> Mike Valenty

Default Interceptor for Unity

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Default: the two sweetest words in the English language – Homer Simpson

Unity interception requires you to specify what kind of interceptor to use for each type you want to intercept. This can be a little painful, so I decided to take a stab at a container extension that will apply a default interceptor with optional matching rules.

This is how I want it to look:

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container
    .AddNewExtension<InterceptOnRegister>()
    .Configure<InterceptOnRegister>()
    .SetDefaultInterceptor<TransparentProxyInterceptor>()
    .AddMatchingRule(new AssemblyNameStartsWith("MyCompany"));

The basic idea here is to subscribe to the register events and configure interception if it meets our criteria. By default it will match everything and apply TransparentProxyInterceptor, but as you can see from the snippet above, you can explicitly supply another interceptor and matching rules.

Here it is:

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public class InterceptOnRegister : UnityContainerExtension {
    private IInstanceInterceptor interceptor;
    private readonly IList<IInterceptRule> rules;

    public InterceptOnRegister() {
        interceptor = new TransparentProxyInterceptor();
        rules = new List<IInterceptRule> {
            new NotUnityInterceptionAssembly(),
            new DoesNotHaveGenericMethods() };
    }

    public InterceptOnRegister AddMatchingRule(IInterceptRule rule) {
        rules.Add(rule);
        return this;
    }

    public InterceptOnRegister AddNewMatchingRule<T>() where T : IInterceptRule, new() {
        rules.Add(new T());
        return this;
    }

    public InterceptOnRegister SetDefaultInterceptor<T>() where T : IInstanceInterceptor, new() {
        interceptor = new T();
        return this;
    }

    protected override void Initialize() {
        AddInterceptionExtensionIfNotExists();

        Context.Registering +=
            (sender, e) => SetInterceptorFor(e.TypeFrom, e.TypeTo ?? e.TypeFrom);
    }

    private void AddInterceptionExtensionIfNotExists() {
        if (Container.Configure<Interception>() == null) {
            Container.AddNewExtension<Interception>();
        }
    }

    private void SetInterceptorFor(Type typeToIntercept, Type typeOfInstance) {
        if (!AllMatchingRulesApply(typeToIntercept, typeOfInstance)) {
            return;
        }

        if (interceptor.CanIntercept(typeOfInstance)) {
            Container
                .Configure<Interception>()
                .SetDefaultInterceptorFor(typeOfInstance, interceptor);
        } else if (interceptor.CanIntercept(typeToIntercept)) {
            Container
                .Configure<Interception>()
                .SetDefaultInterceptorFor(typeToIntercept, interceptor);
        }
    }

    private bool AllMatchingRulesApply(Type typeToIntercept, Type typeOfInstance) {
        foreach (var rule in rules) {
            if (!rule.Matches(typeToIntercept, typeOfInstance)) {
                return false;
            }
        }

        return true;
    }
}

And you’ll need this stuff too:

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public interface IInterceptRule {
    bool Matches(Type typeToIntercept, Type typeOfInstance);
}

public class NotUnityInterceptionAssembly : IInterceptRule {
    public bool Matches(Type typeToIntercept, Type typeOfInstance) {
        return !typeToIntercept.Assembly.Equals(typeof(Interception).Assembly);
    }
}

public class DoesNotHaveGenericMethods : IInterceptRule {
    public bool Matches(Type typeToIntercept, Type typeOfInstance) {
        return typeOfInstance.GetMethods().Count(m => m.IsGenericMethod) == 0;
    }
}

public class AssemblyNameStartsWith : IInterceptRule {
    private string match;

    public AssemblyNameStartsWith(string match) {
        this.match = match;
    }

    public bool Matches(Type typeToIntercept, Type typeOfInstance) {
        return typeOfInstance.Assembly.FullName.StartsWith(match);
    }
}

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