Error declaring verbatim variables after upgrade to asp.net core 3.1

Question

I am experiencing a peculiar error after migrating my Asp.Net Core MVC Webapp solution from .Net Core 2.2 to .Net Core 3.1.
I followed the steps described in the official docs, updating Program, Startup, nuget packages ...

I am using Syncfusion Essential Studio 2 - which I believe is not related to this error - and in a number of views I am declaring variables/creating objects, like creating a DropDownList which I use for a lookup in a Datagrid:

@model CultureMultiModel

@{
    var userCultLookupDDL = new Syncfusion.EJ2.DropDowns.DropDownList()
    {
        FilterType = Syncfusion.EJ2.DropDowns.FilterType.Contains,
        Fields = new Syncfusion.EJ2.DropDowns.DropDownListFieldSettings { Text = "Display", Value = "Name", GroupBy = "Parent" }
        ...
    };
    var userCultParams = new { @params = userCultLookupDDL };

This was working fine in 2.2 but now I get an error on new { @params ...
When hovering on @params the following is shown:
CS1525: invalid expression term 'params'
CS1003: syntax error, ',' expected

I've tried a number of things:

• using round brackets @(params) gives same error
• adding nuget package Microsoft.AspNetCore.Mvc.Razor.Extensions, same error
• added <AddRazorSupportForMvc>true</AddRazorSupportForMvc> to project file, same error
• changing to @Params or @arams gives error CS0103: the name doesn't exist in the current context

As this happens during the immediate compilation/validation while editing the cshtml file I don't think this is related to Startup but I've added this for completeness anyway:

public void ConfigureServices(IServiceCollection services)
        {
            services.AddDbContext<myContext>(options =>
                options.UseSqlServer(
                    Configuration.GetConnectionString("myConnection"), builder => builder.MigrationsAssembly("myDAL")));


            services.AddDefaultIdentity<IdentityUser>()
                .AddRoles<IdentityRole>()
                .AddRoleManager<RoleManager<IdentityRole>>()
                .AddEntityFrameworkStores<ApplicationDbContext>();
            
            services.AddLocalization(options => options.ResourcesPath = "Resources");
            LocalizationSupport localizationSupport = new LocalizationSupport();
            services.Configure<RequestLocalizationOptions>(options =>
            {
                options.DefaultRequestCulture = new RequestCulture(culture: "en", uiCulture: "en");
                options.SupportedCultures = localizationSupport.SupportedCultures;
                options.SupportedUICultures = localizationSupport.SupportedCultures;
            });

            services.AddMemoryCache();

            services
                .AddMvc()
                .AddViewLocalization()
                .AddDataAnnotationsLocalization(options =>
                {
                    options.DataAnnotationLocalizerProvider = (type, factory) =>
                        factory.Create(typeof(SharedModelLocale));
                })
                .AddNewtonsoftJson(options =>
                {
                    options.SerializerSettings.ContractResolver = new DefaultContractResolver();
                });
            ;

            services
                .AddControllersWithViews()
                .AddNewtonsoftJson(
                    options => options.SerializerSettings.ReferenceLoopHandling = Newtonsoft.Json.ReferenceLoopHandling.Ignore
                );
            services.AddRazorPages();

            services.Configure<IdentityOptions>(options =>
            ...

            services.AddHttpContextAccessor();
        }

        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }
            else
            {
                app.UseExceptionHandler("/Home/Error");
                app.UseHsts();
            }

            app.UseHttpsRedirection();
            // << Localization           
            LocalizationSupport localizationSupport = new LocalizationSupport();
            var options = new RequestLocalizationOptions
            {
                DefaultRequestCulture = new RequestCulture("en"),
                SupportedCultures = localizationSupport.SupportedCultures,
                SupportedUICultures = localizationSupport.SupportedCultures,
            };
            app.UseRequestLocalization(options);
            // >> Localization
            app.UseStaticFiles();
            app.UseCookiePolicy();

            app.UseRouting();

            app.UseAuthentication();
            app.UseAuthorization();

            app.UseEndpoints(endpoints => {
                endpoints.MapAreaControllerRoute(name: "IdentityRoute", areaName: "Identity", pattern: "{area:exists}/{controller=Home}/{action=Index}");
                endpoints.MapDefaultControllerRoute();
            });
        }

and my project file

<Project Sdk="Microsoft.NET.Sdk.Web">

  <PropertyGroup>
    <TargetFramework>netcoreapp3.1</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Markdig" Version="0.22.1" />
    <PackageReference Include="Microsoft.AspNetCore.Identity.UI" Version="3.1.15" />
    <PackageReference Include="Microsoft.AspNetCore.Mvc.NewtonsoftJson" Version="3.1.1" />
    <PackageReference Include="Microsoft.VisualStudio.Web.CodeGeneration.Design" Version="3.1.5" PrivateAssets="All" />
    <PackageReference Include="Syncfusion.EJ2.AspNet.Core" Version="19.1.0.59" />
    <PackageReference Include="System.ServiceModel.Http" Version="4.5.3" />
  </ItemGroup>

It might be something trivial but nothing is mentioned about this in any documentation and my searches haven't returned anything.

Answer 1

I was already preparing to rewrite the C# code in JS but Syncfusion support came through with the correct answer: double escaping.
So the correct syntax is:

var userCultParams = new { @@params = userCultLookupDDL };

Although this might seem logic to many I didn't realize that that would be the solution and didn't even try it.
I did read about it in other posts but just couldn't match it with my situation. My reasoning was: I am already escaping the keyword params as @params

Maybe others run into this (and with their head against the wall) like me so I thought it would be wise to share this.