RelayCommand.cs

using System;
using System.Windows.Input;

namespace InvoiceApp
{
    public class RelayCommand : ICommand
    {
        private readonly Action _execute;
        private readonly Predicate<object> _canExecute;

        public RelayCommand(Action execute, Predicate<object> canExecute = null)
        {
            _execute = execute ?? throw new ArgumentNullException(nameof(execute));
            _canExecute = canExecute;
        }

        public event EventHandler? CanExecuteChanged
        {
            add { CommandManager.RequerySuggested += value; }
            remove { CommandManager.RequerySuggested -= value; }
        }

        public bool CanExecute(object? parameter)
        {
            return _canExecute == null || _canExecute(parameter);
        }

        public void Execute(object? _)
        {
            _execute();
        }
    }
}