Response interceptor example

Program.cs

var bld = WebApplication.CreateBuilder(args);
bld.Services
   .SwaggerDocument()
   .AddFastEndpoints();

var app = bld.Build();
app.UseFastEndpoints()
   .UseSwaggerGen();
app.Run();

sealed class ResponseInterceptor : IResponseInterceptor
{
    public Task InterceptResponseAsync(object response,
                                       int statusCode,
                                       HttpContext ctx,
                                       IReadOnlyCollection<ValidationFailure> failures,
                                       CancellationToken ct)
    {
        ctx.Response.Headers.Append("x-some-header", "some value");

        return Task.CompletedTask;
    }
}

sealed class MyEndpoint : Ep.NoReq.Res<object>
{
    public override void Configure()
    {
        Get("hello");
        AllowAnonymous();
        ResponseInterceptor(new ResponseInterceptor()); // can be set globally with endpoint configurator as well
    }

    public override async Task HandleAsync(CancellationToken c)
    {
        await SendInterceptedAsync(new { Messsge = "hello!" }, statusCode: 200);
    }
}