This is an add-in for Fody

Compile time decorator pattern via IL rewriting.

Introduction to Fody

This version is fork of Fody/MethodDecorator with changes I found useful

Differences from original Fody/MethodDecorator:

• No attributes or interfaces in root namespace (actually without namespace) are required
• Interceptor attribute can be declared and implemented in a separate assembly
• Init method is called before any method and receives the method reference and args
• OnEntry/OnExit/OnException methods don't receive the method reference anymore
• IntersectMethodsMarkedByAttribute attribute allows you to intersect a method marked by any attribute

Your Code

// Atribute should be "registered" by adding as module or assembly custom attribute
[module: Interceptor]

// Any attribute which provides OnEntry/OnExit/OnException with proper args
[AttributeUsage(AttributeTargets.Method | AttributeTargets.Constructor | AttributeTargets.Assembly | AttributeTargets.Module)]
public class InterceptorAttribute : Attribute, IMethodDecorator	{
    // instance, method and args can be captured here and stored in attribute instance fields
	// for future usage in OnEntry/OnExit/OnException
	public void Init(object instance, MethodBase method, object[] args) {
		TestMessages.Record(string.Format("Init: {0} [{1}]", method.DeclaringType.FullName + "." + method.Name, args.Length));
	}
	public void OnEntry() {
        TestMessages.Record("OnEntry");
    }

    public void OnExit() {
        TestMessages.Record("OnExit");
    }

    public void OnException(Exception exception) {
        TestMessages.Record(string.Format("OnException: {0}: {1}", exception.GetType(), exception.Message));
    }
}

public class Sample	{
	[Interceptor]
	public void Method()
	{
	    Debug.WriteLine("Your Code");
	}
}

What's gets compiled

public class Sample {
	public void Method(int value) {
	    InterceptorAttribute attribute = 
	        (InterceptorAttribute) Activator.CreateInstance(typeof(InterceptorAttribute));
	    
		// in c# __methodref and __typeref don't exist, but you can create such IL 
		MethodBase method = MethodBase.GetMethodFromHandle(__methodref (Sample.Method), 
														   __typeref (Sample));
	    
		object[] args = new object[1] { (object) value };
		
		attribute.Init((object)this, method, args);

		attribute.OnEntry();
	    try {
	        Debug.WriteLine("Your Code");
	        attribute.OnExit();
	    }
	    catch (Exception exception) {
	        attribute.OnException(exception);
	        throw;
	    }
	}
}

NOTE: this is replaced by null when the decorated method is static or a constructor.

IntersectMethodsMarkedByAttribute

This is supposed to be used as

// all MSTest methods will be intersected by the code from IntersectMethodsMarkedBy 
[module:IntersectMethodsMarkedBy(typeof(TestMethod))] 

You can pass as many marker attributes to IntersectMethodsMarkedBy as you want

[module:IntersectMethodsMarkedBy(typeof(TestMethod), typeof(Fact), typeof(Obsolete))]

Example of IntersectMethodsMarkedByAttribute implementation

[AttributeUsage(AttributeTargets.Module | AttributeTargets.Assembly)]
public class IntersectMethodsMarkedByAttribute : Attribute {
	// Required
	public IntersectMethodsMarkedByAttribute() {}

	public IntersectMethodsMarkedByAttribute(params Type[] types) {
		if (types.All(x => typeof(Attribute).IsAssignableFrom(x))) {
			throw new Exception("Meaningfull configuration exception");
		}
	}
	public void Init(object instance, MethodBase method, object[] args) {}
	public void OnEntry() {}
	public void OnExit() {}
	public void OnException(Exception exception) {}
}

Now all your code marked by [TestMethodAttribute] will be intersected by IntersectMethodsMarkedByAttribute methods. You can have multiple IntersectMethodsMarkedByAttributes applied if you want (don't have idea why). MethodDecorator searches IntersectMethodsMarkedByAttribute by predicate StartsWith("IntersectMethodsMarkedByAttribute")

How to get it

NuGet: https://www.nuget.org/packages/MethodDecoratorEx.Fody/

Planned

• Make Init method optional
• Add "this" as parameter to Init method if method is not static
• Pass return value to "OnExit" if method returns any

Fill free to request for features you want to see in this plugin.