Create delegate from a constructor of unknown type implementing a known interface

Question

I have a type that is created at runtime (through compilation from CodeDOM) and implements a known (at compile time) interface.

Suppose the interface is IMyInterface, and I have the Type instance Type myType that I acquired from the assembly I just compiled from the CodeDOM. The class that myType represents implements IMyInterface.

I would like to get a delegate Func<IMyInterface> that, when Invoked, will return an instance of myType.

Something I would like to call in this way:

Func<IMyInterface> createObject = GetObjectCreator<IMyInterface>(myType);
IMyInterface myObject = createObject();

I know that if I have a MethodInfo m for a parameterless method that returns an instance of myType object, then I could do something like this:

Func<IMyInterface> createObject =
  ( Func<IMyInterface> )Delegate.CreateDelegate(typeof(Func<IMyInterface>), m);

But if I don't have such a method, and the only thing I have is the type's parameterless constructor, then how do I get this delegate?

Update

Though fsimonazzi's answer did exactly what I was asking for, my approach was a bit different.

Since I control the creation and compilation of the myType Type, I added a public static method that returns an instance of that type. Then, after compiling this type, I got a MethodInfo instance for this method, and created the desired delegate calling Delegate.CreateDelegate.

CodeTypeDeclaration type = new CodeTypeDeclaration
{
    Name = "MyClass",
    IsClass = true,
    TypeAttributes = TypeAttributes.Public
};

type.BaseTypes.Add(new CodeTypeReference(typeof(IMyInterface)));

// fullName is the full name of the myType (including namespace)
var staticInstantiator = new CodeMemberMethod
{
    Name = "__Instantiator",
    ReturnType = new CodeTypeReference("MyNamespace.MyClass"),
    Attributes = MemberAttributes.Public | MemberAttributes.Static
};

staticInstantiator.Statements.Add(
   new CodeMethodReturnStatement(
       new CodeObjectCreateExpression("MyNamespace.MyClass")));

    type.Members.Add(staticInstantiator);

The above code generates this code and puts in into the class declaration

public static MyNamespace.MyClass __Instantiator()
{
    return new MyNamespace.MyClass();
}

Now compiling this code and having a myType Type instance for this class, I can do

Func<IMyInterface> createObject = ( Func<IMyInterface> )(
    Delegate.CreateDelegate(typeof(Func<IMyInterface>),
                            myType.GetMethod("__Instantiator")) );

IMyInterface obj = createObject(); // This will call MyClass.__Instantiator()

Answer 1

You can compile a simple lambda expression to get your delegate.

var del = Expression.Lambda<Func<IMyInterface>>(Expression.New(type)).Compile();

Answer 2

You can use Activator.CreateInstance( type ) to actually create an instance of your type. If you want this stuffed in a Func<IMyInterface> then you can just wrap it in a lambda:

Func<IMyInterface> createObject = () => (IMyInterface) Activator.CreateInstance( myType );
IMyInterface myObject = createObject();

UPDATE:

Since Activator.CreateInstance is evidently not what you're looking for (though I'm not entirely sure why), I suppose you could use reflection to find the parameterless constructor of the type:

public Func<T> GetObjectCreator<T>( Type type )
{
    // I'd probably add additional checks here to see that the
    // conversion to T is actually possible.

    var ctor = type.GetConstructor( Type.EmptyTypes );

    if( ctor == null ) throw new ArgumentException( "type", "Public parameterless constructor not found." )

    return () => (T) ctor.Invoke( null );
}