Custom IHttpHandler mapping in ASP.NET MVC

Question

I am trying to configure a custom IHttpHandler in web.config and I don't understand the behavior I'm getting. I'm trying this using IIS express.

This is how my Sytem.WebServer section of web.config looks like:

   <system.webServer>
    <modules runAllManagedModulesForAllRequests="false" />
    <handlers>
      <remove name="ExtensionlessUrlHandler-ISAPI-4.0_32bit" />
      <remove name="ExtensionlessUrlHandler-ISAPI-4.0_64bit" />
      <remove name="ExtensionlessUrlHandler-Integrated-4.0" />
      <add name="ExtensionlessUrlHandler-ISAPI-4.0_32bit" path="*." verb="GET,HEAD,POST,DEBUG,PUT,DELETE,PATCH,OPTIONS" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework\v4.0.30319\aspnet_isapi.dll" preCondition="classicMode,runtimeVersionv4.0,bitness32" responseBufferLimit="0" />
      <add name="ExtensionlessUrlHandler-ISAPI-4.0_64bit" path="*." verb="GET,HEAD,POST,DEBUG,PUT,DELETE,PATCH,OPTIONS" modules="IsapiModule" scriptProcessor="%windir%\Microsoft.NET\Framework64\v4.0.30319\aspnet_isapi.dll" preCondition="classicMode,runtimeVersionv4.0,bitness64" responseBufferLimit="0" />
      <add name="ExtensionlessUrlHandler-Integrated-4.0" path="*." verb="GET,HEAD,POST,DEBUG,PUT,DELETE,PATCH,OPTIONS" type="System.Web.Handlers.TransferRequestHandler" preCondition="integratedMode,runtimeVersionv4.0" />
      <add name="dynamicPngHandler" verb="*" path="*.png" type="ImagServingTest.HttpHandler.PureCustomPngHandler, ImagServingTest"/>
    </handlers>
  </system.webServer>

http ://localhost:48705/default.png (does not work, I get a 404)

http ://localhost:48705/something/default.png (does not work, I get a 404)

http ://localhost:48705/something/something/default.png (does not work, I get a 404)

http ://localhost:48705/something/something/something/default.png (works)

I don't understand why this is happening. Why does the mapping fail until I have a url with three parts after localhost? I imagine the fact that this is an MVC4 app is important (although I can't see why).

UPDATE

I've tried this using web forms instead of MVC and it works as expected.

In the MVC project I tried moving the custom handler so it was the first on the handlers' list in web.config and it still did not work.

Answer 1

The reason for the request failing for those urls is that they were being covered by MVC's default routing rule: "{controller}/{action}/{id}" because all those have defaults.

I've added routes.IgnoreRoute("{ignore}.png"); to RouteConfig and I'm now seeing the behavior that I expected