No parameterless constructor defined for this object

Posted on: 2013-09-09

Few times ago, I open up a solution that was perfectly working previously and realized that this one compile but when I launch the browser raise me an exception :

No parameterless constructor defined for this object

Usually, this exception occur when you are using Asp.Net MVC with Unity and this one cannot resolve the parameters of your constructor. This mean that you request a controller method but the parameters aren't provided. To solve this problem, usually people that develop with Asp.Net MVC and Unity use the Unity.MVC3 code that let you set the Unity dependency resolver to the framework that will resolve controller's parameters when Asp.Net do a http request. The second option is to have constructor without parameter, which is often not possible.

But, in my scenario, I was using Unity.MVC3 and it was working before. The problem reside in the Web.Config file. Visual Studio has inserted an attribute that cause all the chaos of this exception.

The web.config has an attribute called "xmlns" with the value "http://schemas.microsoft.com/.NetConfiguration/v2.0" for the element configuration.

 <configuration xmlns="http://schemas.microsoft.com/.NetConfiguration/v2.0"> <configSections> ... 

By removing the xmlns attribute (and its value) everything worked as before. No more exception "No parameterless constructor defined for this object", no more problem. So, if you have a parameterless constructor exception when using Asp.Net MVC and Unity, check out your configuration file to be sure that this attribute is not set.