FormOrJsonModelBinder: allow both JSON and Form (multipart/form-data or url-encoded) POST to a ASP.net core MVC Action.

FormOrJsonModelBinder.cs

namespace core.Tools {
  using System.Threading.Tasks;
  using Microsoft.AspNetCore.Http;
  using Microsoft.AspNetCore.Mvc;
  using Microsoft.AspNetCore.Mvc.Infrastructure;
  using Microsoft.AspNetCore.Mvc.ModelBinding;
  using Microsoft.AspNetCore.Mvc.ModelBinding.Binders;
  using Microsoft.Extensions.Logging;
  using Microsoft.Extensions.Options;


  public class FormOrJsonAttribute : ModelBinderAttribute {
    public FormOrJsonAttribute() : base(typeof(FormOrJsonModelBinder)) {
      // Optionally, you can set the BindingSource if needed.
      // BindingSource = BindingSource.Custom;
    }
  }

  public class FormOrJsonModelBinder : IModelBinder {


    public async Task BindModelAsync(ModelBindingContext bindingContext) {
      var request = bindingContext.HttpContext.Request;

      // If the request contains form data, delegate to the default binder.
      if (request.HasFormContentType) {
        var metadataProvider = (IModelMetadataProvider)bindingContext.HttpContext.RequestServices
                .GetService(typeof(IModelMetadataProvider));
        var cleanMetadata = metadataProvider.GetMetadataForType(bindingContext.ModelType);

        // Create a new BindingInfo.  
        // IMPORTANT: Set the BindingSource to Form, so that the default binder will look in the form data.
        var bindingInfo = new BindingInfo {  BindingSource = BindingSource.Form };

        // Create a new ModelBinderFactoryContext using the clean metadata.
        var factoryContext = new ModelBinderFactoryContext {
          BindingInfo = bindingInfo,
          Metadata = cleanMetadata,
          CacheToken = bindingContext.ModelType,
        };
        // Get the binder factory and create the default binder.
        var binderFactory = (IModelBinderFactory)bindingContext.HttpContext.RequestServices
            .GetService(typeof(IModelBinderFactory));
        var defaultBinder = binderFactory.CreateBinder(factoryContext);
       
        // Delegate binding to the default binder.
        await defaultBinder.BindModelAsync(bindingContext);
      }
      // If the Content-Type indicates JSON, delegate to the BodyModelBinder.
      else if (request.ContentType?.StartsWith("application/json") == true) {
        // Resolve required services.
        var readerFactory = (IHttpRequestStreamReaderFactory)bindingContext.HttpContext.RequestServices
            .GetService(typeof(IHttpRequestStreamReaderFactory))!;
        var optionsAccessor = (IOptions<MvcOptions>)bindingContext.HttpContext.RequestServices
            .GetService(typeof(IOptions<MvcOptions>))!;
        var mvcOptions = optionsAccessor.Value;
        var loggerFactory = (ILoggerFactory)bindingContext.HttpContext.RequestServices
            .GetService(typeof(ILoggerFactory))!;

        // Note: Order of parameters: inputFormatters, readerFactory, mvcOptions, loggerFactory.
        var bodyBinder = new BodyModelBinder(mvcOptions.InputFormatters, readerFactory, loggerFactory);
        await bodyBinder.BindModelAsync(bindingContext);
      }
      else {
        // Unsupported Content-Type; fail the binding.
        bindingContext.Result = ModelBindingResult.Failed();
      }
    }
  }


}