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How to use Commands in WPF

, 4 Aug 2011
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Today, we will talk about WPF Commands. When I develop a new application in WPF, I try to follow the MVVM Pattern (Model-View-ViewModel).

Today, we will talk about WPF Commands. When I develop a new application in WPF, I try to follow the MVVM Pattern (Model-View-ViewModel).
To help me, I use the toolkit shared by Laurent Bugnion: Mvvm Light Toolkit (Galasoft). If you don’t use it, no matter, the idea is the same with or without a toolkit.

To show you how to use commands, we will create a simple application that save some information about a person.

So a person can be described using this field:

  • LastName
  • FirstName
  • Address
  • ZipCode
  • City

A Save button would call the SaveCommand and a Cancel button would call CancelCommand.

So, we first create our model, adding a Person class in Model directory. Keep things simple:

public class Person
    {
        public string Nom { get; set; }
        public string Prenom { get; set; }
        public string Adresse { get; set; }
        public string CodePostal { get; set; }
        public string Ville { get; set; }
    }

Then we create our ViewModel: I will simply modify the one created by the template for MainWindow. I just edit it to add properties needed:

public class MainViewModel : ViewModelBase
    {
        private readonly Person _newPerson;

        private const string LastNamePropertyName = "LastName";
        public string LastName
        {
            get { return _newPerson.LastName; }
            set
            {
                _newPerson.LastName = value;
                RaisePropertyChanged(LastNamePropertyName);
            }
        }

        private const string FirstNamePropertyName = "FirstName";
        public string FirstName
        {
            get
            {
                return _newPerson.FirstName;
            }
            set
            {
                _newPerson.FirstName = value;
                RaisePropertyChanged(FirstNamePropertyName);
            }
        }

        private const string AddressPropertyName = "Address";
        public string Address
        {
            get
            {
                return _newPerson.Address;
            }
            set
            {
                _newPerson.Address = value;
                RaisePropertyChanged(AddressPropertyName);
            }
        }

        private const string ZipCodePropertyName = "ZipCode";
        public string ZipCode
        {
            get
            {
                return _newPerson.ZipCode;
            }
            set
            {
                _newPerson.ZipCode = value;
                RaisePropertyChanged(ZipCodePropertyName);
            }
        }

        private const string CityPropertyName = "City";
        public string City
        {
            get
            {
                return _newPerson.City;
            }
            set
            {
                _newPerson.City = value;
                RaisePropertyChanged(CityPropertyName);
            }
        }

        ///
        /// Initializes a new instance of the MainViewModel class.
        ///
        public MainViewModel()
        {
            _newPerson = new Person();
        }
    }

Let’s create the UI. We will have a simple form in the MainWindow.xaml:

<Grid Height="195">
        <Grid.RowDefinitions>
            <RowDefinition Height="30" />
            <RowDefinition Height="30" />
            <RowDefinition Height="30" />
            <RowDefinition Height="30" />
            <RowDefinition Height="30" />
            <RowDefinition Height="Auto" />
            <RowDefinition Height="42*" />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="122*" />
            <ColumnDefinition Width="296*" />
        </Grid.ColumnDefinitions>
        <Label Content="LastName" Grid.RowSpan="2" Height="28" HorizontalAlignment="Left"
	 Margin="40,5,0,0" Name="label1" VerticalAlignment="Top" />
        <Label Content="FirstName" Grid.Row="1" Grid.RowSpan="2" Height="28"
	 HorizontalAlignment="Left" Margin="40,5,0,0" Name="label2"
	 VerticalAlignment="Top" />
        <Label Content="Address" Grid.Row="2" Grid.RowSpan="2" Height="28"
	 HorizontalAlignment="Left" Margin="40,5,0,0" Name="label3"
	 VerticalAlignment="Top" />
        <Label Content="Code Postal" Grid.Row="3" Grid.RowSpan="2" Height="28"
	 HorizontalAlignment="Left" Margin="40,5,0,0" Name="label4"
	 VerticalAlignment="Top" />
        <Label Content="City" Grid.Row="4" Grid.RowSpan="3" Height="28"
	 HorizontalAlignment="Left" Margin="40,5,0,0" Name="label5"
	 VerticalAlignment="Top" />
        <TextBox Grid.Column="1" Grid.RowSpan="2" Height="23" HorizontalAlignment="Left"
	 Margin="20,5,0,0" Name="textBox1" VerticalAlignment="Top" Width="120"
	 Text="{Binding Path=LastName}" />
        <TextBox Grid.Column="1" Grid.Row="1" Grid.RowSpan="2" Height="23"
	 HorizontalAlignment="Left" Margin="20,5,0,0" Name="textBox2"
	 VerticalAlignment="Top" Width="120" Text="{Binding Path=FirstName}" />
        <TextBox Grid.Column="1" Grid.Row="2" Grid.RowSpan="2" Height="23"
	 HorizontalAlignment="Left" Margin="20,5,0,0" Name="textBox3"
	 VerticalAlignment="Top" Width="120" Text="{Binding Path=Address}" />
        <TextBox Grid.Column="1" Grid.Row="4" Height="23" HorizontalAlignment="Left"
	 Margin="20,5,0,0" Name="textBox4" VerticalAlignment="Top" Width="120"
	 Text="{Binding Path=City}" />
        <TextBox Grid.Column="1" Grid.Row="3" Grid.RowSpan="2" Height="23"
	 HorizontalAlignment="Left" Margin="20,5,0,0" Name="textBox5"
	 VerticalAlignment="Top" Width="120" Text="{Binding Path=ZipCode}" />
		<Button Content="Save" Grid.Column="1" Grid.Row="6" Height="23"
		 HorizontalAlignment="Left" Margin="25,10,0,0" Name="button1"
		 VerticalAlignment="Top" Width="75" />
        <Button Content="Cancel" Grid.Column="1" Grid.Row="6" Height="23"
	 HorizontalAlignment="Left" Margin="120,10,0,0" Name="button2"
	 VerticalAlignment="Top" Width="75" />
    </Grid>

Here is what it should look like:

Now, the main reason of this article: The Commands.
We will create our two commands and then bind them to our buttons.

How do we define a command?
First, the command should implement ICommand interface.

Then, a Create method will create the command so that we can use it.
Then, we find two methods: the first one, CanExecute, would check if the command can be executed and the second one, Execute, will execute the command.

The Save Command:

public ICommand SaveCommand
        {
            get;
            internal set;
        }

        private bool CanExecuteSaveCommand()
        {
            return !string.IsNullOrEmpty(LastName);
        }

        private void CreateSaveCommand()
        {
            SaveCommand = new RelayCommand(SaveExecute, CanExecuteSaveCommand);
        }

        public void SaveExecute()
        {
            Person.Save(_newPerson);
        }

If we look at the CanExecute method, we can see that the execution would be allowed only if there is a Lastname.
We, now, add the creation of the method in the constructor of our ViewModel:

public MainViewModel()
        {
            _newPerson = new Person();
            CreateSaveCommand();
        }

And we bind our command on the Save button:

<Button Content="Save" Grid.Column="1" Grid.Row="6" Height="23" 
HorizontalAlignment="Left" Margin="25,10,0,0" Name="button1" 
VerticalAlignment="Top" Width="75" Command="{Binding Path=SaveCommand}" />

Let’s launch the application and test this command.

When the application is launched, the LastName is, of course, empty. And you can see, the Save button is disabled and we cannot save:

We add a lastname and set the focus on the following textbox and we can observe that the Save button is now enabled. The condition in the CanExecute returns true!

If we delete the lastname and change focus, the button will be disabled again.

Now, we add the Cancel command. It works the same way as the Save command but there is no requirement to allow the action. So, we will not used the CanExecute method:

public ICommand CancelCommand
        {
            get;
            internal set;
        }

        private void CreateCancelCommand()
        {
            CancelCommand = new RelayCommand(CancelExecute);
        }

        public void CancelExecute()
        {
            LastName = string.Empty;
            FirstName = string.Empty;
            Address = string.Empty;
            ZipCode = string.Empty;
            City = string.Empty;
        }

In the Execute method, we empty all the textboxes. Fill in the information and click on the Cancel button, all the textboxes will be cleared.

Don’t forget to call the CreateCancelCommand method in the constructor and to bind the command to the Cancel command.

The constructor:

public MainViewModel()
        {
            _newPerson = new Person();
            CreateSaveCommand();
            CreateCancelCommand();
        }

Cancel button binding:

<Button Content="Cancel" Grid.Column="1" Grid.Row="6" Height="23" 
HorizontalAlignment="Left" Margin="120,10,0,0" Name="button2" 
VerticalAlignment="Top" Width="75" Command="{Binding Path=CancelCommand}" />

This is a very simple example of how to use Commands in WPF.
Let’s try it!

Source code to download: MvvmCommand

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

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About the Author

Nadege Rouelle
Architect
France France
I'm coding in .Net since 9 years, most with ASP.Net and SharePoint and a little using WPF, MVC, Windows Phone 8 and WinRT technology.
I have learned so much reading others experience and tutorials, or tips so I try to do the same, keeping learning from others of course.
You can also find my blog here : http://sharemstips.wordpress.com/
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Comments and Discussions

 
GeneralNice start Pinmemberkerrywsanders15-Aug-11 3:21 

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