Simple class that listens to mouse clicks and triggers an event.

MouseListener.cs

public class MouseEventArgs : EventArgs
{
    public bool Release { get; private set; }
    public short StateRaw { get; private set; }
    public MouseEventArgs(bool release, short state)
    {
        Release = release;
        StateRaw = state;
    }
}

class MouseListener : IDisposable
{
    [DllImport("user32.dll")]
    static extern short GetKeyState(int vKey);
    
    public delegate void MouseClick(object sender, MouseEventArgs args);
    public event MouseClick OnMouseClick;
    private Thread thread = null;
    private short rawState = short.MinValue;
    
    public MouseListener() => thread = new Thread(() => { while (true) Tick(); });
    
    public void Start() => thread.Start();
    
    private void Tick()
    {
        short rawStateNow = GetKeyState(0x01);
        bool state = rawStateNow < 0;
        if (rawStateNow != rawState)
        {
            OnMouseClick(this, new MouseEventArgs(!state, rawStateNow));
            rawState = rawStateNow;
        }
    }
    
    public void Dispose()
    {
        thread.Abort();
        thread = null;
    }
}