PostSharp.Samples / PostSharp.Samples.Encryption / FilterMethodArgumentsAspect.cs
using System.Reflection;
using PostSharp.Aspects;
using PostSharp.Extensibility;
using PostSharp.Serialization;

namespace PostSharp.Samples.Encryption
{
  [PSerializable]
  [LinesOfCodeAvoided(2)]
  public sealed class FilterMethodArgumentsAspect : MethodInterceptionAspect
  {
    private FilterAttribute[] filters;

    internal FilterMethodArgumentsAspect(MethodBase method)
    {
      filters = new FilterAttribute[method.GetParameters().Length];
    }


    internal void SetFilter(ParameterInfo parameter, FilterAttribute filter)
    {
      if (filters[parameter.Position] != null)
      {
        // If you want to support more than 1 filter, you will need a more complex data structure and to cope with priorities.
        Message.Write(parameter, SeverityType.Error, "MY01", "There cannot be more than 1 filter on parameter {0}.",
          parameter);
        return;
      }

      filters[parameter.Position] = filter;
    }

    public override void OnInvoke(MethodInterceptionArgs args)
    {
      for (var i = 0; i < filters.Length; i++)
      {
        var filter = filters[i];
        if (filter != null)
          args.Arguments[i] = filter.ApplyFilter(args.Arguments[i]);
      }

      args.Proceed();
    }
  }
}