Button not enabled even after providing CommandParameter in WPF

Question

I created a button whose commandparameter is set and command using a class that implements ICommand interface. But my button is disabled. Why is that? I got this code from here: ICommand is like a chocolate cake

<Window x:Class="ICommand_Implementation_CSharp.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:ICommand_Implementation_CSharp"
    Title="MainWindow" Height="350" Width="525">
<Grid>      
    <Grid>
        <Grid.Resources>
            <local:HelloWorldCommand x:Key="hwc"  />
        </Grid.Resources>

        <Button Command="{StaticResource hwc}" CommandParameter="Hello" 
                Height="23" HorizontalAlignment="Left" Margin="212,138,0,0" 
                Name="Button1" VerticalAlignment="Top" Width="75">Button</Button>
    </Grid>
</Grid>

and my class is

class HelloWorldCommand:ICommand
{
    public bool CanExecute(object parameter)
    {
        return parameter != null;
    }

    public event EventHandler CanExecuteChanged;

    public void Execute(object parameter)
    {
        MessageBox.Show(parameter.ToString());
    }
}

Dennis · Accepted Answer · 2016-06-03T14:11:03.260

Well, this is very-very simple implementation of ICommand.

As @JleruOHeP says, partially problem can be solved by swapping setters of Command and CommandParameter. But this is ugly way, because you have to remember the sequence every time.

More correct way is to tell CommandManager to re-query states of command:

public class HelloWorldCommand : ICommand
{
    public bool CanExecute(object parameter)
    {
        return parameter != null;
    }

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

    public void Execute(object parameter)
    {
        MessageBox.Show(parameter.ToString());
    }
}

Now the sequence of setters is indifferent.
To understand how CommandManager works, you can read this nice article from Josh Smith.

+1 I`ve just started to type complex explanation, but you was faster :) — JleruOHeP, Aug 24 '12 at 06:59
@NikhilAgrawal, look at this post: http://waxtadpole.wordpress.com/2009/10/21/wpf-mvvm-in-vb-net-delegating-event-subscriptions-for-routedcommand-or-any-icommand-in-the-viewmodel/ — Dennis, Aug 24 '12 at 07:04

JleruOHeP · Answer 2 · 2012-08-24T06:58:45.137

5

The simplest answer - switch places of Command and Command parameter:

<Button CommandParameter="Hello" Command="{StaticResource hwc}" .../>

But better one is given by @Dennis

edited Aug 24 '12 at 06:58

answered Aug 24 '12 at 06:42

JleruOHeP

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score 0 · Answer 3 · answered Aug 16 '16 at 08:07

In my case it was the CommandParameter type that was causing the issue. My button was simply bound like this:

<Button Content="New" Command="{Binding NewCommand}" CommandParameter="False" />

The underlying NewCommand is a RelayCommand<bool>. Somehow XAML was not able to translate False to bool. (Note that it does work for many built-in types and properties; maybe some TypeConverter or something at action there).

Solution was to simply spoon-feed XAML about the real underlying type of CommandParameter, like this:

<Button Content="New" Command="{Binding NewCommand}">
    <Button.CommandParameter>
        <sys:Boolean>
          False
        </sys:Boolean>
    </Button.CommandParameter>
</Button>

You have to import sys namespace at the top of your XAML file, like this:

xmlns:sys="clr-namespace:System;assembly=mscorlib"

Hope this helps someone down the road.

Button not enabled even after providing CommandParameter in WPF

3 Answers3

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