Interceptors

Creating custom interceptors

Interceptors require the IInterceptor interface. Any public method within the intercepted class will be intercepted (including getters and setters)

public class MyInterceptor : IInterceptor
{
    public void Intercept(IInvocation invocation)
    {
       //Calls the next method in the chain - The last one will be the 
       //original method that was intercepted
       invocation.Proceed();
    }
}

Read this for information about registering an interceptor to components

Interceptors doesn’t have to call Proceed every time

Interceptors are a good tool for implementing cross-cutting concerns such as logging or authentication. Let’s say we have a following service:

public interface IService
{
    string CreateOrder(NetworkCredential credentials, Order orderToCreate);
    string DeleteOrder(NetworkCredential credentials, int orderId);
}

public class Service : IService
{
    public string CreateOrder(NetworkCredential credentials, Order orderToCreate)
    {
        // ...

        return "Order was created succesfully.";
    }

    public string DeleteOrder(NetworkCredential credentials, int orderId)
    {
        // ...

        return "Order was deleted succesfully.";
    }
}

We can create following interceptor:

public class AuthorizationInterceptor : IInterceptor
{
    public void Intercept(IInvocation invocation)
    {
        var userCredentials = invocation.Arguments[0] as NetworkCredential;
        
        if (userCredentials.UserName == "tom" && userCredentials.Password == "pass123")
            // this ^ verification is obviously silly, never do real security like this
        {
            invocation.Proceed();
        }
        else
        {
            invocation.ReturnValue = $"User '{userCredentials.UserName}' was not authenticated.";
        }
    }
}

That can be registered and utilized like this:

var container = new WindsorContainer();
container.Register(
    Component.For<AuthorizationInterceptor>(),
    Component.For<IService>().ImplementedBy<Service>().Interceptors<AuthorizationInterceptor>());

var service = container.Resolve<IService>();

System.Diagnostics.Debug.Assert(
    service.DeleteOrder(new NetworkCredential { UserName = "paul", Password = "pass321" }, 8)
        == "User 'paul' was not authenticated.");

System.Diagnostics.Debug.Assert(
    service.CreateOrder(new NetworkCredential { UserName = "tom", Password = "pass123" }, new Order())
        == "Order was created succesfully.");

The important lesson is that interceptor can decide to pass call and if it doesn’t, it can supply arbitrary return value using ReturnValue property.

Interceptors can be chained in a fixed order

For registration like this:

var container = new WindsorContainer();
container.Register(
    Component.For<FirstInterceptor>(),
    Component.For<SecondInterceptor>(),
    Component.For<ThirdInterceptor>(),

    Component.For<IService>()
        .ImplementedBy<Service>()
        .Interceptors<FirstInterceptor>()
        .Interceptors<SecondInterceptor>()
        .Interceptors<ThirdInterceptor>());

var service = container.Resolve<IService>();
service.CreateOrder(new Order());

With interceptors in the following spirit:

public class FirstInterceptor : IInterceptor
{
    public void Intercept(IInvocation invocation)
    {
        Console.WriteLine("First!");
        invocation.Proceed();
        Console.WriteLine("First.");
    }
}

And with service implemented in the following way:

public interface IService
{
    void CreateOrder(Order orderToCreate);
}

public class Service : IService
{
    public void CreateOrder(Order orderToCreate)
    {
        Console.WriteLine("Creating order...");

        // ...
    }
}

We can observe output that demonstrates interceptors created in a fixed order are going to be executed in that order:

First!
Second!
Third!
Creating order...
Third.
Second.
First.

Interceptors can have dependencies

Interceptors are registered like regular components in Windsor. Like other components, they can depend on another components.

With following service for validating credentials:

public interface ICredentialsVerifier
{
    bool IsAuthorizedForService(NetworkCredential credentials);
}

public class MockCredentialsVerifier : ICredentialsVerifier
{
    public bool IsAuthorizedForService(NetworkCredential credentials)
        => credentials.UserName == "tom" && credentials.Password == "pass123";
        // this ^ verification is obviously silly, never do real security like this
}

We can use the following interceptor:

public class AuthorizationInterceptor : IInterceptor
{
    private readonly ICredentialsVerifier _credentialsVerifier;

    public AuthorizationInterceptor(ICredentialsVerifier credentialsVerifier)
    {
        _credentialsVerifier = credentialsVerifier;
    }

    public void Intercept(IInvocation invocation)
    {
        var userCredentials = invocation.Arguments[0] as NetworkCredential;

        if (_credentialsVerifier.IsAuthorizedForService(userCredentials))
        {
            invocation.Proceed();
        }
        else
        {
            invocation.ReturnValue = $"User '{userCredentials.UserName}' was not authenticated.";
        }
    }
}

We just have to properly register it in the composition root like this:

var container = new WindsorContainer();
container.Register(
    Component.For<AuthorizationInterceptor>(),

    Component.For<ICredentialsVerifier>().ImplementedBy<MockCredentialsVerifier>(),
    Component.For<IService>().ImplementedBy<Service>().Interceptors<AuthorizationInterceptor>());

var service = container.Resolve<IService>();