How to add CamelCasePropertyNamesContractResolver in Startup.cs?

Replace services.AddMvc(); with the following.

services.AddMvc().SetupOptions<MvcOptions>(options =>
{
    int position = options.OutputFormatters.FindIndex(f => 
                                    f.Instance is JsonOutputFormatter);

    var settings = new JsonSerializerSettings()
    {
        ContractResolver = new CamelCasePropertyNamesContractResolver()
    };
    var formatter = new JsonOutputFormatter(settings, false);

    options.OutputFormatters.Insert(position, formatter);
});

UPDATE

With the current version of ASP.NET, this should do.

services.AddMvc().Configure<MvcOptions>(options =>
{
    options.OutputFormatters
               .Where(f => f.Instance is JsonOutputFormatter)
               .Select(f => f.Instance as JsonOutputFormatter)
               .First()
               .SerializerSettings
               .ContractResolver = new CamelCasePropertyNamesContractResolver();
});

UPDATE 2

With ASP.NET 5 beta5, which is shipped with Visual Studio 2015 RTM, the following code works

services.AddMvc().Configure<MvcOptions>(options =>
{
    options.OutputFormatters.OfType<JsonOutputFormatter>()
           .First()
           .SerializerSettings
           .ContractResolver = new CamelCasePropertyNamesContractResolver();
});

UPDATE 3

With ASP.NET 5 beta7, the following code works

services.AddMvc().AddJsonOptions(opt =>
{
    opt.SerializerSettings.ContractResolver = new CamelCasePropertyNamesContractResolver();
});

RC2 UPDATE

MVC now serializes JSON with camel case names by default. See this announcement. https://github.com/aspnet/Announcements/issues/194

The Configure function was removed from services.AddMvc() in either Beta 6 or 7. For Beta 7, in Startup.cs, add the following to the ConfigureServices function:

services.AddMvc().AddJsonOptions(options =>
{
    options.SerializerSettings.ContractResolver = 
        new CamelCasePropertyNamesContractResolver();
});