WPF-Prism CanExecute method not being called

It is most likely that the bound control is never asking for the CanExecute state again. You need to call the RaiseCanExecuteChanged method on the DelegateCommand whenever you detect a condition that changes the command's CanExecute state. This signals the bound control to update the CanExecute state.

Starting with Prism6 the DelegateCommand can "observe" your propertys. Means everytime your property is changing the CanExecute-Method is called. The good thing is you get rid of RaiseCanExecuteChanged in the Propertysetter. You can also chain-call that method if you want to observe more properties:

public LoginViewModel(){    this.LoginCommand =        new DelegateCommand<object>(            this.LoginExecute, this.CanLoginExecute).ObservesProperty(() => Username).ObservesProperty(() => Password);}

Furthermore if you just want your DelegateCommand be called depending on the state of a boolean property you can use .ObservesCanExecute(()=> BoolProp)

public LoginViewModel(){    this.LoginCommand =        new DelegateCommand<object>(            this.LoginExecute).ObservesCanExecute(()=> IsServerOnline).ObservesProperty(() => Username).ObservesProperty(() => Password);}

You dont need this.CanLoginExecute anymore.

Code for RaiseCanExecuteChanged:

    private void RaiseCanExecuteChanged()    {        DelegateCommand<object> command = LoginCommand as DelegateCommand<object>;        command.RaiseCanExecuteChanged();    }    public const string UsernameProperty = "Username";    private String _username;    public String Username    {        get { return _username; }        set        {            _username = value;            this.NotifyPropertyChanged(UsernameProperty);            RaiseCanExecuteChanged();        }    }