Question

I have this class :

public class Repo
{
   public Repo() : this(ConfigurationManager.AppSettings["identity"],       ConfigurationManager.AppSettings["password"])

    {

    }

   public Repo(string identity,string password)
   {
       //Initialize properties.
   }

}

I added a line to web.config so that this type will be automatically constructed by Unity container.

but during the execution of my application, I receive this error message :

  "System.InvalidOperationException : the parameter identity could not be resolved when attempting to call constructor Repo(String identity, String password)  -->Microsoft.Practices.ObjectBuilder2.BuildFailedException : The current Build operation ...."

1) Why isn’t Unity using the default constructor ?

2) Suppose I want Unity to use the second constructor (the parametized one), How do I
pass that information to Unity via the configuration file ?

Answer 1

Unity by default picks the constructor with the most parameters. You have to tell Unity to use a different one explicitly.

One way to do this is with the [InjectionConstructor] attribute like this:

using Microsoft.Practices.Unity;

public class Repo
{
   [InjectionConstructor]
   public Repo() : this(ConfigurationManager.AppSettings["identity"], ConfigurationManager.AppSettings["password"])
   {

   }

   public Repo(string identity,string password)
   {
       //Initialize properties.
   }
}

A second way of doing this, if your opposed to cluttering up classes/methods with attributes, is to specify which constructor to use when configuring your container using an InjectionConstructor:

IUnityContainer container = new UnityContainer();
container.RegisterType<Repo>(new InjectionConstructor());

From the documentation:

How Unity Resolves Target Constructors and Parameters

When a target class contains more than one constructor, Unity will use
the one that has the InjectionConstructor attribute applied. If there
is more than one constructor, and none carries the
InjectionConstructor attribute, Unity will use the constructor with
the most parameters. If there is more than one such constructor (more
than one of the "longest" with the same number of parameters), Unity
will raise an exception.