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

I've just swapped our project from ASP .Net Core 1.0.0-rc2-final to 1.0.0. Our website and client have stopped working because of the capitalization of JSON properties. For example, this line of JavaScript now fails

for (var i = 0; i < collection.Items.length; i++){

because the controller now calls the array "items" instead of "Items". I have made no changes beyond installing the updated packages and editing the project.json file. I have not changed the C# model files which still capitalize their properties.

Why have the ASP.Net Core controllers started returning JSON with lower-cased properties? How do I go back to them honoring the case of the property names from the model?

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);
            ...
        }

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();
    }