C# How do classes communicate in the MVP-Pattern

I think your very close to having a working MVP-like solution. Let me show you how I'd re-structure your Presenter, and give you a partial idea of how the View can interact with the Presenter.

First, let's modify the default Application start-up code in the Program.cs Class so that it invokes a method, 'ApplicationStart in the Presenter which will be declared as a static Class:

using System;
using System.Windows.Forms;

namespace MVP_View
{
    static class Program
    {
        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);

            // modified by bw
            Presenter.DataPresenter.ApplicationStart();
        }
    }
}

The Presenter:

using System.Collections.Generic;
using System.Windows.Forms;

namespace Presenter
{
    // declared static bw
    public static class DataPresenter
    {
        // declared static bw
        static Procedures_Model.DataProcedures dataprocedures = new Procedures_Model.DataProcedures();
        
        // not used bw
        //static DataViewer dataviewer = new DataViewer();

        // not used bw
        //public delegate void SaveEventHandler(string first, string last);
        //public static event SaveEventHandler SaveHandler;

        // not used bw
        // static class parameterless ctor
        //static DataPresenter()
        //{
        //    //SaveHandler = new SaveEventHandler(LoadingData);
        //}

        // added by bw
        public static void ApplicationStart()
        {
            Application.Run(new MVP_View.DataViewer());
        }

        // re-named/modified by bw
        // note: now takes two string arguments
        public static void SaveData(string first, string last)
        {
            dataprocedures.SaveData(first + " " + last);
        }

        // not used bw
        //private static void SaveData()
        //{
        //    dataprocedures.SaveData(dataviewer.getFullname());
        //}

        // added by bw
        // get the list of names from the model
        public static List<string> GetNames()
        {
            return dataprocedures.load_names();
        }
    }
}
</string>

Now, let's look at how one event in the View can interact with the Presenter:

// modified by bw
private void but_saveNames_Click(object sender, EventArgs e)
{
    // send first, and last names to the presenter
    Presenter.DataPresenter.SaveData(textBox_firstname.Text, textBox_surname.Text);
}

And, here's a partial "sketch" of the event handler to get the names shown in the RichTextBox in the View:

// modified by bw
private void but_showAll_Click(object sender, EventArgs e)
{
    richTextBox_allNames.Clear();

    // get names from DataPresenter which gets them from the Model
    // left for you to write
    // List<string> names = ?????;

    if (names.Count == 0)
    {
        richTextBox_allNames.Text = @"No names are currently saved.";
    }
    else
    {
        // left for you to write
    }
}

For you to think about:

0. in this implementation there is a (relatively small) "coupling" of the View and the Presenter caused by the fact that public static methods are exposed in the Presenter ... that means those methods are potentially exposed to other objects/classes ... what would be an alternative to reduce this coupling: to make the View more "stupid:" yes, in MVP, it's a good thing for the View to be stupid.

1. doesn't a real MVP architecture require use of an Interface ?

2. isn't a real pay-off of having an MVP architecture the ability to create multiple Views, and enable automatic updating of multiple Views as the data changes ... the change perhaps originating from any/all Views ... for which using an Interface comes in handy ? Where would an Interface be defined in an MVP implementation ?

3. what would supporting multiple views, and automatic updates of multiple views, involve ?

4. how would you create a Form to be used as a template for creating multiple views, and how would you then alter instances of this template Form to remove UI elements you didn't want ?

Below you will se my code so far.

I want to fire an event which have to get catched by the presenter so that he know that he now must call the method to save the name

The View:

namespace MVP_View
{
    public partial class DataViewer : Form
    {
        public delegate void SaveEventHandler ();
        public event SaveEventHandler button_save_clicked;

        public List<string> names = new List<string>();

        public DataViewer()
        {
            InitializeComponent();
        }

        public string getFullname()
        {
            string fullname = textBox_firstname.Text + " " + textBox_surname.Text;
            return fullname;
        }
        
        private void but_saveNames_Click(object sender, EventArgs e)
        {
            // Save full name over the presenter
        }
       
        private void but_showAll_Click(object sender, EventArgs e)
        {
            // Load a List in 'names' with all names from the Model over the presenter 

            foreach (var name in names)
            {
                richTextBox_allNames.Text = richTextBox_allNames.Text + "\n" + name;
            } 
        }
    }
}
</string></string>

The Presenter:

using MVP_View;
using Procedures_Model;

namespace Presenter
{

    public class DataPresenter
    {
        private DataProcedures dataprocedures = new DataProcedures();
        private DataViewer dataviewer = new DataViewer();

        private delegate void SaveEventHandler();
        private event SaveEventHandler SaveHandler;

        public DataPresenter()
        {
            this.SaveHandler += new SaveEventHandler(LoadingData);
        }
        
        private void LoadingData()
        {
           
        }

        private void SaveData()
        {
			dataprocedures.SaveData(dataviewer.getFullname());
        }
    }
}

The Model:

namespace Procedures_Model
{
    public class DataProcedures
    {
        public List<string> names = new List<string>();

        public List<string> load_names()
        {
            return names;
        }

        public void SaveData(string fullName)
        {
            names.Add(fullName);
        }
    }
}</string></string></string>

And I don't know how to fire the event from the View and how to catch this event in the Presenter!?