There is already a listener on IP endpoint error

The error message "There is already a listener on IP endpoint 0.0.0.0:9999" is a characteristic error on WCF services when another instance of host or other service is running on the same port, causing a conflict.

The problem cause appears to be simple, if in fact there is another service running on the same port, otherwise, this post will help you to find the real cause of the problem.

Check the symptoms bellow, everyone should occur:

1. The process hosting the service is interrupted.
2. Apparently the requests are been attended, but never returns, until a timeout exception is thrown.
3. Whenever you restarts the Service Host, an exception is thrown: There is already a listener on IP endpoint 0.0.0.0:10101.  Make sure that you are not trying to use this endpoint multiple times in your application and that there are no other applications listening on this endpoint. 
4. Probably you have Microsoft Visual Studio installed on the machine, check on Windows Process (Task Manager) if exists a process called “vsjitdebbuger.exe” running.
   

The priority is reestablish your services, then kill the “vsjitdebbuger.exe” process, see bellow:

Now your services should run normally, but you need identify why the service stayed unavailable.

The “vsjitdebbuger.exe” is a Visual Studio tool for debug .NET applications, and runs automatically when a process throws an unhandled exception killing the current process, capturing the instance for debug it.

This process should be terminated, because it keeps active the instance of failed process in debug mode, because of this, the service accepts requests still, however no have return and prevents that another service instance is created.

Possible Causes of the problem:

1. Out of Memory. The service may be using much resources, make sure that you are releasing resources, disposing variables correctly and using the appropriate instance context mode.
2. Implementations on service class Construtor. Any fail in service class constructor, without a proper treatment, rethrowing the exception, will cause a fatal fault in the service, killing him.
3. Threads. Any fail in thread scope, without a proper treatment, rethrowing the exception, will cause a fatal fault in the service, killing him.

I don't recommend implementations in service class constructor, except in a very specific case.

In both cases, service class constructor and threads, always put your code into a try-catch block. In catch scope you need handle the exception and make the appropriated treatment, like log it. Never allow the exception to be rethrow.