TestCommand.cs

public class TestCommand : ICommand
{
    private bool _whatShouldWeReturnForCanExecute;

    public bool WhatShouldWeReturnForCanExecute
    {
        get => _whatShouldWeReturnForCanExecute;
        set
        {
            if (_whatShouldWeReturnForCanExecute == value)
            {
                return;
            }

            _whatShouldWeReturnForCanExecute = value;
            CanExecuteChanged?.Invoke(this, EventArgs.Empty);
        }
    }

    public bool CanExecute(object parameter)
    {
        return _whatShouldWeReturnForCanExecute;
    }

    public void Execute(object parameter)
    {
        MessageBox.Show("TestCommand executed");
    }

    public event EventHandler CanExecuteChanged;
}