If you would like to know more details on it you could check our official document here