How can I set JSON Serializer in ASP.NET Core (.NET 5)?

You can using JSON serializer in ASP.NET Core (Using .NET 5), by adding Nuget package first

Install-Package Microsoft.AspNetCore.Mvc.NewtonsoftJson

and then in your Startup.cs, use the below code

 public void ConfigureServices(IServiceCollection services)
        {
            services.AddControllersWithViews();
            services.AddControllers().AddNewtonsoftJson(options =>
            {
                // Use the default property (Pascal) casing
                options.SerializerSettings.ContractResolver = new DefaultContractResolver();
            });
        }

or, if you want to add Custom Serializer, you can add this code, in above configuration

   // Configure a custom converter
    options.SerializerOptions.Converters.Add(new YourCustomJsonConverter());

Using .NET Core 3

Simply use this (Should work in .NET 5 also)

services.AddMvc().AddJsonOptions(o =>
{
    o.JsonSerializerOptions.PropertyNamingPolicy = null;
    o.JsonSerializerOptions.DictionaryKeyPolicy = null;
});

This should work.