Putting C# attributes to use

Recently in an interview I was asked if I had worked with attributes, I had, and I told them how, I don’t know why I didn’t put it on my blog, anyways now I will do it.

What I have done is that, I have created 2 attribues [notNULL] and [notNIL] these can be applied to any function parameter, what happens is, if you pass null to a function that has the parameter decorated as [notNULL] then a ArgumentNullException is thrown and for [notNIL] ArgumentException is thrown.

To achieve this, the class which is going to use these attributes must be decorated with another attribute ([the] in our case) this attribute adds a property for itself and this property adds a sink where you get a hook and can query the method/parameter/etc and execute any business logic you want – in my case I am simply checking if the parameter has the said attributes and throw exception.

The Client Program (Program.cs)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Text;

using System.Runtime.Remoting.Activation;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            theClass theObject = new theClass();
            theObject.operate("a",3,null);

            theChild theChildObject = new theChild();
            theChildObject.working("");
        }
    }       
}

The Class (theClass.cs)

using System;
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication1
{
    [the]
    class theClass:ContextBoundObject
    {
        public void operate([notNULL]string s, int a, [notNIL]string ss)
        {
                Console.WriteLine("theClass did its work");
        }
    }
    [the]
    class theChild : theClass
    {
        public void working([notNULL] string s) 
        {
            Console.WriteLine("theChild did its work");
        }
    }
}

NotNULL/NotNIL Attributes (notNULL.cs)

using System;
using System.Collections.Generic;
using System.Text;

namespace ConsoleApplication1
{
    [AttributeUsage(AttributeTargets.Parameter)]
    class notNULLAttribute:Attribute
    {
        public notNULLAttribute() { }

    }
    [AttributeUsage(AttributeTargets.Parameter)]
    class notNILAttribute : Attribute
    {
        public notNILAttribute() { }

    }
}

The Attribute (theAttribute.cs)

using System;
using System.Collections.Generic;
using System.Runtime.Remoting.Contexts;
using System.Runtime.Remoting.Activation;
using System.Text;

namespace ConsoleApplication1
{
    [AttributeUsage(AttributeTargets.Class,Inherited=true)]
    class theAttribute:Attribute,IContextAttribute
    {
        public theAttribute()
        { }

        public void GetPropertiesForNewContext(IConstructionCallMessage msg)
        {
            theProperty property = new theProperty();
            msg.ContextProperties.Add(property);
        }

        public bool IsContextOK(Context ctx, IConstructionCallMessage msg)
        {
            return false;
        }
    }
}

The Property (theProperty.cs)

using System;
using System.Collections.Generic;
using System.Runtime.Remoting.Contexts;
using System.Runtime.Remoting.Messaging;
using System.Text;

namespace ConsoleApplication1
{
    class theProperty:IContextProperty,IContributeServerContextSink
    {
        public theProperty() { }

        public string Name { get { return "MyProperty"; } }

        public bool IsNewContextOK(Context newctx) { return true; }

        public void Freeze(Context newContext){}

        public IMessageSink GetServerContextSink(IMessageSink nextSink)
        {
            return new theSink(nextSink);
        }
    }
}

The Sink (theSink.cs)

using System;
using System.Collections.Generic;
using System.Runtime.Remoting.Contexts;
using System.Runtime.Remoting.Activation;
using System.Runtime.Remoting.Messaging;
using System.Reflection;
using System.Text;

namespace ConsoleApplication1
{
    class theSink:IMessageSink
    {
        private IMessageSink next;

        public theSink(IMessageSink next) { this.next = next; }

        public IMessage SyncProcessMessage(IMessage msg)
        {
            if (msg is IMethodMessage)
            {
                IMethodMessage methodMessage = msg as IMethodMessage;
                
                MethodBase methodInfo = methodMessage.MethodBase;
                
                ParameterInfo[] parameters = methodInfo.GetParameters();
                foreach (ParameterInfo p in parameters)
                {
                    object[] attributes = p.GetCustomAttributes(true);
                    foreach (Attribute a in attributes)
                    {
                        if (a.GetType() == typeof(notNULLAttribute))
                        {
                            //Console.WriteLine("found " + p.ToString() + " at position "+p.Position+" which has notnull attribute");                          
                            if (null == methodMessage.GetArg(p.Position))
                            {
                                Console.WriteLine("Error : " + p.ToString() + " cannot be null in "+methodInfo.Name);
                                throw new ArgumentNullException("Not null attribute violated");
                            }
                        }
                        if (a.GetType() == typeof(notNILAttribute))
                        {
                            //Console.WriteLine("found " + p.ToString() + " at position "+p.Position+" which has notnull attribute");
                            if(null == methodMessage.GetArg(p.Position) ||
                            "" == methodMessage.GetArg(p.Position).ToString())
                            {
                                Console.WriteLine("Error : " + p.ToString() + " cannot be nil in "+methodInfo.Name);
                                throw new ArgumentException("Not nil attribute violated");
                            }
                        }
                    }
                }
            }
            IMessage replyMessage = next.SyncProcessMessage(msg);
            return replyMessage;
        }

        public IMessageSink NextSink { get { return this.next; } }

        public IMessageCtrl AsyncProcessMessage(IMessage msg, IMessageSink replySink)
        {
            return null;
        }
    }
}