---

Es bastante más complicado que el pseudo-código que has mostrado de ejemplo.

Debes crear un hook de bajo nivel o bien puedes registrar el dispositivo del mouse mediante el modelo RAWINPUT.

Ejemplo de uso:

using Microsoft.VisualBasic;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Data;
using System.Diagnostics;
public sealed class Form1 : Form
{
 
	private MouseHook withEventsField_mouseEvents = new MouseHook(Install: false);
	private MouseHook mouseEvents {
		get { return withEventsField_mouseEvents; }
		set {
			if (withEventsField_mouseEvents != null) {
				withEventsField_mouseEvents.MouseLeftDown -= MouseEvents_MouseLeftDown;
			}
			withEventsField_mouseEvents = value;
			if (withEventsField_mouseEvents != null) {
				withEventsField_mouseEvents.MouseLeftDown += MouseEvents_MouseLeftDown;
			}
		}
 
	}
 
	private void Form1_Load(object sender, EventArgs e)
	{
		// Install Mouse Hook on the System.
		this.mouseEvents.Install();
 
		// Start processing mouse events.
		this.mouseEvents.Enable();
 
	}
 
 
	private void Form1_FormClosing(object sender, FormClosingEventArgs e)
	{
		// Stop processing mouse events.
		this.mouseEvents.Disable();
 
		// Uninstall the mouse hook from system.
		this.mouseEvents.Uninstall();
 
	}
 
 
	private void MouseEvents_MouseLeftDown(object sender)
	{
		Trace.WriteLine(string.Format("Mouse Left Down"));
 
	}
	public Form1()
	{
		FormClosing += Form1_FormClosing;
		Load += Form1_Load;
	}
 
}
 
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Código fuente destripado para cumplir la función que mencionas, y convertido a C# (puede no haberse convertido correctamente):

using Microsoft.VisualBasic;
using System;
using System.Collections;
using System.Collections.Generic;
using System.Data;
using System.Diagnostics;
#region " Instructions "
 
// Go to page:
// Project > Properties > Debug
//
// Then uncheck the option:
// "Enable the Visual Studio Hosting Process"
 
#endregion
 
#region " Imports "
 
using System.Reflection;
using System.Runtime.InteropServices;
 
#endregion
 
#region " MouseHook "
 
/// <summary>
/// A low level mouse hook that processes mouse input events.
/// </summary>
internal sealed class MouseHook : IDisposable
{
 
	#region " P/Invoke "
 
	protected sealed class NativeMethods
	{
 
		#region " Methods "
 
		[DllImport("user32.dll", CallingConvention = CallingConvention.StdCall, CharSet = CharSet.Auto)]
		public static extern IntPtr CallNextHookEx(IntPtr idHook, int nCode, IntPtr wParam, IntPtr lParam);
 
		[DllImport("user32.dll", CallingConvention = CallingConvention.StdCall, CharSet = CharSet.Auto)]
		public static extern IntPtr SetWindowsHookEx(HookType idHook, LowLevelMouseProcDelegate lpfn, IntPtr hInstance, uint threadId);
 
		[DllImport("user32.dll", CallingConvention = CallingConvention.StdCall, CharSet = CharSet.Auto)]
		public static extern bool UnhookWindowsHookEx(IntPtr idHook);
 
		#endregion
 
		#region " Enumerations "
 
		public enum WindowsMessages : uint
		{
 
			WMlButtonDown = 0x201u
 
		}
 
		public enum HookType : uint
		{
 
			WHMouseLL = 14u
 
		}
 
		[Flags()]
		public enum MsllHookStructFlags : int
		{
 
			LlmhfInjected = 1,
 
			LlmhfLowerILInjected = 2
 
		}
 
		#endregion
 
		#region " Delegates "
 
		public delegate int LowLevelMouseProcDelegate(int nCode, NativeMethods.WindowsMessages wParam, IntPtr lParam);
 
		#endregion
 
	}
 
	#endregion
 
	#region " Properties "
 
	/// <summary>
	/// Handle to the hook procedure.
	/// </summary>
	private IntPtr MouseHook { get; set; }
 
	/// <summary>
	/// The mouse hook delegate.
	/// </summary>
	private NativeMethods.LowLevelMouseProcDelegate MouseHookDelegate { get; set; }
 
	#endregion
 
	#region " Events "
 
	/// <summary>
	/// Occurs when the mouse left button is pressed.
	/// </summary>
	public event MouseLeftDownEventHandler MouseLeftDown;
	public delegate void MouseLeftDownEventHandler(object sender);
 
	#endregion
 
	#region " Constructors "
 
	/// <summary>
	/// Initializes a new instance of the <see cref="MouseHook"/> class.
	/// </summary>
	/// <param name="Install">
	/// If set to <c>true</c>, the Hook starts initialized for this <see cref="MouseHook"/> instance.
	/// </param>
 
	public MouseHook(bool install = false)
	{
		if (install) {
			this.Install();
		}
 
	}
 
	#endregion
 
	#region " Public Methods "
 
	/// <summary>
	/// Installs the Mouse Hook, then start processing messages to fire events.
	/// </summary>
 
	public void Install()
	{
		if (this.IsVisualStudioHostingProcessEnabled()) {
			throw new Exception("Visual Studio Hosting Process should be deactivated.");
			return;
		}
 
		this.MouseHookDelegate = new NativeMethods.LowLevelMouseProcDelegate(LowLevelMouseProc);
 
		try {
			this.MouseHook = NativeMethods.SetWindowsHookEx(NativeMethods.HookType.WHMouseLL, this.MouseHookDelegate, new IntPtr(Marshal.GetHINSTANCE(Assembly.GetExecutingAssembly.GetModules()(0)).ToInt32), 0);
 
		} catch (Exception ex) {
			throw;
 
		}
 
	}
 
	/// <summary>
	/// Uninstalls the Mouse Hook and free all resources, then stop processing messages to fire events.
	/// </summary>
 
	public void Uninstall()
	{
		this.Finalize();
 
	}
 
	#endregion
 
	#region " Private Methods "
 
	private bool IsVisualStudioHostingProcessEnabled()
	{
		return AppDomain.CurrentDomain.FriendlyName.EndsWith("vshost.exe", StringComparison.OrdinalIgnoreCase);
	}
 
	private int LowLevelMouseProc(int nCode, NativeMethods.WindowsMessages wParam, IntPtr lParam)
	{
 
 
		if (nCode == 0) {
			switch (wParam) {
 
				case NativeMethods.WindowsMessages.WMlButtonDown:
					if (MouseLeftDown != null) {
						MouseLeftDown(this);
					}
 
 
					break;
				default:
					// Do Nothing
					break; // TODO: might not be correct. Was : Exit Select
 
 
					break;
			}
 
			return Convert.ToInt32(NativeMethods.CallNextHookEx(MouseHook, nCode, new IntPtr(wParam), lParam));
 
		} else if (nCode < 0) {
			return Convert.ToInt32(NativeMethods.CallNextHookEx(MouseHook, nCode, new IntPtr(wParam), lParam));
 
		// nCode > 0
		} else {
			return Convert.ToInt32(NativeMethods.CallNextHookEx(MouseHook, nCode, new IntPtr(wParam), lParam));
 
		}
 
	}
 
	#endregion
 
	#region "IDisposable Support"
 
	/// <summary>
	/// Flag to detect redundant calls at <see cref="Dispose"/> method.
	/// </summary>
 
	private bool disposedValue;
 
	protected void Dispose(bool disposing)
	{
 
		if (!this.disposedValue) {
			// Dispose managed state (managed objects).
			if (disposing) {
 
			// Free unmanaged resources (unmanaged objects).
			} else {
				NativeMethods.UnhookWindowsHookEx(this.MouseHook);
 
			}
 
		}
 
		this.disposedValue = true;
 
	}
 
 
	protected override void Finalize()
	{
		// Do not change this code. Put cleanup code in method: Dispose(ByVal disposing As Boolean)
 
		this.Dispose(disposing: false);
		base.Finalize();
 
	}
 
 
	private void Dispose()
	{
		// Do not change this code. Put cleanup code in method: Dispose(ByVal disposing As Boolean)
 
		this.Dispose(disposing: true);
		GC.SuppressFinalize(obj: this);
 
	}
 
	#endregion
 
}
 
#endregion
 
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Código fuente original y documentado de pies a cabeza, en VB.Net:
➢ http://pastebin.com/Bjr06jGR

Si quieres ver un ejemplo de como sería usando RAWINPUT, puedes hacerte una idea aquí:
➢ [SOURCE] Algoritmo KeyLogger (RawInput) - By Elektro

Saludos.