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Help with MVVM in WPF C#

I've been working on learning MVVM.  I know VB.NET a lot better than C#, but most of the resources out there are C#, so I'm trying to polish up my C# while learning MVVM... fun stuff.

I've found a bunch of tutorials out there, including this one: http://www.c-sharpcorner.com/UploadFile/raj1979/simple-mvvm-pattern-in-wpf/

I follow most of it - and I have it working.  However, I kind of want to understand the code a little bit better.  The view model: UserViewModel.cs has the following code (excerpt):

private ICommand mUpdater;
        private ICommand UpdateCommand
        {
            get
            {
                if (mUpdater == null)
                    mUpdater = new Updater();
                return mUpdater;
            }
            set
            {
                mUpdater = value;
            }
        }

        private class Updater : ICommand
        {
            #region ICommand Members

            public bool CanExecute(object parameter)
            {
                return true;
            }

            public event EventHandler CanExecuteChanged;

            public void Execute(object paramater)
            {
               
            }

            #endregion
        }

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So I understand that first we create a property of type iCommand called mUpdater.
Then we create another property called UpdateCommand. (this is bound in the view)

the get and set properties of UpdateCommand is what is really throwing me:
if mUpdater == null (I'm assuming it's null initially), it creates a new instance of Updater.

What exactly is mUpdater returning?  It looks like basically that get and set for UpdateCommand makes it into an instance of Updater with a bool property, an event and a method Execute that is empty.

But what is causing the user information to change when the button linked to UpdateCommand is clicked?
When I look at the UpdateCommand here, it doesn't look like it really should be doing anything.

Sorry if the question seems vague, I just feel like I almost have this, if I can just get my head around this last part.
My entire code is the tutorial in the link above, I didn't re-post it here because it's clearer in the tutorial.

Thanks for your help!!!
0
slightlyoff
Asked:
slightlyoff
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1 Solution
 
Éric MoreauSenior .Net ConsultantCommented:
>>But what is causing the user information to change when the button linked to UpdateCommand is clicked?

When you look at the full sample, you can find that the Command property is bound to the UpdateCommand. This is how the button is linked.

>>What exactly is mUpdater returning?  

mUpdater is the pointer to a Command (action and properties on this action)
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slightlyoffAuthor Commented:
Thanks for the reply.  I see how its linked, what I dont see is where its being told what action to perform "on-click".

Its probably obvious,  but I'm missing it.

Thanks again for your help!
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Éric MoreauSenior .Net ConsultantCommented:
mUpdater is assigned to the Updater class:
 if (mUpdater == null)
                    mUpdater = new Updater();

Open in new window


And the Updater class has an Excute Method.
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ambienceCommented:
This tutorial is actually not a good example of, at least, how to use ICommand. The whole magic is really the two way data-binding in action - because the SelectedItem of the List is bound to the Form on top. You should see that the functionality doesn't change even if you don't click the Update button or just remove it altogether. Due to two-way binding (the default) WPF will automatically reflect changes to the SelectedItem of the List as soon as you change focus across text fields.

Why don't you try to add some of your own code to the Execute method of the Updater class, a simple MessageBox would do just to see how Command routing works?

Another useful exercise that would definitely help understand the stuff better would be to try to add "Add new User" button that should allow you to Add a new item to list. A "Remove" button for deletion.
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slightlyoffAuthor Commented:
I see what you mean.  In effect, the Update button isn't really doing anything.  When you click it, it takes the focus, and when the focus is changed, the ListView is updated regardless of what's going on in the ViewModel.

I took your suggestion to add a messagebox to the execute method.  My code looks like this:

        private class Updater : ICommand
        {
            #region ICommand Members

            public bool CanExecute(object parameter)
            {
                return true;
            }

            public event EventHandler CanExecuteChanged;

            public void Execute(object paramater)
            {
                MessageBox.Show("Hey!");
            }

            #endregion
        }

Open in new window


When I run the application and click the button, nothing happens.
Am I missing something?

Thanks for your response!
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ambienceCommented:
Theres a typo in XAML "UpdateCommad" should be "UpdateCommand"
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slightlyoffAuthor Commented:
Thanks again for the response, and I appreciate your time.  I fixed the typo, but still nothing happens on click.
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ambienceCommented:
Thats odd, just attached a working version of the code from the same tutorial that works.
WpfApplication2.zip
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slightlyoffAuthor Commented:
Thank you for your help.
A couple of things...

1. I misspelled "parameter"
2. I had the methods private, where you had them public.  Once I changed that it worked.

Thanks for your help!!!
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