Question

I am building a .NET solution with a GUI mode(one project) and a console mode (another project). Each of these projects references a shared class library that holds the shared logical guts behind each app. I want the DLL to be as isolated as possible from each client application. how can I have the DLL report progress to the gui? Right now, the GUI project references the DLL. VS2010 / .NET won’t let me reference the GUI from the DLL because it would be a circular reference.

How can I do this? Are there any general principles for how to handle this in .NET design, or software design in general? Is there a way to use a callback without a reference to the calling project?

Answer 1

It sounds to me like you need a custom event in your DLL that you then attach to in your GUI.

Here’s some code to make it more understandable.

First off lets create a mock of our DLL that does something similar to loading and also defines an event we can attach to in our GUI that received the completed percentage:

public class MyDLLType
{
  public decimal PercentageComplete = 5;

  public event EventHandler<CustomEventArgs> StatusChanged;

  public virtual void OnStatusChanged(CustomEventArgs e)
  {
   if(StatusChanged != null)
   {
      StatusChanged(this, e);
   }
  }

  public void DoSomething()
  {

     var eventArgs = new CustomEventArgs();
     eventArgs.PercentageComplete = PercentageComplete;

     OnStatusChanged(eventArgs);

     if(PercentageComplete < 100)
     {
       Thread.Sleep(500);
       PercentageComplete++;
       DoSomething();
     }
  }
}

Now we have a mock DLL that exposes a StatusChanged event for our GUI to hook into. Here’s a mock GUI taking advantage of this:

public class MyGUI
{
void Main()
{
    var myNiceDLLType = new MyDLLType();
    myNiceDLLType.StatusChanged += UpdateStatusChange;
    myNiceDLLType.DoSomething();
}

public void UpdateStatusChange(object sender, CustomEventArgs e)
{
 var percentageComplete = e.PercentageComplete;
 Console.WriteLine(percentageComplete);
}
}

and Bingo, our GUI is receiving the complete percentage from the DLL without the DLL needing to know anything about the GUI.

The CustomEventArgs I used above looks like this:

public class CustomEventArgs : EventArgs
{
    public decimal PercentageComplete{get;set;}
}