C#

1. SomeClass is a Control (inherited from PictureBox) that exposes public events:
   you can just subscribe to the mousedown event in the MainForm class (just as you set its location):

public class MainForm : Form
{
    // ...

    public MainForm()
    {   
        // ...
        someObject.MouseDown += mouseDown;
    }

    private void mouseDown(object sender, MouseEventArgs e)
    {
        panel1.BackColor = Color.Red;
    }
}

public class SomeClass : PictureBox
{
    Panel panel1;

    public SomeClass(Panel mainPanel)
    {
        panel1 = mainPanel;
        //...
    }
}

SomeClass someObject = new SomeClass(panel1);

public class SomeClass : PictureBox
    {
    public SomeClass()
        {
        this.BackColor = Color.Green;
        this.MouseDown += new System.Windows.Forms.MouseEventHandler(mouseDown);
        }
    public void mouseDown(object sender, MouseEventArgs e)
        {
        OnFocussed(e);
        }
    /// <summary>
    /// Event to indicate Focussed on
    /// </summary>
    public event EventHandler Focussed;
    /// <summary>
    /// Called to signal to subscribers that Focussed on
    /// </summary>
    /// <param name="e"></param>
    protected virtual void OnFocussed(EventArgs e)
        {
        EventHandler eh = Focussed;
        if (eh != null)
            {
            eh(this, e);
            }
        }
    }

    {
    MyPictureBox1.Focussed += MyPictureBox_Focussed;
    MyPictureBox2.Focussed += MyPictureBox_Focussed;
    MyPictureBox3.Focussed += MyPictureBox_Focussed;
    }

void MyPictureBox_Focussed(object sender, EventArgs e)
    {
    SomeClass sc = sender as SomeClass;
    if (sc != null)
        {
        BackColor = Color.Red;
        }
    }

using System;
using System.Drawing;
using System.Windows.Forms;

namespace CPWorking
{
    public partial class PBX : PictureBox
    {
        public PBX()
        {
            InitializeComponent();

            this.BackColor = Color.Green;

            this.MouseDown += OnMouseDown;
        }

        private void OnMouseDown(object sender, MouseEventArgs mouseEventArgs)
        {
            this.Parent.BackColor = Color.Red;
        }
    }
}

public partial class PBX : PictureBox
{
    public PBX()
    {
        InitializeComponent();
        this.MouseDown += OnMouseDown;
    }

    public Action<string, Color> PBXOnColorChanged { set; get; }

    private void OnMouseDown(object sender, MouseEventArgs mouseEventArgs)
    {
        // always check to be sure "something is listening" !
        if (PBXOnColorChanged != null) PBXOnColorChanged((sender as PBX).Name, this.BackColor);
    }
}

private void Form1_Load(object sender, EventArgs e)
{
    pictureBox1.PBXOnColorChanged += PbxOnColorChanged;
    pictureBox2.PBXOnColorChanged += PbxOnColorChanged;
    pictureBox3.PBXOnColorChanged += PbxOnColorChanged;
}

private void PbxOnColorChanged(string pbxName, Color pbxBackColor)
{
    // do something with the data in 'pbxName and 'pbxBackColor

}

public partial class PBX : PictureBox
{
    public PBX()
    {
        InitializeComponent();
        this.BackColor = Color.Green;
        this.MouseDown += OnMouseDown;
    }

    public Func<string, Color, Color> PBXOnColorChanged { set; get; }

    private void OnMouseDown(object sender, MouseEventArgs mouseEventArgs)
    {
        if (PBXOnColorChanged != null)
        {
            Color result = PBXOnColorChanged((sender as PBX).Name, this.BackColor);

            if (result != this.BackColor && result != Color.Empty)
            {
                this.BackColor = result;
            }
            else
            {   // switch it back to green
                this.BackColor = Color.Green;
            }
        }
    }
}

private Color PbxOnColorChanged(string pbxName, Color pbxBackColor)
{
    Color NextColor = Color.Empty;

    switch (pbxName)
    {
        case "pictureBox1":
            if (pbxBackColor == Color.Green) NextColor = Color.Yellow;
            break;
        case "pictureBox2":
            if (pbxBackColor == Color.Green) NextColor = Color.Red;
            break;
        case "pictureBox3":
            break;
    }

    return NextColor;
}

Peter Vegter wrote:

Is there a reason you don't just subscribe to the 'original' event (MouseDown)?

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>netstandard1.4</TargetFramework>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="System.Collections.Immutable" Version="1.4.0" />
    <PackageReference Include="System.Net.Sockets" Version="4.3.0" />
    <PackageReference Include="System.ValueTuple" Version="4.4.0" />
    <PackageReference Include="System.Interactive.Async" Version="3.1.1" />
    <PackageReference Include="System.Reactive.Core" Version="3.1.1" />
  </ItemGroup>

  <ItemGroup>
    <ProjectReference Include="..\..\Galador.Reflection\Galador.Reflection.PCL\Galador.Reflection.PCL.csproj" />
  </ItemGroup>

</Project>

Super Lloyd wrote:

And also updated them to lastest .csproj format