Using Attributes to Call Methods

Question

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.

Answer 1

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

Answer 2

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.