How to validate selected item is null or not

How to validate selected item is null or not

I wrote MVVM code in WPF. I want that if user selects nothing and click on
edit button, it should display Message to user to select item first. With
thing i am not getting clear is that how to put this validation before
UpdateCommand. I need your suggestions. Thank You. Here Code is:
class UserViewModel
{
private IList<User> _UsersList;
public User SelectedUser { get; set; }
public UserViewModel()
{
_UsersList = new List<User>
{
new User{UserId =
1,FirstName="Raj",LastName="Beniwal",City="Delhi",State="DEL",Country="INDIA"},
new User{UserId=2,FirstName="Mark",LastName="henry",City="New
York", State="NY", Country="USA"},
new
User{UserId=3,FirstName="Mahesh",LastName="Chand",City="Philadelphia",
State="PHL", Country="USA"},
new
User{UserId=4,FirstName="Vikash",LastName="Nanda",City="Noida",
State="UP", Country="INDIA"},
new
User{UserId=5,FirstName="Harsh",LastName="Kumar",City="Ghaziabad",
State="UP", Country="INDIA"},
new
User{UserId=6,FirstName="Reetesh",LastName="Tomar",City="Mumbai",
State="MP", Country="INDIA"},
new
User{UserId=7,FirstName="Deven",LastName="Verma",City="Palwal",
State="HP", Country="INDIA"},
new
User{UserId=8,FirstName="Ravi",LastName="Taneja",City="Delhi",
State="DEL", Country="INDIA"}
};
if(SelectedUser == null) { ShowNoUserSelectedWarning(); } // How to put
this before UpdateCommand To Warn User
}
public IList<User> Users
{
get { return _UsersList; }
set { _UsersList = value; }
}
public void ShowNoUserSelectedWarning()
{
System.Windows.MessageBox.Show("Warning");
}
private ICommand mUpdater;
public ICommand UpdateCommand
{
get
{
if (mUpdater == null)
mUpdater = new Updater();
return mUpdater;
}
set
{
mUpdater = value;
}
}
private class Updater : ICommand
{
#region ICommand Members
public bool CanExecute(object parameter)
{
return true;
}
public event EventHandler CanExecuteChanged;
public void Execute(object parameter)
{
}