<% @ServiceHost Service="MyNamespace.MyServiceImplementationTypeName" %>
It consists of the @ServiceHost directive and a single attribute, Service. The value of the Service attribute is the common language runtime (CLR) type name of the service implementation. Using this directive is basically equivalent to creating a service host using the following code.
new ServiceHost( typeof( MyNamespace.MyServiceImplementationTypeName ) );
Additional hosting configuration, such as creating a list of base addresses for the service can also be done. You can also use a custom ServiceHostFactory to extend the directive for use with custom hosting solutions.
The IIS applications that host WCF services are not responsible for managing the creation and lifetime of ServiceHost instances. The managed WCF hosting infrastructure creates the necessary ServiceHost instance dynamically when the first request is received for the .svc file. The instance is not released until either it is closed explicitly by code or when the application is recycled.
NOTE: This content has been picked from http://msdn.microsoft.com/en-us/library/aa751792.aspx
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