MVVM and Silverlight help

Question

I am trying to learn the MVVM pattern using Silverlight. There are tons of videos and blogs. I understand it in a high level, but cant seem to get my own implementation of it.

I got the following View:

<UserControl x:Class="SilverlightApplication1.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    xmlns:vm="clr-namespace:SilverlightApplication1.ViewModel"
    mc:Ignorable="d"
    d:DesignHeight="300" d:DesignWidth="400">

    <Grid x:Name="LayoutRoot" Background="White">
        <TextBlock Text="{Binding Name, Mode=TwoWay}" Width="200" Height="200"/>
        <Button Name="btn1" Width="200" Height="20" Margin="100,268,100,12" Click="btn1_Click"/>

    </Grid> </UserControl>

In my VM, I have: namespace SilverlightApplication1.ViewModel

{
    public class ViewModel : INotifyPropertyChanged
    {
        private Model.UserModel m_model;

        public ViewModel()
        {
            m_model = new Model.UserModel();
        }

        public string Name 
        {
            get
            {
                return m_model.Name;
            }
            set
            {
                if (value != m_model.Name)
                {
                    m_model.Name = value;
                    InvokePropertyChanged("Name");
                }
            }
        }

        public event PropertyChangedEventHandler PropertyChanged;

        private void InvokePropertyChanged(string propertyName)
        {
            if (this.PropertyChanged != null)
            {
                this.PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
            }

        }


    }
}

and in my Model:

namespace SilverlightApplication1.Model
{
    public class UserModel
    {
        public string Name { get; set; }    
    }
}

Basically, I want to simulate a button click and update the property so that it can fire the property changed event. I tried hard coding the property on the MainPage code behind like this:

private void btn1_Click(object sender, RoutedEventArgs e)
        {
            ViewModel.ViewModel vm = new ViewModel.ViewModel();
            vm.Name = "Test";
        }

Shouldnt this update the property (for Name) and raise the propertychanged event? I have seen other examples do something similar to this. I dont understand as to what is subscribing to the event

Can anyone shine some light?

Things look right on the surface, however, did you set breakpoints or do any troubleshooting? what were your findings? What happens when you run the app? Everything works except your NotifyPropertyChange ? — RyBolt
The PropertyChanged event is always Null. When I set breakpoints to the property, it updates, however the event never gets raised — NewGuyIntown

RyBolt RyBolt · Accepted Answer · 2011-01-23T17:24:32

In this example, to fix it, try this code:

private void btn1_Click(object sender, RoutedEventArgs e)
        {
            ViewModel.ViewModel vm = new ViewModel.ViewModel();
            this.LayoutRoot.DataContext = vm;
            vm.Name = "Test";
        }

Note: I know you just learning MVVM, but after you're comfortable with this paradigm, you should adopt a framework so you can focus on things besides the MVVM plumbing. In saying that, doing things in the code-behind is not the MVVM way, like setting the data context. However, I assume you're just experimenting and learning, so it's fine for now.

MVVM and Silverlight help

2 Answers