ServiceHostFactory lifetime by Mark Seemann

For a while I've been wondering about the lifetime behavior of custom ServiceHostFactory classes hosted in IIS. Does IIS create an instance per request? Or a single instance to handle all requests?

I decided to find out, so I wrote a little test service. The conclusion seems to be that there is only a single instance that servers as a factory for all requests. This is very fortunate, since it gives us an excellent place to host a DI Container. The container can then manage the lifetime of all components, including Singletons that will live for the duration of the process.

If you are curious how I arrived at this conclusion, here's the code I wrote. I started out with this custom ServiceHostFactory:

public class PocServiceHostFactory : ServiceHostFactory
{
    private static int number = 1;
 
    public PocServiceHostFactory()
    {
        Interlocked.Increment(
            ref PocServiceHostFactory.number);
    }
 
    protected override ServiceHost CreateServiceHost(
        Type serviceType, Uri[] baseAddresses)
    {
        return new PocServiceHost(
            PocServiceHostFactory.number, serviceType,
            baseAddresses);
    }
}

The idea is that every time a new instance of ServiceHostFactory is created, the static number is incremented.

The PocServiceHostFactory just forwards the number to the PocServiceHost:

public class PocServiceHost : ServiceHost
{
    public PocServiceHost(int number, Type serviceType,
        Uri[] baseAddresses)
        : base(serviceType, baseAddresses)
    {
        foreach (var cd in 
            this.ImplementedContracts.Values)
        {
            cd.Behaviors.Add(
                new NumberServiceInstanceProvider(
                    number));
        }
    }
}

The PocServiceHost just forwards the number to the NumberServiceInstanceProvider:

public class NumberServiceInstanceProvider : 
    IInstanceProvider, IContractBehavior
{
    private readonly int number;
 
    public NumberServiceInstanceProvider(int number)
    {
        this.number = number;
    }
 
    #region IInstanceProvider Members
 
    public object GetInstance(
        InstanceContext instanceContext,
        Message message)
    {
        return this.GetInstance(instanceContext);
    }
 
    public object GetInstance(
        InstanceContext instanceContext)
    {
        return new NumberService(this.number);
    }
 
    public void ReleaseInstance(
        InstanceContext instanceContext,
        object instance)
    {
    }
 
    #endregion
 
    #region IContractBehavior Members
 
    public void AddBindingParameters(
        ContractDescription contractDescription,
        ServiceEndpoint endpoint,
        BindingParameterCollection bindingParameters)
    {
    }
 
    public void ApplyClientBehavior(
        ContractDescription contractDescription,
        ServiceEndpoint endpoint,
        ClientRuntime clientRuntime)
    {
    }
 
    public void ApplyDispatchBehavior(
        ContractDescription contractDescription,
        ServiceEndpoint endpoint,
        DispatchRuntime dispatchRuntime)
    {
        dispatchRuntime.InstanceProvider = this;
    }
 
    public void Validate(
        ContractDescription contractDescription,
        ServiceEndpoint endpoint)
    {
    }
 
    #endregion
}

The relevant part of NumberServiceInstanceProvider is the GetInstanceMethod that simply forwards the number to the NumberService:

public class NumberService : INumberService
{
    private readonly int number;
 
    public NumberService(int number)
    {
        this.number = number;
    }
 
    #region INumberService Members
 
    public int GetNumber()
    {
        return this.number;
    }
 
    #endregion
}

As you can see, NumberService simply returns the injected number.

The experiment is now to host NumberService in IIS using PocServiceHostFactory. If there is only one ServiceHostFactory per application process, we would expect that the same number (2) is returned every time we invoke the GetNumber operation. If, on the other hand, a new instance of ServiceHostFactory is created per request, we would expect the number to increase for every request.

To test this I spun up a few instances of WcfTestClient.exe and invoked the operation. It consistently returns 2 across multiple clients and multiple requests. This supports the hypothesis that there is only one ServiceHostFactory per service process.

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