Question

How to create an aspect checking for null references on all methods in a class in postsharp.

using System; using System.Collections.Generic; using System.Linq; using System.Text;  namespace test {     [MethodParameterNullCheck]     internal class Class     {         public Class()         {          }          public void MethodA(int i, ClassA a, ClassB b)         {               //Some business logic         }     } } 

The aspect [MethodParameterNullCheck] should then unfold to the following code:

using System; using System.Collections.Generic; using System.Linq; using System.Text;  namespace test {     [MethodParameterNullCheck]     internal class Class     {         public Class()         {          }          public void MethodA(int i, ClassA a, ClassB b)         {             if (a == null) throw new ArgumentNullException('Class->MethodA: Argument a of ClassA is not allowed to be null.');             if (b == null) throw new ArgumentNullException('Class->MethodA: Argument b of ClassB is not allowed to be null.');             // Some Business Logic         }     } } 

I will appreciate if you can give me a sample implementation on this to get me startet on AOP with postsharp.

Answer 1

An alternative approach is an extension method:

public static void ThrowIfNull<T>(this T obj, string parameterName) where T : class {     if(obj == null) throw new ArgumentNullException(parameterName); } 

then call:

foo.ThrowIfNull('foo'); bar.ThrowIfNull('bar'); 

The T : class pervents us accidentally boxing ints etc.

Re AOP; Jon Skeet has a sample for something similar here – but covering a single method/parameter.

Here’s the aspect reproduced; note that this aspect covers only 1 argument at a time, and is method-specific, but in general I’d argue that this is perfectly reasonable… however, you could probably change it.

using System; using System.Reflection; using PostSharp.Laos;  namespace IteratorBlocks {     [Serializable]     class NullArgumentAspect : OnMethodBoundaryAspect     {         string name;         int position;          public NullArgumentAspect(string name)         {             this.name = name;         }          public override void CompileTimeInitialize(MethodBase method)         {             base.CompileTimeInitialize(method);             ParameterInfo[] parameters = method.GetParameters();             for (int index = 0; index < parameters.Length; index++)             {                 if (parameters[index].Name == name)                 {                     position = index;                     return;                 }             }             throw new ArgumentException('No parameter with name ' + name);         }          public override void OnEntry(MethodExecutionEventArgs eventArgs)         {             if (eventArgs.GetArguments()[position] == null)             {                 throw new ArgumentNullException(name);             }         }     } }