Manipulates the default behavior of Model Binding to initialize strings to String.Empty. (as in ASP.NET MVC 1.0)

EmptyStringModelBindingAspect.cs

using System;
using System.Linq;
using System.Reflection;
using MySolution;
using PostSharp.Aspects;
using PostSharp.Extensibility;

[assembly: EmptyStringModelBindingAspect(
    AttributeTargetTypes = @"regex:[^\.]*\.Controllers\..*Controller",
    AttributeTargetTypeAttributes = MulticastAttributes.Public,
    AttributeTargetElements = MulticastTargets.Method,
    AttributeTargetMemberAttributes = MulticastAttributes.Public)]

namespace MySolution
{
    [Serializable]
    public class EmptyStringModelBindingAspect : MethodInterceptionAspect
    {
        public override void OnInvoke(MethodInterceptionArgs args)
        {
            for (int i = 0; i < args.Arguments.Count; i++)
            {
                FixString(args, i);
                FixStringsInObjects(args.Arguments[i]);
            }
            args.Proceed();
        }

        private static void FixString(MethodInterceptionArgs args, int i)
        {
            if (args.Arguments[i] is string &&
                args.Arguments[i] == null)
            {
                args.Arguments.SetArgument(i, string.Empty);
            }
        }

        private static void FixStringsInObjects(object obj)
        {
            if (obj == null)
            {
                return;
            }

            Type type = obj.GetType();
            PropertyInfo[] properties = (from p in type.GetProperties()
                                         where p.PropertyType == typeof(string) &&
                                               p.CanRead &&
                                               p.CanWrite &&
                                               p.GetValue(obj, null) == null
                                         select p).ToArray();

            foreach (PropertyInfo item in properties)
            {
                item.SetValue(obj, string.Empty, null);
            }
        }

        public override bool CompileTimeValidate(MethodBase method)
        {
            return !(method.Name.StartsWith("get_") || method.Name.StartsWith("set_"));
        }
    }
}

Manipulates the default behavior of Model Binding to initialize strings to String.Empty. (as in ASP.NET MVC 1.0)

In ASP.NET MVC 1.0 the default behavior of Model Binding was to initialize strings to String.Empty. However, later versions of ASP.NET MVC initialize Strings to null. This results in some nasty NullReferenceExceptions all over the code. This solution makes use of the Aspect Oriented Programming (AOP) framework PostSharp.

See the complete introduction here: http://blog.johanneshoppe.de/2013/01/gist-asp-net-mvc-bind-string-property-as-string-empty-instead-of-null/

how could you tell the type of an argument if its value is null? For me (using MVC 4) "if(args.Arguments[i] is string)" always returns false

I solved it like this:

   private static void FixString(MethodInterceptionArgs args, int i)
    {
        ParameterInfo[] paramInfo = args.Method.GetParameters();          
        if (paramInfo[i].ParameterType == typeof(string) && args.Arguments[i] == null)
            {
                args.Arguments.SetArgument(i, string.Empty);
            }
    }