Silverlight icommand when is canexecute called

No question too simple! Sign in to vote. Hi there. Can someone help me out? Wednesday, April 20, AM. Hi, There is one last step step six that's missing from that article to fully implement commanding. I hope this answers your question!

Quick and to the point, excellent. Y'all fellas have saved my bacon again. Imports System. ObjectModel Imports System. Students End Sub. SubmitChanges End Sub. End Class End Namespace. Note that in this example, the parameter is not used. Since the entire grid is saved in a save operation in this example, there is no processing needed for the selected student. The last step is the easiest: just bind the Command property of the Button or Hyperlink to the command property created in the prior step.

This method ultimately calls the SubmitChanges method, saving any changes. This passes the currently selected Student to the command.

In this specific example, the parameter is not used so you only really need to set the Command property, not the CommandParameter property. Use this technique any time you need to process commands from the Button or Hyperlink controls. Or any other control that inherits from ButtonBase. If you need to process commands from other controls, such as a click on a grid row, you need more code.

But that is for a later post. Deborah Kurata, very helpfull post. I have a registration form just 4 fields. Can u help me wht type of code is in the view model so that i can proceed my proof of concept a stuck me for two days. Once binded to the UI, it will automatically load the information.

This will give you clean, maintainable code with separation of the view from the business logic. To implement Command binding, you have to create a DelegateCommand which implements the ICommand interface.

The ICommand interface is available in System. Input namespace in the System. It defines the contract for commanding. Now we have to implement the methods defined in the ICommand interface to our DelegateCommand class. Here is the simple implementation of the same:. Let us now implement the ViewModelBase for our application. Though for this sample application you can directly use the ViewModel , it is recommended to create a base class implementation by inheriting the INotifyPropertyChanged interface so that if you are creating multiple ViewModel s, it will be easier to inherit the base class.

Here is the code for that:. Now all the base classes are ready to use. Hence, we can go further to create our first ViewModel. In this example, we are going to load Customer information, so we will name it as CustomerViewModel. It takes two parameters. The first one is the Action which fires when the command executes and the second one is a Function pointer which returns whether the command can be executed. If it returns true , the command binded to the element will be enabled to do the operation and if it returns false , the command binded to the element will be disabled by default.

Once it becomes true by any other operation, the UI thread automatically makes the element enabled. As of now, our back end code implementation is ready and now we have to create our UI to show the customer information.

First of all, we will create the static instance of the viewmodel as a resource of the UserControl. Once we click on the button, it should execute the command and load the data in the ListBox. The button will have the LoadCustomersCommand associated with it. If the canExecute method returns false , you will notice the button as disabled and when it returns true , it will become enabled.

Our full code implementation is ready. We can now run the application to see the code to execute. Press F5 to run it. Once the view is loaded, you will see the ListBox is empty and there is a button just below the empty ListBox. Now press the button. It will fire the command to the viewmodel and will fetch the customer information from the provider to load it in the collection. Once the collection is modified, this will automatically trigger the PropertyChanged event to update the UI.

This ensures that the business logic is totally separated from the UI implementation. This gives higher readability and maintainability of the code. You are welcome to make any queries, comments or suggestions here.

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Sign in Email. Forgot your password? Search within: Articles Quick Answers Messages. Improve this answer. Paul Wheeler Paul Wheeler Instead of using a private local member to store your value you use the DependencyObject.

See msdn. Ozan Ozan 4, 2 2 gold badges 21 21 silver badges 35 35 bronze badges. Actually, there isn't even a PropertyChangedCallback on the CommandParameter dependency property, so nothing special happens when the parameter value changes. Ugh, that's some really bad news Thomas.

It is hard to belive they missed that use case. Thomas: we are talking about the Silverlight framework here. Jonathan: DependencyProperty, CommandParameter is one. CanExecute, make sure the Binding works. Ozan, you are talking about the Silverlight framework ;. Thomas Levesque Thomas Levesque k 62 62 gold badges silver badges bronze badges. That doesn't seem to do anything.

I even tried having my command explicitly register for the CommmandManager. RequerySuggested event.