Oh boy, this one just drove me insane for over two hours.

First of all, I have a ViewModel that is bound to a user control on a “start page” of my application which is designed to show a Recent Files list.  The displaying of this list worked just fine.  The ViewModel encapsulates a command member whose execution method is within the ViewModel class itself.

The command is declared and initialized in my view model like this:

public override void InitializeViewModel()
{
    FileSelectedCommand = new DelegateCommand<Uri>(FileSelectedCommand_Execute);
    base.InitializeViewModel();
}

public DelegateCommand<Uri> FileSelectedCommand { get; protected set; }

The command’s execution method looks like this, but what it’s doing is not what’s important here.

void FileSelectedCommand_Execute(Uri arg)
{
    if (arg != null)
    {
        _RecentDocManager.NotifyDocumentOpened(arg);
        OnFileSelected(new FileLocationEventArgs(arg));
    }
}

What is important is the argument that it’s set up to receive.  Notice that in this case, it’s a URI, which is the exact type to which the CommandParameter argument in my view is bound:

<src:StartPageViewBase.Resources>
    <shared:CommandReference x:Key="fileSelectedCommand" Command="{Binding Path=FileSelectedCommand}" />
    <ResourceDictionary x:Key="dict1">
        <ResourceDictionary.MergedDictionaries>
            <ResourceDictionary Source="/Views/SharedResources.xaml" />
        </ResourceDictionary.MergedDictionaries>
    </ResourceDictionary>
</src:StartPageViewBase.Resources>
<Grid>
    <ItemsControl ItemsSource="{Binding Path=Documents}" DataContext="{Binding}" Grid.IsSharedSizeScope="True" Margin="5,5,5,5">
        <ItemsControl.ItemTemplate>
            <DataTemplate>
                <Button Style="{DynamicResource {x:Static ribbon:RibbonStyles.ButtonKey}}" HorizontalContentAlignment="Left"
                        Command="{StaticResource fileSelectedCommand}" CommandParameter="{Binding Path=Location}">
                    <Grid>
                        <Grid.ColumnDefinitions>
                            <ColumnDefinition Width="Auto" SharedSizeGroup="Name" />
                            <ColumnDefinition Width="*" />
                        </Grid.ColumnDefinitions>
                        <TextBlock Grid.Column="0" Text="{Binding Path=Name}" Margin="0,0,3,0" />
                        <TextBlock Grid.Column="1" Text="{Binding Path=Location}" ToolTip="{Binding Path=Location}" Margin="0,0,3,0" />
                    </Grid>
                </Button>
            </DataTemplate>
        </ItemsControl.ItemTemplate>
    </ItemsControl>
</Grid>

The problem that drove me crazy was the fact that originally my command was declared with a string in the generic type, and in the case of a delegate-command (or a relay-command as it is otherwise known), the generic type defines the type that the potential argument will be.  The CommandParameter argument shown above is bound to a data context member called Location, and this member is a URI, not a string.

The simple mistake of having declared FileSelectedCommand as DelegateCommand<string>, completely disbled the button to which the command is bound.

Uhhhhhhg!

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On another topic, I haven’t done a very good job regularly posting the “This Week In Code” series.  I’m not going to stop doing it, and I promise to try to get back to making it weekly as in the first four, but I’ve been traveling a lot lately and could never guarantee where I’m going to be from one week to the next.

Until next time…