Command to open a second window in MVVM

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2.00/5 (1 vote)

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Hi,
I have been banging my head against the wall on this one.

Until now I have used the code behind for button logic (I know bad.). so I decided to creat a command for my buttons. My command code:

<Button Name="AddUser" Command="{Binding Path=AddUserCommand}" CommandParameter="AddUser" Content="New User" Margin="20" />

And My command code:

public ICommand AddUserCommand { get; set; }

public bool CanExecute(object parameter)
{
if (parameter != null)
{
return true;
}
else
{
return false;
}
}

public event EventHandler CanExecuteChanged
{
add { CommandManager.RequerySuggested += value; }
remove { CommandManager.RequerySuggested -= value; }
}

public void Execute(string parameter)
{
if (parameter == "AddUser")
{
var window = new AddUser();
window.Show();
}
}
I have set break points and my command code never gets touched.

I don't see where My problem is.

Thanks in advance!

Posted 27-Oct-13 13:36pm

David C# Hobbyist.

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tgrt 27-Oct-13 19:46pm

Is that all your code? Because, I don't see where your command is being created. You have a property, but you're not setting it. You're binding to that property which is a null reference... So I'm not sure what you're expecting.

David C# Hobbyist. 27-Oct-13 19:57pm

Hmmmm maybe I haven't learned enough yet. Should I be setting this in my ViewModel constructor?

tgrt 27-Oct-13 19:59pm

See solution below.

1 solution

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tgrt · Accepted Answer · 2013-10-27T13:57:00

Solution 1

In your ctor do the following (you might have to implement the DelegateCommand - it's present in Prism):

C#

AddUserCommand = new DelegateCommand(Execute, CanExecute);

Also change the following:
* Change parameter type in Execute to object and cast it within the function
* Get rid of CanExecuteChanged
* Consider not using the parameter at all in Execute and XAML, because you don't need it

Posted 27-Oct-13 13:57pm

tgrt

Comments

tgrt 27-Oct-13 20:00pm

The following site has a good implementation of the DelegateCommand if you're not using Prism. http://blog.rdeverett.com/post/812093504/wpf-delegatecommand

David C# Hobbyist. 27-Oct-13 20:03pm

There is more to the if statement then I included. I was trying to reuse the Execute method for more than 1 command.
Prism available from nuget i presume.
I will try Your suggestion.

tgrt 27-Oct-13 20:11pm

You can name the Execute method anything you want. That is, your command instantiation can be: AddUserCommand = new DelegateCommand(AddUser, CanAddUser);

In that case, you'd have two methods with the same signature as Execute and CanExecute currently have.

The only reason to use a parameter in your scenario is if you want poor maintainability. In other words, there's no good reason to do it that way.

You can also pass in lambdas to the instantiation if you have encapsulated an object that handles this type of stuff. For example:

AddUserCommand = new DelegateCommand(parm => userManager.AddUser(), parm => userManager.CanAddUser);