OnMethodBoundaryAspect Class

The OnMethodBoundaryAspect aspect results in the target method to be wrapped into a try ... catch ... finally block. You can implement four advices: OnEntry(MethodExecutionArgs), executed at the beginning of the block; OnSuccess(MethodExecutionArgs), executed only when the method is successful (i.e. does not result in an exception); OnException(MethodExecutionArgs), invoked when the method results in an exception; and OnExit(MethodExecutionArgs), always executed after method execution (whether the method resulted in an exception or not).

Schematically, the aspect transforms the original method as follows:

int MyMethod(object arg0, int arg1)
{
   OnEntry();
   try
   {
    // Original method body. 
    OnSuccess();
    return returnValue;
  }
  catch ( Exception e )
  {
    OnException();
  }
  finally
  {
    OnExit();
  }
}

Note that this code is only schematic; actually generated instructions are more complex because they have to cope with parameter boxing and control flow modification, among others.

An object of type MethodExecutionArgs is passed to every advice of this aspect. This object allows you to:

• Get or set arguments . Input and output arguments are available on the property Arguments . You can change output ( out ) and by-reference ( ref ) arguments, but not input arguments. If you need to modify arguments passed by value, consider using a MethodInterceptionAspect . (see property );
• Get the current exception . The current exception is available on property Exception , but from the OnException(MethodExecutionArgs) advice. You cannot replace the exception by setting this property. Throw a new exception from the aspect if you need to replace the current exception.
• Get or set the return value . The return value is available on the ReturnValue property.
• Change the method control flow . You can change the value of the FlowBehavior property to specify whether the target method should execute after the execution of the current advice. This may be useful to implement a caching aspect or an exception handler.
• Share state between advices . You can use the MethodExecutionTag property to store state between the execution of different advices related to the same execution of a method. For instance, you can store a cache key in OnEntry(MethodExecutionArgs) and find it back on OnSuccess(MethodExecutionArgs) . Using MethodExecutionTag is the only way to share state that is both thread-safe and reentrant. All other implementations are buggy.

Note

All classes implementing IAspect should typically be marked as serializable using the SerializableAttribute or PSerializableAttribute custom attribute . Fields that are only used at runtime (and unknown at compile-time) should be carefully marked with the NonSerializedAttribute or PNonSerializedAttribute custom attribute. When PostSharp is used on a platform that does not support aspect serialization (such as .NET Compact Framework, Silverlight, or Windows Phone), or when another aspect serializer is used, it is not necessary to mark the aspect class as serializable. For more information, see Aspect Serialization .

Reference

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