MethodBoundaryAspect.Fody

A Fody weaver which allows to decorate methods and hook into method start, method end and method exceptions. Additionally you have access to useful method parameters.

You can easily write your own aspects for

• transaction handling
• logging
• measuring method execution time
• exception wrapping
• displaying wait cursor
• and much more ...

Supported features

• Hook into method start and end
• Hook into raised exceptions in a method
• Access method information like
  • applied object instance
  • the method itself (System.Reflection.MethodBase)
  • the passed method arguments
  • the thrown exception
  • some custom object, which can be set at the start of the method and accessed at the end (e.g. useful for transactions or timers)
• Apply aspects at different levels
  • globally in AssemblyInfo.cs
  • on class
  • on method
• Change method behavior
  • Overwrite return value to be returned from the method.

Feel free to make a Fork of this repository.

Quickstart

1. Install the MethodBoundaryAspect.Fody NuGet package (Install-Package MethodBoundaryAspect.Fody)
2. Create FodyWeavers.xml in your project and add the Weaver MethodBoundaryAspect to it (further details)
3. Write your custom aspects by deriving from OnMethodBoundaryAspect and decorate your methods (see sample below)

Sample

Short sample how a transaction aspect could be implemented.

The aspect code

using MethodBoundaryAspect.Fody;

public sealed class TransactionScopeAttribute : OnMethodBoundaryAspect
{
    public override void OnEntry(MethodExecutionArgs args)
    {
        args.MethodExecutionTag = new TransactionScope();
    }

    public override void OnExit(MethodExecutionArgs args)
    {
        var transactionScope = (TransactionScope)args.MethodExecutionTag;

        transactionScope.Complete();
        transactionScope.Dispose();
    }

    public override void OnException(MethodExecutionArgs args)
    {
        var transactionScope = (TransactionScope)args.MethodExecutionTag;

        transactionScope.Dispose();
    }
}

The applied aspect

public class Sample
{
    [TransactionScope]
    public void Method()
    {
        Debug.WriteLine("Do some database stuff isolated in surrounding transaction");
    }
}

Additional Sample

Consider an aspect written like this.

The aspect code

using static System.Console;
using MethodBoundaryAspect.Fody;

public sealed class LogAttribute : OnMethodBoundaryAspect
{
    public override void OnEntry(MethodExecutionArgs args)
    {
        WriteLine("On entry");
    }

    public override void OnExit(MethodExecutionArgs args)
    {
        WriteLine("On exit");
    }

    public override void OnException(MethodExecutionArgs args)
    {
        WriteLine("On exception");
    }
}

The applied aspect

Suppose the aspect is applied to this method:

using static System.Console;

public class Sample
{
    [Log]
    public void Method()
    {
        WriteLine("Entering original method");
        OtherClass.DoSomeOtherWork();
        WriteLine("Exiting original method");
    }
}

We would expect the method call to output this:

On entry
Entering original method
Exiting original method
On exit

If, however, the call to OtherClass.DoSomeOtherWork() throws an exception, then it will look like this instead:

On entry
Entering original method
On exception

Note that the OnExit handler is not called when an exception is thrown.

Asynchronous Sample

Consider the same aspect as above but now applied to this method:

using static System.Console;

public class Sample
{
    [Log]
    public async Task MethodAsync()
    {
        WriteLine("Entering original method");
        await OtherClass.DoSomeOtherWorkAsync();
        WriteLine("Exiting original method");
    }
}

The On entry line will be written when MethodAsync is first called on the main thread.

The Entering original method line will be written shortly thereafter, on the main thread.

The Exiting original method line will be written only after the task returned by OtherClass.DoSomeOtherWorkAsync has completed. Depending on your context and whether the given task was already complete when it was returned, this may or may not be on the main thread.

The On exit line will be written when MethodAsync returns to its caller, which may be slightly after or long before the long-running task has completed, but it will occur synchronously on the main thread.

The On exception line will be written if OtherClass.DoSomeOtherWorkAsync throws an exception, returns null (thus causing a NullReferenceException when it is awaited), or returns a faulted task. As such, this may occur long after MethodAsync itself has returned its Task to its caller. The call to OnException will take place on whichever thread the synchronization context was running when the exception occurred.

Note that, unlike for synchronous methods, an aspect for an asynchronous method will have its OnExit handler called whether or not its OnException is called. Furthermore, unlike synchronous methods, the call to OnException may take place long after the call to OnExit. If this behavior is undesirable, consider using the MethodExecutionTag to track whether the OnExit has run before the OnException. One such solution looks like this.

using static System.Console;
using MethodBoundaryAspect.Fody;

public sealed class LogAttribute : OnMethodBoundaryAspect
{
    public override void OnEntry(MethodExecutionArgs args)
    {
        WriteLine("On entry");
        arg.MethodExecutionTag = false;
    }

    public override void OnExit(MethodExecutionArgs args)
    {
        WriteLine("On exit");
        arg.MethodExecutionTag = true;
    }

    public override void OnException(MethodExecutionArgs args)
    {
        if ((bool)arg.MethodExecutionTag)
          return;
        WriteLine("On exception");
    }
}

One additional note about the asynchronous behavior: the OnExit handler runs when the MethodAsync returns to its caller, not when the asynchronous code finishes running, which may be some time later. If you need code to be run when the method's asynchronous code finishes instead of when the actual method exits, consider a solution like the following:

using static System.Console;
using MethodBoundaryAspect.Fody;

public sealed class LogAttribute : OnMethodBoundaryAspect
{
    public override void OnEntry(MethodExecutionArgs args)
    {
        WriteLine("On entry");
    }

    public override void OnExit(MethodExecutionArgs args)
    {
        if (args.ReturnValue is Task t)
            t.ContinueWith(task => WriteLine("On exit"));
    }

    public override void OnException(MethodExecutionArgs args)
    {
        WriteLine("On exception");
    }
}

Altering Method Behavior

In order to change the return value of a method, hook into its OnExit handler and set the ReturnValue property of the MethodExecutionArgs.

using MethodBoundaryAspect.Fody.Attributes;
using System;
using System.Linq;
using static System.Console;

public sealed class IndentAttribute : OnMethodBoundaryAspect
{
    public override void OnExit(MethodExecutionArgs args)
    {
        args.ReturnValue = String.Concat(args.ReturnValue.ToString().Split('\n').Select(line => "  " + line));
    }
}

public class Program
{
    [Indent]
    public static string GetLogs() => @"Detailed Log 1
Detailed Log 2";

    public static void Main(string[] args)
    {
        WriteLine(GetLogs()); // Output: "  Detailed Log 1\n  Detailed Log 2";
    }
}

This can also be used on async methods to add additional processing or exception-handling.

using MethodBoundaryAspect.Fody.Attributes;
using System;
using System.Linq;
using static System.Console;

public sealed class HandleExceptionAttribute : OnMethodBoundaryAspect
{
    public override void OnExit(MethodExecutionArgs args)
    {
        if (args.ReturnValue is Task<string> task)
        {
          args.ReturnValue = task.ContinueWith(t =>
          {
            if (t.IsFaulted)
              return "An error happened: " + t.Exception.Message;
            return t.Result;
          });
        }
    }
}

public class Program
{
    [HandleException]
    public static async Task<string> Process()
    {
      await Task.Delay(10);
      throw new Exception("Bad data");
    }

    public static async Task Main(string[] args)
    {
        WriteLine(await Process()); // Output: "An error happened: Bad data"
    }
}