How is it possible to access the GUI elements from another unreferenced project?

Question

To make it simple, I have two projects:

• project A : windows form application

• project B : a C# class library project

B is referenced on A, so A can not be referenced on B. B got the main business of the whole program, and I need to display some information (feedbacks, warnings, errors ..) on the GUI (project A).

It is very annoying the sending back the information from the project B to the project A everytime to display it.

Any ideas?

Answer 1

The class library should notify the UI project of a change that it can react to and draw the correct UI.

There are a number of aproaches from a method that the UI layer passes to the project B (callbacks) to events the UI layer can subscribe to.

Below is an example with events. See here for more details on raising custom events.

  public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            var projectB = new ProjectB();
            projectB.OnUpdateStatus += projectB_OnUpdateStatus;
            projectB.Run();
        }

        private void projectB_OnUpdateStatus(string message)
        {
            MessageBox.Show(message);
        }
    }



public class ProjectB
{
    public delegate void StatusUpdateHandler(string message);
    public event StatusUpdateHandler OnUpdateStatus;

    public void Run()
    {
        OnUpdateStatus("Updated");
    }
}

From you comemnts you have mention that you have huge methods inside the business, and need to display some information on the GUI while the business method is not finished yet the processing.

That become all about threads. With the example I have given above it will work however the UI may not update as the currect thread is too busy doing the work. The UI may even lock up while the background task happens.

If you use just mulitpule threads and the aprach above you will find the issue that you cannot update the UI on another thread.

The background worker thread gets around this issue by doing the work on a 2nd thread but the events fire back to the main UI thread. This keeps the UI respoisive and updated.

 public partial class Form1 : Form
    {
        private BackgroundWorker _backgroundfWorker;

        public Form1()
        {
            InitializeComponent();
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            _backgroundfWorker = new BackgroundWorker();
            _backgroundfWorker.ProgressChanged += OnUpdateStatus;
            _backgroundfWorker.DoWork += backgroundWorker1_DoWork;
            _backgroundfWorker.WorkerReportsProgress = true;
            _backgroundfWorker.RunWorkerAsync();
        }

        private void backgroundWorker1_DoWork(object sender, System.ComponentModel.DoWorkEventArgs e)
        {
            var b = new ProjectB();
            b.OnUpdateStatus += ProjectBOnUpdateStatus;
            b.Run();
        }

        private void ProjectBOnUpdateStatus(string message)
        {
            _backgroundfWorker.ReportProgress(0, message);
        }

        private void OnUpdateStatus(object sender,ProgressChangedEventArgs progressChangedEventArgs)
        {
            MessageBox.Show(progressChangedEventArgs.UserState.ToString());
        }
    }

Answer 2

A class library shouldn't reference an assembly that contains user controls and forms. It should be the other way around. Messaging between controls and libraries in WinForms generally happens using events or commands. You definitely shouldn't have code like form1.ErrorMessage = "Something bad happened" in a class library.

i have huge methods inside the business, and i need to display some information on the GUI while the business method is not finished yet the processing

Then you create a ReportProgress event or whatever in the class library, and subscribe to that from the GUI to display the progress.