.NET Core Web API key

There is a nice article on using api keys in header requests on this link: http://www.mithunvp.com/write-custom-asp-net-core-middleware-web-api/

To summarise, in ASP.NET Core , you can use Middleware to control the http pipeline configuration. Middleware effectively replaces HttpHandlers, which were used in ealier versions of asp.net MVC.

This is what I did in the end:

 public static void ApiKeyMiddlewear(this IApplicationBuilder app, IServiceProvider serviceProvider)
    {
        app.Use(async (context, next) =>
        {
            if (context.Request.Path.StartsWithSegments(new PathString("/api")))
            {
                // Let's check if this is an API Call
                if (context.Request.Headers["ApiKey"].Any())
                {
                    // validate the supplied API key
                    // Validate it
                    var headerKey = context.Request.Headers["ApiKey"].FirstOrDefault();
                    await ValidateApiKey(serviceProvider, context, next, headerKey);
                }
                else if (context.Request.Query.ContainsKey("apikey"))
                {
                    if (context.Request.Query.TryGetValue("apikey", out var queryKey))
                    {
                        await ValidateApiKey(serviceProvider, context, next, queryKey);
                    }
                }
                else
                {
                    await next();
                }
            }
            else
            {
                await next();
            }
        });
    }

    private static async Task ValidateApiKey(IServiceProvider serviceProvider, HttpContext context, Func<Task> next, string key)
    {
        // validate it here
        var valid = false;
        if (!valid)
        {
            context.Response.StatusCode = (int)HttpStatusCode.Unauthorized;
            await context.Response.WriteAsync("Invalid API Key");
        }
        else
        {
            var identity = new GenericIdentity("API");
            var principal = new GenericPrincipal(identity, new[] { "Admin", "ApiUser" });
            context.User = principal;
            await next();
        }
    }

This has changed quite a bit since I answered the original question (Answer is still valid). But you can read about this here: https://tidusjar.github.io/2019/03/25/net-core-jwt-api-key/