RelayCommand.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Input;

namespace ModelessWindowSolution
{
    public class RelayCommand : ICommand
    {
        private readonly Action<object> _execute;
        private readonly Func<object, bool> _canExecute;
        public event EventHandler CanExecuteChanged
        {
            add => CommandManager.RequerySuggested += value;
            remove => CommandManager.RequerySuggested -= value;
        }
        public RelayCommand(Action<object> Execute, Func<object, bool> CanExecute)
        {
            _execute = Execute ?? throw new ArgumentNullException(nameof(Execute));
            _canExecute = CanExecute;
        }
        public bool CanExecute(object parameter)
        {
            return _canExecute?.Invoke(parameter) ?? true;
        }

        public void Execute(object parameter)
        {
            _execute(parameter);
        }
    }
}