JSON properties now lower case on swap from ASP .Net Core 1.0.0-rc2-final to 1.0.0

Question

MVC now serializes JSON with camel case names by default

Use this code to avoid camel case names by default

  services.AddMvc()
        .AddJsonOptions(options => options.SerializerSettings.ContractResolver = new DefaultContractResolver());

Source: https://github.com/aspnet/Announcements/issues/194

In case you found this from Google and looking for a solution for Core 3.

Core 3 uses System.Text.Json, which by default does not preserve the case. As mentioned with this GitHub issue, setting the PropertyNamingPolicy to null will fix the problem.

public void ConfigureServices(IServiceCollection services)
{
...
    services.AddControllers()
            .AddJsonOptions(opts => opts.JsonSerializerOptions.PropertyNamingPolicy = null);

and if you don't want to change the global settings, for one action only it's like this:

return Json(obj, new JsonSerializerOptions { PropertyNamingPolicy = null });

You can change the behavior like this:

services
    .AddMvc()
    .AddJsonOptions(options => options.SerializerSettings.ContractResolver = new DefaultContractResolver());

See the announcement here: https://github.com/aspnet/Announcements/issues/194

For those who migrated to Core 3.1 and have Core MVC project can use following setup code in Startup.cs:

---

        public void ConfigureServices(IServiceCollection services)
        {
            ...
            services.AddControllersWithViews().AddJsonOptions(opts => opts.JsonSerializerOptions.PropertyNamingPolicy = null);
            ...
        }

For someone who does not want to set it globally, it is possible to use ContractResolver also to return as Json result:

public IActionResult MyMethod()
{
    var obj = new {myValue = 1};
    return Json(obj, new JsonSerializerSettings {ContractResolver = new DefaultContractResolver()});
}

This will fix it in dotnet core 3 webapi, so that it doesn't change your property names at all, and you return to your client exactly what you intended to.

In Startup.cs:

public void ConfigureServices(IServiceCollection services)
    {
        services.AddControllers().AddJsonOptions(options => options.JsonSerializerOptions.PropertyNamingPolicy = null);
        services.AddHttpClient();
    }

For some one who is using ASP.net WEB API ( rather than ASP.NET Core).

Add this line in your WebApiConfig.

//Comment this jsonFormatter.SerializerSettings.ContractResolver = new CamelCasePropertyNamesContractResolver();

jsonFormatter.SerializerSettings.ContractResolver = new DefaultContractResolver();

Adding this as an answer here because this comes up first in google search for web api as well.