Conditionally use custom middleware - asp.net-core

Question

I created my custom authentication middleware in asp. net core project, and registered it as shown below:
public class MyAuthenticationMidleware
{
private readonly RequestDelegate _next;
public ConnectAuthenticationMidleware(RequestDelegate next)
{
_next = next;
}
public async Task Invoke(HttpContext context)
{
if (!UserIsAuthenticated())
{
context.Response.StatusCode = 401;
return;
}
...
await _next.Invoke(context);
}
}
public static class MyAuthenticationMidlewareExtensions
{
public static IApplicationBuilder UseMyAuthentication(this IApplicationBuilder builder)
{
return builder.UseMiddleware<MyAuthenticationMidleware>();
}
}
In Startup:
public void Configure(...)
{
app.UseStaticFiles();
app.UseMyAuthentication();
app.UseMvc();
}
This works correctly - authentication middleware is run for each request. If user is not authenticated, 401 is returned. Otherwise specific mvc action is invoked.
What I tried to do was to prevent the authentication middleware from running for some specific actions. I used MapWhen method to create another extension method and used it as follows:
public static class MyAuthenticationMidlewareExtensions
{
public static IApplicationBuilder UseMyAuthentication(this IApplicationBuilder builder)
{
return builder.UseMiddleware<MyAuthenticationMidleware>();
}
public static IApplicationBuilder UseMyAuthenticationWhen(this IApplicationBuilder builder, Func<HttpContext, bool> predicate)
{
return builder.MapWhen(predicate, applicationBuilder => applicationBuilder.UseMyAuthentication());
}
}
public void Configure(...)
{
app.UseStaticFiles();
app.UseMyAuthenticationWhen(context => context.Request.Path != "xyz");
app.UseMvc();
}
Unfortunately, this doesn't work as expected. The middleware is invoked only when path is different than "xyz", but it seems that it short-circuts the whole chain - no mvc specific actions or filters are invoked.
Probably my understanding of MapWhen is incorrect. Is there any way to get the result I want?

Answer 1

MapWhen creates a new pipeline branch when the supplied predicate is true, and that branch does not rejoin with the main branch where you have UseMvc().
You can change your extension method to use UseWhen instead of MapWhen. UseWhen rejoins with the main pipeline so that your UseMvc() will still get called.
Note: While the above link references aspnet-contrib, the UseWhen extension method is now part of Microsoft.AspNetCore.Http.Abstractions.
This allows you to keep UseMvc() explicit in your Configure method instead of hidden away in your authentication extension method, where it really has no business being.

Answer 2

MapWhen is used to seperate middleware pipeline. If you want to use mvc for branced pipeline you need to add separetely. So you should use .UseMvc(); in extension method like below:
public static IApplicationBuilder UseMyAuthenticationWhen(this IApplicationBuilder builder, Func<HttpContext, bool> predicate)
{
return builder.MapWhen(predicate, applicationBuilder =>
{
applicationBuilder.UseMyAuthentication();
applicationBuilder.UseMvc();
});
}
However i wouldn't go with your way. For authentication middleware i would implement my own middleware like Simple token based authentication/authorization in asp.net core for Mongodb datastore and use Authorize attribute for authorization mvc actions.