Upload files and JSON in ASP.NET Core Web API

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.

https://i.stack.imgur.com/xuMRp.png

---

I'm working with Angular 7 on the front-end, so I make use of the FormData class, which allows you to append strings or blobs to a form. They can be pulled out of the form in the controller action using the [FromForm] attribute. I add the file to the FormData object, and then I stringify the data I wish to send together with the file, append it to the FormData object, and deserialize the string in my controller action.

Like so:

//front-end:
let formData: FormData = new FormData();
formData.append('File', fileToUpload);
formData.append('jsonString', JSON.stringify(myObject));

//request using a var of type HttpClient
http.post(url, formData);

//controller action
public Upload([FromForm] IFormFile File, [FromForm] string jsonString)
{
    SomeType myObj = JsonConvert.DeserializeObject<SomeType>(jsonString);

    //do stuff with 'File'
    //do stuff with 'myObj'
}

You now have a handle on the file and the object. Note that the name you provide in the params list of your controller action must match the name you provide when appending to the FormData object on the front-end.

---

Apparently there is no built in way to do what I want. So I ended up writing my own ModelBinder to handle this situation. I didn't find any official documentation on custom model binding but I used this post as a reference.

Custom ModelBinder will search for properties decorated with FromJson attribute and deserialize string that came from multipart request to JSON. I wrap my model inside another class (wrapper) that has model and IFormFile properties.

IJsonAttribute.cs:

public interface IJsonAttribute
{
    object TryConvert(string modelValue, Type targertType, out bool success);
}

FromJsonAttribute.cs:

using Newtonsoft.Json;
[AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
public class FromJsonAttribute : Attribute, IJsonAttribute
{
    public object TryConvert(string modelValue, Type targetType, out bool success)
    {
        var value = JsonConvert.DeserializeObject(modelValue, targetType);
        success = value != null;
        return value;
    }
}

JsonModelBinderProvider.cs:

public class JsonModelBinderProvider : IModelBinderProvider
{
    public IModelBinder GetBinder(ModelBinderProviderContext context)
    {
        if (context == null) throw new ArgumentNullException(nameof(context));

        if (context.Metadata.IsComplexType)
        {
            var propName = context.Metadata.PropertyName;
            var propInfo = context.Metadata.ContainerType?.GetProperty(propName);
            if(propName == null || propInfo == null)
                return null;
            // Look for FromJson attributes
            var attribute = propInfo.GetCustomAttributes(typeof(FromJsonAttribute), false).FirstOrDefault();
            if (attribute != null) 
                return new JsonModelBinder(context.Metadata.ModelType, attribute as IJsonAttribute);
        }
        return null;
    }
}

JsonModelBinder.cs:

public class JsonModelBinder : IModelBinder
{
    private IJsonAttribute _attribute;
    private Type _targetType;

    public JsonModelBinder(Type type, IJsonAttribute attribute)
    {
        if (type == null) throw new ArgumentNullException(nameof(type));
        _attribute = attribute as IJsonAttribute;
        _targetType = type;
    }

    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;
            bool success;
            var result = _attribute.TryConvert(valueAsString, _targetType, out success);
            if (success)
            {
                bindingContext.Result = ModelBindingResult.Success(result);
                return Task.CompletedTask;
            }
        }
        return Task.CompletedTask;
    }
}

Usage:

public class MyModelWrapper
{
    public IList<IFormFile> Files { get; set; }
    [FromJson]
    public MyModel Model { get; set; } // <-- JSON will be deserialized to this object
}

// Controller action:
public async Task<IActionResult> Upload(MyModelWrapper modelWrapper)
{
}

// Add custom binder provider in Startup.cs ConfigureServices
services.AddMvc(properties => 
{
    properties.ModelBinderProviders.Insert(0, new JsonModelBinderProvider());
});

---

Following the excellent answer by @bruno-zell, if you have only one file (I didn't test with an IList<IFormFile>) you can also just declare your controller as this :

public async Task<IActionResult> Create([FromForm] CreateParameters parameters, IFormFile file)
{
    const string filePath = "./Files/";
    if (file.Length > 0)
    {
        using (var stream = new FileStream($"{filePath}{file.FileName}", FileMode.Create))
        {
            await file.CopyToAsync(stream);
        }
    }

    // Save CreateParameters properties to database
    var myThing = _mapper.Map<Models.Thing>(parameters);

    myThing.FileName = file.FileName;

    _efContext.Things.Add(myThing);
    _efContext.SaveChanges();


    return Ok(_mapper.Map<SomeObjectReturnDto>(myThing));
}

Then you can use the Postman method shown in Bruno's answer to call your controller.

---

I had a similar issue and I solved the problem by using [FromForm] attribute and FileUploadModelView in the function as follows:

[HttpPost("Save")]
public async Task<IActionResult> Save([FromForm] ProfileEditViewModel model)
{          
  return null;
}

---

I wanted to do the same using Vue frontend and .net core api. But for some weird reason IFormFile always returned null. So then I had to change it to IFormCollection and got it sorted out. Here is the code for anyone facing the same issue :)

public async Task<IActionResult> Post([FromForm]IFormCollection files)

---

An updated version for .net 5 based on @bruno-zell 's answer with added support to multiple files

using System;
using System.Collections;
using System.Text.Json;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.ModelBinding;
using Microsoft.Extensions.Options;

public class JsonModelBinder : IModelBinder
{
    private readonly JsonOptions _jsonOptions;
    public JsonModelBinder(IOptions<JsonOptions> jsonOptions)
    {
        _jsonOptions = jsonOptions.Value;
    }
    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);

            string toSerialize;
            // Attempt to convert the input value
            if (typeof(IEnumerable).IsAssignableFrom(bindingContext.ModelType))
            {
                toSerialize = "[" + string.Join<string>(',', valueProviderResult.Values) + "]";
            }
            else
            {
                toSerialize = valueProviderResult.FirstValue;
            }
            var result = JsonSerializer.Deserialize(toSerialize, bindingContext.ModelType, _jsonOptions.JsonSerializerOptions);
            if (result != null)
            {
                bindingContext.Result = ModelBindingResult.Success(result);
                return Task.CompletedTask;
            }
        }

        return Task.CompletedTask;
    }
}

---

I am not sure if you can do the two things in a single step.

How I have achieved this in the past is by uploading the file through ajax and returning the file url back in the response and then pass it along with post request to save the actual record.

---

I had a similar problem when posting from angular to asp core api.

Chrome: Form Data

------WebKitFormBoundarydowgB6BX0wiwKeOk
Content-Disposition: form-data; name="file1"

undefined
------WebKitFormBoundarydowgB6BX0wiwKeOk
Content-Disposition: form-data; name="file2"

undefined
------WebKitFormBoundarydowgB6BX0wiwKeOk
Content-Disposition: form-data; name="reportData"; filename="blob"
Content-Type: application/json

{"id":2,"report":3,"code":"XX0013","business":"01","name":"Test","description":"Description"}
------WebKitFormBoundarydowgB6BX0wiwKeOk--

Here is how I do it:

I use reportData as an uploaded file data, then I read the file's contents.

[HttpPost]
public async Task<IActionResult> Set([FromForm] IFormFile file1, [FromForm] IFormFile file2, [FromForm] IFormFile reportData)
{
    try
    {
        ReportFormModel.Result result = default;

        if (reportData != null)
        {
            string reportJson = await reportData.ReadFormFileAsync();
            ReportFormModel.Params reportParams = reportJson.JsonToObject<ReportFormModel.Params>();

            if (reportParams != null)
            {
                //OK
            }
        }
        return Ok(result);
    }
    catch (Exception ex)
    {
        return BadRequest();
    }
}


public static class Utilities
{
    public static async Task<string> ReadFormFileAsync(this IFormFile file)
    {
        if (file == null || file.Length == 0)
        {
            return await Task.FromResult((string)null);
        }

        using var reader = new StreamReader(file.OpenReadStream());
        return await reader.ReadToEndAsync();
    }
}

Although this way is not appreciated, but it worked.

---

You don't need "JsonModelBinder" and other custom stuff, I have model

public class UpdateAdminProfileInfoRequest
{
    
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public string Mobile { get; set; }
    public IFormFile Image { get; set; }
}

and in Controller action method only one argument

[FromForm]UpdateAdminProfileInfoRequest request

and everything works fine. If you need multiple file upload just change

IFormFile 

with

List<IFormFile> files {get; set}

Be careful I don't know why but currently I use .net6 and nor JsonProperty nor JsonPropertyName don't work on IFormFile, if you decorate Image property with

[JsonProperty("imageFile")] 

attribute or something like this asp.net don't map client "imageFile" field to "Image" property.

---

I'm just adding my two cents.

The Angular app function for uploading file with json

 uploadFileWithJson = (files) => {
if (files.length === 0) {
  return;
}

let fileToUpload = <File>files[0];
const formData = new FormData();
formData.append('file', fileToUpload, fileToUpload.name);

const user = {
  "name":"Vikram",
  "age":35
}
const userTxt = JSON.stringify(user);
formData.append('userData',userTxt);    

this.http.post('https://localhost:5001/api/upload/UploadFileWithJson', formData, {reportProgress: true, observe: 'events'})
  .subscribe({
    next: (event) => {
    if (event.type === HttpEventType.UploadProgress)
      this.progress = Math.round(100 * event.loaded / event.total);
    else if (event.type === HttpEventType.Response) {
      this.message = 'Upload success.';
      this.onUploadFinished.emit(event.body);
    }
  },
  error: (err: HttpErrorResponse) => console.log(err)
});

}

And .NET Core API

[HttpPost("UploadFileWithJson"), DisableRequestSizeLimit]        
    public IActionResult UploadFileWithJson([FromForm]UserWithFile model)
    {
        try
        {
            if (model == null)
                throw new Exception($"{nameof(model)} is null");

            if (model.File == null)
                throw new Exception("File is null");

            var folderName = Path.Combine("Resources", "Images");
            var pathToSave = Path.Combine(Directory.GetCurrentDirectory(), folderName);
            if (model.File.Length > 0)
            {
                var fileName = ContentDispositionHeaderValue.Parse(model.File.ContentDisposition).FileName.Trim('"');
                var fullPath = Path.Combine(pathToSave, fileName);
                var dbPath = Path.Combine(folderName, fileName);
                using (var stream = new FileStream(fullPath, FileMode.Create))
                {
                    model.File.CopyTo(stream);
                }
                return Ok(new { dbPath });
            }
            else
            {
                return BadRequest();
            }                
        }
        catch (Exception ex)
        {
            return StatusCode(500, $"Internal server error: {ex}");
        }
    }

as well as model class.

public class UserWithFile
{
    public string UserData { get; set; }

    public IFormFile File { get; set; }
}

Now notice, in Angular app, the FormData attribute file name first letter f is small-cased. However, in .NET Core same is File (or upper-cased). Same is case for userData.

I simply changed file and userData in Angular app to File and UserData respectively.

https://i.stack.imgur.com/swplF.png

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