How to set json serializer settings in ASP.NET Core

In ASP.NET Core, the AddMvc method returns an IMvcBuilder implementation, which provides an extension method called AddJsonOptions to configure JSON serialization settings. The newer methods AddControllers, AddControllersWithViews, and AddRazorPages also return an IMvcBuilder and can be chained with AddJsonOptions in the same way as AddMvc. This allows you to configure JSON serialization settings for your controllers, Razor Pages, or MVC views.

To configure the JSON serialization globally, you need to modify the AddControllers or AddMvc method in the ConfigureServices method of your Startup.cs or Program.cs file (for ASP.NET Core 6 and later).

using Microsoft.AspNetCore.Builder;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;
using Newtonsoft.Json; // If you want to use Newtonsoft.Json

var builder = WebApplication.CreateBuilder(args);

// Configure JSON serialization settings
builder.Services.AddControllers()
    .AddJsonOptions(options =>
    {
        // Example: Use camel case for property names
        options.JsonSerializerOptions.PropertyNamingPolicy = System.Text.Json.JsonNamingPolicy.CamelCase;
        
        // Example: Ignore null values
        options.JsonSerializerOptions.DefaultIgnoreCondition = System.Text.Json.JsonIgnoreCondition.WhenWritingNull;

        // Example: Configure custom converters if needed
        // options.JsonSerializerOptions.Converters.Add(new YourCustomConverter());
    });

// If you want to use Newtonsoft.Json instead of System.Text.Json
builder.Services.AddControllers()
    .AddNewtonsoftJson(options =>
    {
        // Example: Camel case property names using Newtonsoft.Json
        options.SerializerSettings.ContractResolver = new Newtonsoft.Json.Serialization.CamelCasePropertyNamesContractResolver();
        
        // Example: Ignore null values using Newtonsoft.Json
        options.SerializerSettings.NullValueHandling = NullValueHandling.Ignore;
    });

var app = builder.Build();

// Use the application
app.MapControllers();

app.Run();

For versions prior to ASP.NET Core 6, you would typically use Startup.cs. Here's how to configure JSON serialization settings in ConfigureServices.

public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers()
        .AddJsonOptions(options =>
        {
            // Example: Use camel case for property names
            options.JsonSerializerOptions.PropertyNamingPolicy = System.Text.Json.JsonNamingPolicy.CamelCase;
            
            // Example: Ignore null values
            options.JsonSerializerOptions.DefaultIgnoreCondition = System.Text.Json.JsonIgnoreCondition.WhenWritingNull;

            // Example: Add custom converters
            // options.JsonSerializerOptions.Converters.Add(new YourCustomConverter());
        });

    // If you want to use Newtonsoft.Json instead of System.Text.Json
    services.AddControllers()
        .AddNewtonsoftJson(options =>
        {
            // newtonsoft json serializer settings
            // Example: Camel case property names
            options.SerializerSettings.ContractResolver = new Newtonsoft.Json.Serialization.CamelCasePropertyNamesContractResolver();

            // Example: Ignore null values
            options.SerializerSettings.NullValueHandling = NullValueHandling.Ignore;
        });
}

You can control the behavior of JSON serialization on a per-action basis or globally via middleware.

[HttpGet]
public IActionResult Get()
{
    var result = new { Name = "John", Age = 30 };
    
    return new JsonResult(result, new JsonSerializerOptions
    {
        PropertyNamingPolicy = JsonNamingPolicy.CamelCase,
        DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull
    });
}

To set JSON serializer settings globally in ASP.NET Core, you generally modify the AddJsonOptions method in your Program.cs or Startup.cs file. You can use System.Text.Json or Newtonsoft.Json based on your preference.