Implementing a DelegateCommand with reason why is disabled

The following link point out how to show Tooltip ONLY when the control is DISABLED: http://stackoverflow.com/questions/2890872/show-wpf-tooltip-on-disabled-item-only

That's nice. However, I need something more.

I want to show a tooltip explaning why the BUTTON is disabled. Most of the time, my button is using Commands, so there are a lot of code like this:

<Button Command="{Binding Authenticate}" IsDefault="True" Content="ENTRAR" />

public class LoginViewModel
{
public DelegateCommand Authenticate { get; private set; }

public LoginViewModel()
{
Authenticate = new DelegateCommand(ExecuteLogin, CanExecuteLogin);
}

private bool CanExecuteLogin()
        {
            return ClientId.HasValue && !string.IsNullOrEmpty(UserName) && UserName.Contains("@") && !string.IsNullOrEmpty(Password);
        }
}

FYA, I'm using PRISM framework (that's where DelegateCommand come from). So, using command is awesome because without changing the XAML, I can enable or disable the button. That's beautifull. I wonder if it is possible to do similar thing with tootip.

I mean, without having a property on ViewModel bound to Button's ToolTip property. I don't want that. I wonder if its is possible to put the tooltip text as a part of the command, so I would have something like that:

Authenticate = new DelegateCommand(ExecuteLogin, CanExecuteLogin, ReasonCantExecute);

private string ReasonCantExecute()
        {
            return "Provide a password";
        }

Take a look at WPF FlashMessage
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