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WCF: The type provided as the Service attribute could not be found

This had me stumped for a little while today. Whilst trying to host a WCF service in IIS we got this message when we tried to access the .svc file:

The type 'YourType, YourAssembly', provided as the Service attribute value in the ServiceHost directive could not be found.

At first the message seemed to be very specific about not being able to find the type - almost as though it had found the assembly so I changed the YourAssembly to utter nonsense to see if the error changed. Alas no, so it was possible that the assembly couldn't be 'found', let alone the type.

Everything seemed to be in order as far as the assembly was concerned. He was in the bin folder and security was setup appropriately. The type, namespace and assembly were quadruple checked and definitely correct. We even hooked up WinDbg and used the sxe ld YourAssembly command in the hope that we'd see the module attempt to load and fail. But nothing.

So I did what I always do in these situations and reverted to my ol' skool Response.Write-ASP-classic-style debugging techniques. First step was to drop in an aspx page that used the Activator to create the type dynamically. Here's the ASPX page (no .aspx.cs required):

<%@ Page Language="C#" %>
<%@ Import Namespace="System.Runtime.Remoting" %>

ObjectHandle oh = Activator.CreateInstance("YourAssembly", "YourType");
Response.Write(oh.Unwrap().ToString() + " loaded OK");

Just drop this page next to your .svc file and navigate to it in the browser. If it works and you can see the name of your type then you have a different problem to ours. I got an error that looked like this:

Could not load file or assembly 'SomeOtherAssembly' or one of its dependencies. The system cannot find the file specified.

Aha! So we could find the type specified in the Service attribute but we couldn't find some of its dependences. That's better - turns out I was missing an assembly that had to be GAC'd for this to work.

Tags: WCF

Josh Post By Josh Twist
10:25 AM
12 Aug 2008

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Comments are closed for this post.

Posted by Darren Jefford @ 12 Aug 2008 3:12 PM
Fusion Log Viewer (FusLogVw) can also help in these circumstances - always one of the first tools I bring out to see what Fusion is up to..

Posted by Matt McGregor @ 06 Jan 2009 3:22 AM
Thanks Josh. This was really stumping me, using your ASPX route pointed out the missing reference I had. You gotta love WCF error messaging!

Posted by Charles @ 12 Jan 2009 1:08 PM
Thanks Josh! In trying to find out why couldn't find my assembly, I tried your code - and the surprising thing is that it didn't throw an exception! I'm still getting a "The type 'YourType', provided as the Service attribute value in the ServiceHost directive could not be found." error, and I have no idea why.

I see that you used the both the service type and assembly name in your ServiceHost directive, separated by ", " - I have tried that also, but I end up getting the following error:
"An error occurred during the parsing of a resource required to service this request ... The required directive 'ServiceHost' is missing."

Any suggestions? My .svc looks like this:
<%@ServiceHost Service=”BWC.NetServices.Ordering.OrderService, BWC.NetServices.Ordering” %>

Thanks in advance!

Posted by Charles @ 12 Jan 2009 1:17 PM
Doh! Nevermind... Somehow I was using quotes like ”, as opposed to " :D.


Thanks again!

Posted by Robert Hellestrae @ 26 Feb 2009 9:16 AM
Thanks for the insight!

What is the missing assembly that is required in the GAC?

Best regards, Rob

Posted by James @ 07 Apr 2009 5:46 PM
Actually, the parameters are ("Assembly", "Type"). :)

Posted by pdschuller @ 22 May 2009 10:54 AM
I had a similar problem that required me to do two things in order to resolve it:

1.) the dll that my web service project created had to be dragged over to the bin directory of the web-site-to-which-I-was-deploying. (The reason I hadnt done this was that "Microsoft .NET Framework 3.5 - Windows Communication Foundation" (p.106) had said that was optional.

2.) when it came time to create a Service Reference in the VS 2008 I was using the IP number like and I needed to use
btw, I got that correct url when I browsed to The IP worked in that case.

Posted by ongle @ 29 May 2009 4:48 PM
thanks, useful tip

Posted by Smith @ 27 Jul 2009 2:11 PM
Does this method really work?

I just used in my WCF test and all in GAC. I compiled a DLL called "IHelloWorldService.ll"
using System;
using System.Runtime.Serialization;
using System.ServiceModel;

and test.aspx always complianed
<%@ Page Language="C#" Debug="true" %>
<%@ Import Namespace="System.Runtime.Remoting" %>

ObjectHandle oh = Activator.CreateInstance("IHelloWorldService.dll", "IHelloWorldService");
Response.Write(oh.Unwrap().ToString() + " loaded OK");

also tried
("IHelloWorldService", "IHelloWorldService.dll");

Compile error:
System.IO.FileNotFoundException: Could not load file or assembly 'IHelloWorldService.dll' or one of its dependencies. The system cannot find the file specified.

Posted by kapil @ 04 Nov 2009 4:22 AM
I added your code and I got a message
Assembly loaded OK
Not sure why the error is coming while I am trying to run WCF

Posted by vicent @ 15 Nov 2009 5:07 PM
hi ,i met the erroe today,may be you can try add the the the refrence of dll file which implement the ServiceContract

Posted by Henrik @ 15 Sep 2010 2:31 PM
Thanks! You saved my day.

Posted by Bryan @ 12 Oct 2010 9:11 PM
This post just kept me from insanity. Thanks

Posted by Ram @ 29 Nov 2010 6:22 AM
Thanks , it solved my problem

Posted by Sundar Rajan @ 19 Aug 2011 5:44 AM
I also had the same issue. I checked with the namespace, interface, te class wich implements that particular service to check if there is any name mismatch, but everything was correct and still i had the issue. Finally i identified that the DLL(assembly whic has thhe implementation for this service) reference was missing in the project where i had the .SVC file ten i added the dll reference and the problem was resolved. Basically this is an example. i ave 2 projects 1 is the servicehost project which host all the services and the 2nd project has the implementation for all services. So, in the 1st project there should be a ALL reference to the second project when the service (.SVC) is hosted in browser as it expects the reference to the original class which implements the service.

Posted by Richard @ 06 Dec 2011 3:18 PM
Parameters to the method CreateInstance are in the wrong order. CreateInstance("YourAssemblyName", "YourTypeName")

Posted by josh @ 17 Jan 2012 5:51 AM
Thanks to Richard (commenter) and Clark Roberts for the fix to the CreateInstance parameter order.

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