EscribePassword/App.xaml.cs

using System;
using System.Threading;
using System.Windows;
using Application = System.Windows.Application;

namespace EscribePassword
{
    public partial class App : Application
    {
        private static Mutex _mutex = null;

        public KeyboardHelper PasteLogic { get; } = new KeyboardHelper();

        protected override void OnStartup(StartupEventArgs e)
        {
            const string appName = "EscribePasswordApp";
            bool createdNew;

            _mutex = new Mutex(true, appName, out createdNew);

            if (!createdNew)
            {
                // Hay otra instancia en ejecución, cerramos esta instancia
                Application.Current.Shutdown();
                return;
            }

            base.OnStartup(e);
            PasteLogic.SaveCurrentWindow();
            SetInitialWindowPosition();
        }

        public void SetInitialWindowPosition()
        {
            var screenRect = PasteLogic.GetScreenOfPreviousWindow();
            var mainWindow = Application.Current.MainWindow;

            if (mainWindow != null)
            {
                var cursorPosition = System.Windows.Forms.Cursor.Position;
                var screen = Screen.FromPoint(cursorPosition);

                double newLeft = screenRect.Right;
                double newTop = screenRect.Top;

                if (newLeft < screenRect.Left)
                {
                    newLeft = screenRect.Left;
                }

                if (newLeft + mainWindow.Width > screen.WorkingArea.Right)
                {
                    newLeft = screen.WorkingArea.Right - mainWindow.Width;
                }

                mainWindow.Left = newLeft;
                mainWindow.Top = newTop;
            }
        }
    }
}