PostSharpDeveloping Custom AspectsValidating Aspect Usage
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Validating Aspect Usage

Some aspects make sense only on a specific subset of targets. For instance, an aspect may require being applied to non-static methods only. Another aspect may not be compatible with methods that have ref or out parameters. If these constraints are not respected, these aspects will fail at run time. However, defects detected by the compiler are always cheaper to fix than ones detected later. So, as the developer of an aspect, you should ensure that the build will fail if your aspect is being used on an invalid target.

Using [MulticastAttributeUsage]

The first level of protection is to configure multicasting properly with [MulticastAttributeUsageAttribute], as described in the article Adding Aspects Declaratively Using Attributes. However, this approach can only filter based on characteristics that are supported by the multicasting component.

Implementing CompileTimeValidate

The best way to validate aspect usage is to override the CompileTimeValidate(Object) method of your aspect class.

In this example, we will show how an aspect RequirePermissionAttribute can require being applied only to methods of types that implement the ISecurable interface.

  1. Inherit from one of the pre-built aspects. In this case, OnMethodBoundaryAspect.

    public class RequirePermissionAttribute: OnMethodBoundaryAspect
  2. Override the CompileTimeValidate(Object) method.

    public override bool CompileTimeValidate(MethodBase target) 
  3. Perform a check to see if the target class implements the interface in question.

    Type targetType = target.DeclaringType; 
    if (!typeof(ISecurable).IsAssignableFrom(targetType)) 
  4. If the target does not implement the interface you must signal the compilation process that this target should not have the aspect applied to it. There are two ways to do this. The first option is to throw an InvalidAnnotationException.

    if (!typeof(ISecurable).IsAssignableFrom(targetType)) 
      throw new InvalidAnnotationException("The target type does not implement ISecurable."); 
  5. The second option is to emit an error message to the compilation process.

    if (!typeof(ISecurable).IsAssignableFrom(targetType)) 
      Message.Write(SeverityType.Error, "Custom01", 
                    "The target type does not implement ISecurable.", target); 
     return false; 

You may have noticed that CompileTimeValidate(Object) returns a boolean value. If you only return false from this method the compilation process will silently ignore it. You must either throw the InvalidAnnotationException or emit an error message to not silently ignore the false return value.

Making use of the CompileTimeValidate(Object) method is a great way to encode custom rules for applying aspects to target code. While it could be used to duplicate the functionality of the AttributeTargetTypeAttributes or AttributeTargetMemberAttributes, its real power is to go beyond those filtering techniques. By using CompileTimeValidate(Object) you are able to filter aspect application in any manner that you can interrogate your codebase using reflection.

Using Message Sources

If you plan to raise many messages, you may prefer to define your own MessageSource. A MessageSource is backed by a managed resource mapping error codes to error messages.

In order to create your own MessageSource, you should:

  • Create an implementation of the IMessageDispenser. Typically, implement the GetMessage(String) method using a large switch statement. To each message will correspond a string

  • Create a static instance of the MessageSource class for your message source. For instance, the following code defines a message source based on a message dispenser:

    internal class ArchitectureMessageSource : MessageSource
        public static readonly ArchitectureMessageSource Instance = new ArchitectureMessageSource();
        private ArchitectureMessageSource() : base( "PostSharp.Architecture", new Dispenser() )
        private class Dispenser : MessageDispenser
            public Dispenser() : base( "CUS" )
            protected override string GetMessage( int number)
                switch ( number )
                    case 1:
                        return "Interface {0} cannot be implemented by {1} because of the [InternalImplement] constraint.";
                    case 2:
                        return "{0} {1} cannot be referenced from {2} {3} because of the [ComponentInternal] constraint.";
                    case 3:
                        return "Cannot use [ComponentInternal] on {0} {1} because the {0} is not internal.";
                    case 4:
                        return "Cannot use [Internal] on {0} {1} because the {0} is not public.";
                        return null;
  • Then you can use a convenient set of methods on your MessageSource object:

    MyMessageSource.Instance.Write( classType, SeverityType.Error, "CUS001", new object[] { interfaceType, classType } );

You can also emit information and warning messages.


Use ReflectionSearch to perform complex queries over System.Reflection.

Validating Attributes That Are Not Aspects

You can validate any attribute derived from <xref:System.Attribute> by implementing the interface IValidableAnnotation.

See Also