[Question] Help with a InvalidCastException when calling wcf services

Question

[Question] Help with a InvalidCastException when calling wcf services

jpareias opened this issue 5 years ago · comments

I have two plugins that both call wcf services. Both are referring System.ServiceModel with version 4.7. I'm marking both plugins as unloadable.

Only one of the plugins can call his service (always the first to be executed). Whenever the other plugin calls a service, the InvalidCastException is thrown (Exception: Unable to cast object of type 'generatedProxy_2' to type 'System.ServiceModel.Channels.ServiceChannelProxy').

The only way to get around the problem is to have each plugin reference a different version of System.Model. I also tried to make System.ServiceModel has PrivateAssemblies.

it is necessary to do something different to better isolate each plugin?

Nate McMaster · Answer 1 · Tue Jan 28 2020 14:51:01 GMT+0800 (China Standard Time)

It's hard to say. Can you share steps to reproduce this error? It's hard to tell what is happening from your description.

jpareias · Answer 2 · Sat Feb 01 2020 21:15:23 GMT+0800 (China Standard Time)

Sorry, sometimes it all makes sense just in my head.

I have an application that at a given point in the user registration process sends the user data to other systems/application. Each user data synchronization is done by a separate plugin.
My application provides an interface with a method that each plugin must implement.
After the user registers, the application loads all the plugin's that implement this interface and executes the method for each one.
After successful loaded the plugin, the plugin is not unloaded. They are kept in memory.

If more than one of the plugins makes a calls to soap services using these libraries (System.ServiceModel and his dependencies). Only the first plugin to run does it successfully. Any of the following gives the error 'generatedProxy_2' to type 'System.ServiceModel.Channels.ServiceChannelProxy'.

If I have each plugin using a different version of the libraries. The error no longer occurs.

Small code sample:

var basicHttpBinding = new BasicHttpBinding(BasicHttpSecurityMode.Transport);
var factory = new ChannelFactory<UserServices>(basicHttpBinding, new EndpointAddress("https://services/user.svc"));
var serviceClient = factory.CreateChannel();

The error occurs in the last line of code.

Nate McMaster · Answer 3 · Mon Feb 03 2020 04:01:05 GMT+0800 (China Standard Time)

Thanks for more detail. I'm still not sure how to go about finding a solution here. Based on what you said, I have a starting point, but no definite answers.

Hint 1: you said the error says something about a generated proxy. Is WCF using System.Reflection.Emit to generate new types? Or using Microsoft.CodeAnalysis to do runtime complications?

Hint 2: error doesn't occur if using different WCF versions. Maybe WCF had a bug? Try upgrading?

jpareias · Answer 4 · Wed Feb 12 2020 17:40:02 GMT+0800 (China Standard Time)

My first thought was precisely that. I validate with other web services, but I always got the same error.
I believe WCF is using System.Reflection.Emit. The plugin's are generating their code through visual studio 2019. Visual studio is using Microsoft.Tools.ServiceModel.Svcutil v2.0.2.

I ended up definitely solving the problem by passing the creation of the wcf client to the host. The plug-ins will use a class made available by the host through DI to create the wcf client. The code that i show above will be executed in this class.

Nate McMaster · Answer 5 · Thu Feb 13 2020 13:33:09 GMT+0800 (China Standard Time)

Ok. Closing because I think your solution to move WCF into the host is a good move. .NET assembly load context and system.ref.emit apparently don't play nice. There's only so much this library can do to coerce certain plugin behaviors out of .NET's runtime.

Mertsch · Answer 6 · Tue Aug 18 2020 22:03:02 GMT+0800 (China Standard Time)

@natemcmaster I was running into the same issue and found a solution thanks to this post.
Maybe add this to the FAQ, as it seems to be a general thing with WCF services.

jpareias · Answer 7 · Sat Oct 02 2021 20:13:47 GMT+0800 (China Standard Time)

Hi. The idea will be to pass the creation of the channel to a service. At the host I have a service (IPluginServiceFactory) with a method ( CreateChannel). I have a class that implements this service ( WcfServiceFactory). The method is responsible for creating the channel. In the plugin I just have to add the service (IPluginServiceFactory ) in the constructor. And call the method whenever necessary. Host code: IPluginServiceFactory.cs TChannel CreateChannel<TChannel>(ChannelFactory<TChannel> factory); WcfServiceFactory.cs public TChannel CreateChannel<TChannel>(ChannelFactory<TChannel> factory) { return factory.CreateChannel(); } Plugin code: var channelFactory = new ChannelFactory<Users>(basicHttpBinding, new EndpointAddress(new Uri("Http://somehost/webservices/userservice.svc"))); var serviceClient = pluginServiceFactory.CreateChannel(channelFactory); it is also possible to have the (IPluginServiceFactory) create the ChannelFactory. in this case the method would only receive the information to create the EndpointAddress. After this initial problem I ended up having another situation. There were two plugins sharing the same business. Each of the plugins invoked the exact same wcf service and the exact same methods. To solve this, in the build of each plugin, I define a different assembly version.