I have various individual methods which all need to perform the same functions before continuing on with their own implementation. Now I could implement these functions in each method, but I was wondering if there's a way to exploit attributes to do this? As a very simple example, all network calls have to check for a network connection.
public void GetPage(string url)
{
if(IsNetworkConnected())
...
else
...
}
This would work, but I'd have to call the IsNetworkConnected method for each method that uses the network and handle it individually. Instead, I'd like to do this
[NetworkCall]
public void GetPage(string url)
{
...
}
If the network is unavailable, an error method is called instead and GetPage is ignored, otherwise GetPage is invoked.
This sounds very much like Aspect Orientated Programming, but I don't want to implement an entire framework for a few calls. This is more of a learning exercise than an implementation one, so I was curious as to how something like this would be best implemented.
You can use PostSharp, it is aspect-oriented framework for .NET, it seems quite easy to use:
static void Main(string[] args)
{
Foo();
}
[IgnoreMethod(IsIgnored=true)]
public static void Foo()
{
Console.WriteLine("Executing Foo()...");
}
[Serializable]
public class IgnoreMethodAttribute : PostSharp.Aspects.MethodInterceptionAspect
{
public bool IsIgnored { get; set; }
public override void OnInvoke(PostSharp.Aspects.MethodInterceptionArgs args)
{
if (IsIgnored)
{
return;
}
base.OnInvoke(args);
}
}
Method-Level Aspects feature is available in the free edition: http://www.sharpcrafters.com/purchase/compare
Run-Time Performance:
Because PostSharp is a compiler technology, most of the expensive work is done at build time, so that applications start quickly and execute fast. When generating code, PostSharp takes the assumption that calling a virtual method or getting a static field is an expensive operation. Contrary to rumor, PostSharp does not use System.Reflection at run time.
http://www.sharpcrafters.com/postsharp/performance
I don't think you can do this with attributes only, because they are not executed by the runtime if you're not actively doing something with them. A lightweight approach would be Ninject with Interceptions extension, it is a framework, but a very thin one, and one you might already be using for DI anyway.
Another option, but a bit more involved, could be based on MEF, and then you can use attributes and do something during with them during activation.
You're right, it sounds a lot like AOP.
What you're after sounds like compile time weaving? I.e. the attribute is turned into additional code by the compiler.
You could look at how to implement this...
Generating additional code through a custom attribute
http://www.cs.columbia.edu/~eaddy/wicca/ &
http://www.sharpcrafters.com/aop.net/compiletime-weaving
all refer to tools and techniques for doing this.
Or you could use an AOP framework. IMHO, you should look at AOP frameworks.
Related
I have a bunch of methods with varying signatures. These methods interact with a fragile data connection, so we often use a helper class to perform retries/reconnects, etc. Like so:
MyHelper.PerformCall( () => { doStuffWithData(parameters...) });
And this works fine, but it can make the code a little cluttery. What I would prefer to do is decorate the methods that interact with the data connection like so:
[InteractsWithData]
protected string doStuffWithData(parameters...)
{
// do stuff...
}
And then essentially, whenever doStuffWithData is called, the body of that method would be passed in as an Action to MyHelper.PerformCall(). How do I do this?
So, I just went to a AOP session this weekend, and here's a way to do it with PostSharp:
[Serializable]
public class MyAOPThing : MethodInterceptionAspect
{
public override void OnInvoke(MethodInterceptionArgs args)
{
Console.WriteLine("OnInvoke! before");
args.Proceed();
Console.WriteLine("OnInvoke! after");
}
}
And then decorate methods with [MyAOPThing]. Easy!
.NET Attributes are meta-data, not decorators / active components that automatically get invoked. There is no way to achieve this behaviour.
You could use attributes to implement decorators by putting the decorator code in the Attribute class and call the method with a helper method that invokes the method in the Attribute class using Reflection. But I'm not sure this would be a big improvement over just calling the "decorator-method" directly.
"Decorator-Attribute":
[AttributeUsage(AttributeTargets.Method)]
public class MyDecorator : Attribute
{
public void PerformCall(Action action)
{
// invoke action (or not)
}
}
Method:
[MyDecorator]
void MyMethod()
{
}
Usage:
InvokeWithDecorator(() => MyMethod());
Helper method:
void InvokeWithDecorator(Expression<Func<?>> expression)
{
// complicated stuff to look up attribute using reflection
}
Have a look at frameworks for Aspect Oriented Programming in C#. These may offer what you want.
This type of problem is pretty much what AOP (aspect oriented programming) aims to solve.
Tools such as PostSharp can provide cross-cutting concerns by re-writing the compiled code.
Scott Hanselman's podcast recently discussed AOP, so it might be worth having a listen.
Without the use of code generation, you can't do much against it. You could probably make the syntax better.
But what about using an extension method?
class static MyHelper
{
Wrap<T>(this object service, Action<T> action)
{
// check attribute and wrap call
}
}
usage:
RawFoo foo = ...
foo.Wrap(x => x.doStuffWithData(parameters...));
This is trivial, but you can't make sure that Wrap had been used.
You could implement a generic decorator. This decorator would be used once to wrap the service and then you can't call it without wrapping.
class Decorator<T>
{
private T implementor;
Decorator(T implementor)
{
this.implementor = implementor;
}
void Perform<T>(Action<T> action)
{
// check attribute here to know if wrapping is needed
if (interactsWithData)
{
MyHelper.PerformCall( () => { action(implementor) });
}
else
{
action(implementor);
}
}
}
static class DecoratorExtensions
{
public static Decorator<T> CreateDecorator<T>(T service)
{
return new Decorator<T>(service);
}
}
Usage:
// after wrapping, it can't be used the wrong way anymore.
ExtendedFoo foo = rawFoo.CreateDecorator();
foo.Perform(x => x.doStuffWithData(parameters...));
Check out aspect oriented frameworks. But be aware that while they hide complexity in each method, the existence of AoP features could make your program harder to maintain. It's a tradeoff.
It seems like what you want is similar to behavior of an IoC container or test runner framework, where it isn't actually executing from your assembly, but is running a dynamically emitted assembly built around your code. (Smarter folks than I have called this AOP in other answers)
So maybe in a stub for your app you could scan the other assemblies, build those emitted assemblies (which call MyHelper.PerformCall with the body of the decorated methods) then your program runs against the emitted code.
By no means would I start down the road of trying to write this without evaluating whether some existing AOP framework could accomplish what you need. HTH>
Seeing that you're willing to add a line of code to every method that needs this, why not just make the call to MyHelper from within the method itself, like this?
protected string doStuffWithData(parameters...)
{
MyHelper.PerformCall( () => { doStuffWithDataCore(parameters...) });
}
private string doStuffWithDataCore(parameters...) {
//Do stuff here
}
How can I Invoke a method in a class whenever a method in the same class is called?
Instead of doing this:
public class MyClass
{
private void InvokeMe() { }
public void Method1()
{
this.InvokeMe();
// some codes
}
public void Method2()
{
this.InvokeMe();
// some codes
}
public void Method3()
{
this.InvokeMe();
// some codes
}
// more methods
}
I want to automatically invoke the InvokeMe private method instead of putting it on each of the public methods in MyClass because we have too many method in that class and that class always change.
My code is in C#, Framework 4.0, build in Visual Studio 2010 Pro.
Please help, Thanks in advance.
This could be accomplished using some Aspect programming (take a look at PostSharp or one of its alternatives). Alternatively since you are using .NET 4 you could create a DynamicObject implementation to act as a proxy for your class and when a method is called on it have it call the InvokeMe method first.
UPDATE
I've added a link to the DynamicObject documentation above for further reading. There's a good MSDN blog available here that discusses the relevant points.
Postsharp is an option but it has a button that says "Purchase" (unless you are happy with their free starter edition).
If you are looking for other, less expensive options, try for example Ninject or Spring.NET.
In some of my projects I have used Ninject exactly as you described and it was easy to use. Please be aware that interception (or Aspect-Oriented Programming) introduces a new set of concepts and it pays off to be aware of that.
Also note that just because it worked for my projects doesn't mean that it will work for your projects as well as other factors may influence your choice.
Is there a way to wrap methods in other methods transparently in C#? I want to achieve what is done by Moose's around functionality: http://search.cpan.org/perldoc?Moose::Manual::MethodModifiers
EDIT: And by transparent I mean without modifying the original method.
I think you're looking for what's termed Aspect Oriented Programming. There are many C# libraries to help with this. One is called PostSharp (The free version of PostSharp supports this functionality). Here is an example similar to the moose example. This creates a Trace Attribute which you can use on other methods to tack on this extra functionality:
[Serializable]
public class TraceAttribute : OnMethodBoundaryAspect
{
public override void OnEntry( MethodExecutionArgs args )
{
Trace.WriteLine("about to call method");
}
public override void OnExit(MethodExecutionArgs args)
{
Trace.WriteLine("just finished calling method");
}
}
You would add it to method "Foo" by placing the Trace attribute right before it:
[Trace]
public void Foo() { /* ... */ }
Now when Foo executes, the above OnEntry method will run before it, and OnExit will run right after.
Indeed, they're called "delegates" in .NET. See:
http://alexdresko.com/2010/07/25/using-idisposable-objects-responsibly-the-easy-way/
http://alexdresko.com/2010/07/27/using-delegates-to-eliminate-duplicate-code/
for help.
You can achieve the same effect by utilizing a dynamic proxy. An example is the Castle Dynamic Proxy.
Such frameworks leverage the C# reflection facilities to construct 'proxy' or 'wrapper' classes. So, keep that in mind. There is a certain amount of overhead because of this. Alternatively you can use frameworks that can create classes statically via code generation.
No, not the way it's done in Moose. You might want to look into some AOP library.
Some isolation libraries implement functionality that allows replacing calls to methods with "detours" or mock methods. You may be able to use the same functionality to implement the interception you are referring to. For more details, check the following:
Rhino Mocks stubs and mocks are only good for interfaces?
http://research.microsoft.com/en-us/projects/moles/
http://www.typemock.com/typemock-isolator-product3/
I have a bunch of methods with varying signatures. These methods interact with a fragile data connection, so we often use a helper class to perform retries/reconnects, etc. Like so:
MyHelper.PerformCall( () => { doStuffWithData(parameters...) });
And this works fine, but it can make the code a little cluttery. What I would prefer to do is decorate the methods that interact with the data connection like so:
[InteractsWithData]
protected string doStuffWithData(parameters...)
{
// do stuff...
}
And then essentially, whenever doStuffWithData is called, the body of that method would be passed in as an Action to MyHelper.PerformCall(). How do I do this?
So, I just went to a AOP session this weekend, and here's a way to do it with PostSharp:
[Serializable]
public class MyAOPThing : MethodInterceptionAspect
{
public override void OnInvoke(MethodInterceptionArgs args)
{
Console.WriteLine("OnInvoke! before");
args.Proceed();
Console.WriteLine("OnInvoke! after");
}
}
And then decorate methods with [MyAOPThing]. Easy!
.NET Attributes are meta-data, not decorators / active components that automatically get invoked. There is no way to achieve this behaviour.
You could use attributes to implement decorators by putting the decorator code in the Attribute class and call the method with a helper method that invokes the method in the Attribute class using Reflection. But I'm not sure this would be a big improvement over just calling the "decorator-method" directly.
"Decorator-Attribute":
[AttributeUsage(AttributeTargets.Method)]
public class MyDecorator : Attribute
{
public void PerformCall(Action action)
{
// invoke action (or not)
}
}
Method:
[MyDecorator]
void MyMethod()
{
}
Usage:
InvokeWithDecorator(() => MyMethod());
Helper method:
void InvokeWithDecorator(Expression<Func<?>> expression)
{
// complicated stuff to look up attribute using reflection
}
Have a look at frameworks for Aspect Oriented Programming in C#. These may offer what you want.
This type of problem is pretty much what AOP (aspect oriented programming) aims to solve.
Tools such as PostSharp can provide cross-cutting concerns by re-writing the compiled code.
Scott Hanselman's podcast recently discussed AOP, so it might be worth having a listen.
Without the use of code generation, you can't do much against it. You could probably make the syntax better.
But what about using an extension method?
class static MyHelper
{
Wrap<T>(this object service, Action<T> action)
{
// check attribute and wrap call
}
}
usage:
RawFoo foo = ...
foo.Wrap(x => x.doStuffWithData(parameters...));
This is trivial, but you can't make sure that Wrap had been used.
You could implement a generic decorator. This decorator would be used once to wrap the service and then you can't call it without wrapping.
class Decorator<T>
{
private T implementor;
Decorator(T implementor)
{
this.implementor = implementor;
}
void Perform<T>(Action<T> action)
{
// check attribute here to know if wrapping is needed
if (interactsWithData)
{
MyHelper.PerformCall( () => { action(implementor) });
}
else
{
action(implementor);
}
}
}
static class DecoratorExtensions
{
public static Decorator<T> CreateDecorator<T>(T service)
{
return new Decorator<T>(service);
}
}
Usage:
// after wrapping, it can't be used the wrong way anymore.
ExtendedFoo foo = rawFoo.CreateDecorator();
foo.Perform(x => x.doStuffWithData(parameters...));
Check out aspect oriented frameworks. But be aware that while they hide complexity in each method, the existence of AoP features could make your program harder to maintain. It's a tradeoff.
It seems like what you want is similar to behavior of an IoC container or test runner framework, where it isn't actually executing from your assembly, but is running a dynamically emitted assembly built around your code. (Smarter folks than I have called this AOP in other answers)
So maybe in a stub for your app you could scan the other assemblies, build those emitted assemblies (which call MyHelper.PerformCall with the body of the decorated methods) then your program runs against the emitted code.
By no means would I start down the road of trying to write this without evaluating whether some existing AOP framework could accomplish what you need. HTH>
Seeing that you're willing to add a line of code to every method that needs this, why not just make the call to MyHelper from within the method itself, like this?
protected string doStuffWithData(parameters...)
{
MyHelper.PerformCall( () => { doStuffWithDataCore(parameters...) });
}
private string doStuffWithDataCore(parameters...) {
//Do stuff here
}
What I am looking for is a way to call a method after another method has been invoked but before it is entered. Example:
public class Test {
public void Tracer ( ... )
{
}
public int SomeFunction( string str )
{
return 0;
}
public void TestFun()
{
SomeFunction( "" );
}
}
In the example above I would like to have Tracer() called after SomeFunction() has been invoked by TestFun() but before SomeFunction() is entered. I'd also like to get reflection data on SomeFunction().
I found something interesting in everyone's answers. The best answer to the question is to use Castle's DynamicProxy; however, this is not that I'm going to use to solve my problem because it requires adding a library to my project. I have only a few methods that I need to "trace" so I've chosen to go with a modified "core" methodology mixed with the way Dynamic Proxy is implemented. I explain this in my answer to my own question below.
Just as a note I'm going to be looking into AOP and the ContextBoundObject class for some other applications.
You can use a dynamic proxy (Castle's DynamicProxy for example) to intercept the call, run whatever code you wish, and then either invoke your method or not, depending on your needs.
Use a *Core method:
public int SomeFunction(string str)
{
Tracer();
return SomeFunctionCore(str);
}
private int SomeFunctionCore(string str)
{
return 0;
}
A number of the .NET APIs use this (lots do in WPF).
Use delegates!
delegate void SomeFunctionDelegate(string s);
void Start()
{
TraceAndThenCallMethod(SomeFunction, "hoho");
}
void SomeFunction(string str)
{
//Do stuff with str
}
void TraceAndThenCallMethod(SomeFunctionDelegate sfd, string parameter)
{
Trace();
sfd(parameter);
}
You want to look into Aspect Oriented Programming. Here's a page I found for AOP in .NET: http://www.postsharp.org/aop.net/
Aspect Oriented Programming involves separating out "crosscutting concerns" from code. One example of this is logging - logging exists (hopefully) across all of your code. Should these methods all really need to know about logging? Maybe not. AOP is the study of separating these concerns from the code they deal with, and injecting them back in, either at compile-time or run-time. The link I posted contains links to several tools that can be used for both compile-time and run-time AOP.
.NET has a class called ContextBoundObject that you can use to setup message sinks to do call interception as long as you don't mind deriving from a base class this will give you what you are looking for without taking an library dependency.
You would have to use some form of AOP framework like SpringFramework.NET to do that.
If you need to do this on large scale (i.e. for every function in a program) and you don't want to hugely alter the source, you might look into using the .NET Profiling API. Its a little hairy to use since you have to build free-threaded COM objects to do so, but it gives you an enormous amount of control over the execution of the program.
This is the solution I've choosen to solve my problem. Since there is no automatic (attribute like) way to make this work I feel it is the least obtrusive and allows the functionality to be turned on and off by choosing what class get instantiated. Please note that this is not the best answer to my question but it is the better answer for my particular situation.
What's going on is that we're simply deriving a second class that sometimes or always be instantiated in place of its parent. The methods that we want to trace (or otherwise track) are declared virtual and reimplemented in the derived class to perform whatever actions we want to trace and then the function is called in the parent class.
public class TestClass {
public virtual void int SomeFunction( string /*str*/ )
{
return 0;
}
public void TestFun()
{
SomeFunction( "" );
}
}
public class TestClassTracer : TestClass {
public override void int SomeFunction( string str )
{
// do something
return base.SomeFunction( str );
}
}