c# - Upload files and JSON in ASP.NET Core Web API

Question

How can I upload a list of files (images) and json data to ASP.NET Core Web API controller using multipart upload?

I can successfully receive a list of files, uploaded with multipart/form-data content type like that:

public async Task<IActionResult> Upload(IList<IFormFile> files)

And of course I can successfully receive HTTP request body formatted to my object using default JSON formatter like that:

public void Post([FromBody]SomeObject value)

But how can I combine these two in a single controller action? How can I upload both images and JSON data and have them bind to my objects?

Answer 1

Simple, less code, no wrapper model

There is simpler solution, heavily inspired by Andrius' answer. By using the ModelBinderAttribute you don't have to specify a model or binder provider. This saves a lot of code. Your controller action would look like this:

public IActionResult Upload(
    [ModelBinder(BinderType = typeof(JsonModelBinder))] SomeObject value,
    IList<IFormFile> files)
{
    // Use serialized json object 'value'
    // Use uploaded 'files'
}

Implementation

Code behind JsonModelBinder (see GitHub or use NuGet package):

using System;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc.ModelBinding;

public class JsonModelBinder : IModelBinder {
    public Task BindModelAsync(ModelBindingContext bindingContext) {
        if (bindingContext == null) {
            throw new ArgumentNullException(nameof(bindingContext));
        }

        // Check the value sent in
        var valueProviderResult = bindingContext.ValueProvider.GetValue(bindingContext.ModelName);
        if (valueProviderResult != ValueProviderResult.None) {
            bindingContext.ModelState.SetModelValue(bindingContext.ModelName, valueProviderResult);

            // Attempt to convert the input value
            var valueAsString = valueProviderResult.FirstValue;
            var result = Newtonsoft.Json.JsonConvert.DeserializeObject(valueAsString, bindingContext.ModelType);
            if (result != null) {
                bindingContext.Result = ModelBindingResult.Success(result);
                return Task.CompletedTask;
            }
        }

        return Task.CompletedTask;
    }
}

Example request

Here is an example of a raw http request as accepted by the controller action Upload above.

A multipart/form-data request is split into multiple parts each separated by the specified boundary=12345. Each part got a name assigned in its Content-Disposition-header. With these names default ASP.Net-Core knows which part is bound to which parameter in the controller action.

Files that are bound to IFormFile additionally need to specify a filename as in the second part of the request. Content-Type is not required.

Another thing to note is that the json parts need to be deserializable into the parameter types as defined in the controller action. So in this case the type SomeObject should have a property key of type string.

POST http://localhost:5000/home/upload HTTP/1.1
Host: localhost:5000
Content-Type: multipart/form-data; boundary=12345
Content-Length: 218

--12345
Content-Disposition: form-data; name="value"

{"key": "value"}
--12345
Content-Disposition: form-data; name="files"; filename="file.txt"
Content-Type: text/plain

This is a simple text file
--12345--

Testing with Postman

Postman can be used to call the action and test your server side code. This is quite simple and mostly UI driven. Create a new request and select form-data in the Body-Tab. Now you can choose between text and file for each part of the reqeust.