WPF/Silverlight: How do you make a Button call ICommand.CanExecute when the command parameter is changed?

Question

How do you make a Button call ICommand.CanExecute when the command parameter is changed?

This is my current XAML.

<Button Content="Delete" Command="{Binding DeleteItemCommand}" CommandParameter="{Binding SelectedItem, ElementName=DaGrid}" />

EDIT It appears this is only an issue in WPF.

Answer 1

Strange. Normally OnCommandParameterChanged calls UpdateCanExecute (both internal methods). Does the Binding to CommandParameter work as expected?

Answer 2

You need to call CommandManager.InvalidateRequerySuggested to re-evaluate CanExecute. Note that it will re-evaluate it for all commands, not just the one your want...

Answer 3

I'm not sure what you're doing wrong, but here is an example of a Button being controlled both by a BindingParameter and a CanExecute Flag. Perhaps your binding parameter isn't a DependencyProperty, and therefore, when it changes the Button isn't being notified.

<UserControl x:Class="SilverlightICommandTest.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:ct="clr-namespace:SilverlightICommandTest"
         mc:Ignorable="d"
         d:DesignHeight="300" d:DesignWidth="400">
<UserControl.Resources>
    <ct:TestModel x:Key="Model" />
</UserControl.Resources>

<StackPanel x:Name="LayoutRoot" Orientation="Vertical" Background="White" DataContext="{StaticResource Model}">
    <CheckBox Content="Enable" IsChecked="{Binding TestCmd.CanDoCommand, Mode=TwoWay}" />
    <Grid HorizontalAlignment="Stretch">
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" />
            <ColumnDefinition />
        </Grid.ColumnDefinitions>
        <TextBlock Text="{Binding ElementName=testSlider, Path=Value}" Width="40" Grid.Column="0" />
        <Slider Name="testSlider" Minimum="0" Maximum="100" SmallChange="1" Grid.Column="1" />
    </Grid>
    <Button Command="{Binding TestCmd}" CommandParameter="{Binding ElementName=testSlider, Path=Value}" Content="Do Something" />
</StackPanel>
</UserControl>

And the code file:

using System;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Input;

namespace SilverlightICommandTest
{
    public partial class MainPage : UserControl
    {
        public MainPage()
        {
            InitializeComponent();
        }
    }

    public class TestModel : DependencyObject
    {
        TestCommand _testCmd = new TestCommand();

        public TestCommand TestCmd { get { return _testCmd; } }

        public TestModel()
        {
        }
    }

    public class TestCommand : DependencyObject, ICommand
    {
        public static readonly DependencyProperty CanDoCommandProperty = DependencyProperty.Register("CanDoCommand", typeof(Boolean), typeof(TestCommand), new PropertyMetadata(false, new PropertyChangedCallback(CanDoCommandChanged)));

        public Boolean CanDoCommand
        {
            get { return (Boolean)GetValue(CanDoCommandProperty); }
            set { SetValue(CanDoCommandProperty, value); }
        }

        public event EventHandler CanExecuteChanged;

        public TestCommand()
        {
        }

        public Boolean CanExecute(Object parameter)
        {
            return this.CanDoCommand && (((Int32)(Double)parameter) % 2 == 0);
        }

        public void Execute(Object parameter)
        {
            MessageBox.Show("Oh Hai!");
        }

        private void OnCanDoCommandChanged(DependencyPropertyChangedEventArgs args)
        {
            if (this.CanExecuteChanged != null)
            {
                this.CanExecuteChanged(this, new EventArgs());
            }
        }

        private static void CanDoCommandChanged(DependencyObject sender, DependencyPropertyChangedEventArgs args)
        {
            ((TestCommand)sender).OnCanDoCommandChanged(args);
        }
    }
}

In the future I recommend doing a little more research on the pattern first (http://www.silverlightshow.net/items/Model-View-ViewModel-in-Silverlight.aspx), and if you still can't figure it out, post more of your source code.