async2.cs

private string username;  
public string Username  
{ 
    get 
    { 
        return username; 
    } 
    set 
    { 
        username = value; 
        RaisePropertyChanged("Username"); 
    } 
}

// Assume this gets called from a command bound to a button, for example
public void UpdateUsername()  
{ 
    var task = ThreadPool.RunAsync(t => 
    { 
        // set the private property to prevent raising property changed, which has to be done on the UI thread
        username = GetUsername(); 
    }); 
    task.Completed = RunOnComplete(RefreshUI); 
} 

public void RefreshUI()  
{
    // set the public property to private one which raises property changed and triggers the UI to update
    Username = username;   
}

public AsyncActionCompletedHandler RunOnComplete(Action method)  
{ 
    return new AsyncActionCompletedHandler((IAsyncAction source, AsyncStatus status) => 
    { 
        DispatcherHelper.CheckBeginInvokeOnUI(() => { method(); }); 
    }); 
}