Circular dependency in composition root

Question

I have a MDI MyMDIWinForm that needs an instance of IGUIErrorHanlder for error handling, At the same time IGUIErrorHanlder depends on the MDI MyMDIWinForm for having a place from where to launch pop-ups and other stuff.

public class MyMDIWinForm 
{
   readonly IGUIErrorHanlder  GUIErrorHandler;
   public MyMDIWinForm(IGUIErrorHanlder guiErrorHandler)
   {
       GUIErrorHandler=guiErrorHandler;
   }

   public void DumbMethod()
   {
     try
     {
        //dumb code
     }
     catch(Exception e)
     {
         GUIErrorHandler.OnError(e.Message);
     }
   }

}

public FormGUIErrorHanlder:IGUIErrorHanlder
{
     readonly  Form Parent;
     public FormGUIErrorHanlder(Form form)
     {
         //this form should be the MDI form
         Parent=form;
     }

     public OnError(string message)
     {
          Parent.ShowDialog(new PopUpForm(message));
     }
}

I want that MyMDIWinForm has a instance of FormGUIErrorHanlder and that FormGUIErrorHanlder has the same instance of MyMDIWinForm

How do I remove that circular dependency?

Many thanks

Answer 1

Can you not just use delegation?

public class MyMDIWinForm 
{
   ...

   public void DumbMethod()
   {
     try
     {
        //dumb code
     }
     catch(Exception e)
     {
         GUIErrorHandler.OnError(this, e.Message);
     }
   }

}

public FormGUIErrorHanlder:IGUIErrorHanlder
{
     ...

     public OnError(Form parentForm, string message)
     {
          parentForm.ShowDialog(new PopUpForm(message));
     }
}

Answer 2

The simple solution here is to not have the constructor injection in FormGUIErrorHandler. Instead, have an extra parameter in OnError of type Form and pass in the form from the MyMDIWinForm class.

The key here is that the dependency you were trying to model is actually too tight. The FormGUIErrorHandler class should be able to deal with any Form. You might even want to consider having only one IGUIErrorHandler in your application.

Answer 3

I finally solved by injecting a factory:

 public interface  IErrorGUIHandlerFactory
 {
        IErrorGUIHandler Create(Form form);
 }



public class MyMDIWinForm 
{
   readonly IErrorGUIHandlerFactory GUIErrorHandlerFactory;

   public MyMDIWinForm(IErrorGUIHandlerFactory guiErrorHandlerFact)
   {
       GUIErrorHandlerFactory=guiErrorHandlerFact;
   }

   public void DumbMethod()
   {
     try
     {
        //dumb code
     }
     catch(Exception e)
     {
         //use the factory passing Form as parameter
         GUIErrorHandlerFactory.Create(this).OnError(e.Message);
     }
   }

}