ASP.NET Core MVC - Return ViewResult or JSON from controller action

You can implement MVC Action filter to easily achieve such functionality.

using System;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Filters;
using Microsoft.Net.Http.Headers;

[AttributeUsage(AttributeTargets.Method | AttributeTargets.Class)]
public class ViewJsonActionFilterAttribute : Attribute, IActionFilter
{
    public void OnActionExecuted(ActionExecutedContext context)
    {
        //result must be OkObjectResult
        if (context.Result is OkObjectResult okResult)
        {
            var accept = context.HttpContext.Request.Headers[HeaderNames.Accept];
            switch (accept)
            {
                //changing result to ViewResult
                case "text/html":
                    var controller = (Controller)context.Controller;
                    context.Result = controller.View(okResult.Value);
                    break;
                case "application/json":
                default:
                    //do nothing
                    break;
            }
        }
    }

    public void OnActionExecuting(ActionExecutingContext context)
    {
        //do nothing
    }
}

Then apply the attribute to the specific action/controller or globally. Always return Ok from controller as a successful result so filter can process it as expected.

//important: controller should be derived from Controller class to contain View method
public class TestController : Controller
{
    [HttpGet]
    [ViewJsonActionFilter]
    public IActionResult TestViewJson()
    {
        var model = new ViewJsonTestViewModel
        {
            Id = 55,
            Name = "I'm a test model"
        };

        return Ok(model);
    }
}

If you set request Accept header to text/html you will get html, otherwise it will return json.