Disable WPF buttons during longer running process, the MVVM way

Ok the CanExecute method will not work because the click will immediately put you into your long-running task.
So here's how I would do it:

1. Make your view model implement INotifyPropertyChanged

2. Add a property called something like:

   public bool IsBusy
   {
       get
       {
           return this.isBusy;
       }
       set
       { 
           this.isBusy = value;
           RaisePropertyChanged("IsBusy");
       }
   }
   

3. Bind your buttons to this property in this manner:

   <Button IsEnabled="{Binding IsBusy}" .. />
   

4. In your ShowMessage/CallExternal device methods add the line

   IsBusy = true;
   

Should do the trick

I think this is a bit more elegant:

XAML:

<Button IsEnabled="{Binding IsGuiEnabled}" Content="Simulate external device" Command="{Binding DeviceCommand}" Height="50" Margin="0 10"></Button>

C# (using async & await):

public class MainWindowViewModel : INotifyPropertyChanged
{
    private bool isGuiEnabled;

    /// <summary>
    /// True to enable buttons, false to disable buttons.
    /// </summary>
    public bool IsGuiEnabled 
    {
        get
        {
            return isGuiEnabled;
        }
        set
        {
            isGuiEnabled = value;
            OnPropertyChanged("IsGuiEnabled");
        }
    }

    public ICommand DeviceCommand
    {
        get
        {
            return new RelayCommand(this.CallExternalDevice, this.IsGuiEnabled);
        }
    }

    private async void CallExternalDevice(object obj)
    {
        IsGuiEnabled = false;
        try
        {
            await Task.Factory.StartNew(() => Thread.Sleep(3000));
        }
        finally
        {
            IsGuiEnabled = true; 
        }
    }
}

Try this:

//Declare a new BackgroundWorker
BackgroundWorker worker = new BackgroundWorker();
worker.DoWork += (o, ea) =>
{
    try
    {
        // Call your device

        // If ou need to interact with the main thread
       Application.Current.Dispatcher.Invoke(new Action(() => //your action));
    }
    catch (Exception exp)
    {
    }
};

//This event is raise on DoWork complete
worker.RunWorkerCompleted += (o, ea) =>
{
    //Work to do after the long process
    disableGui = false;
};

disableGui = true;
//Launch you worker
worker.RunWorkerAsync();