Binding visibility of a control to 'Count'

问题:

I have a list of objects contained in an IEnumerable<>. I would like to set the visibility of a control based on the count of this list. I have tried:

 Visibility="{Binding MyList.Count>0?Collapsed:Visible, Mode=OneWay}"

But this doesn't work. I tried binding MyList.Count to the text in a text block to ensure that the count value was correct, and it is. It just doesn't seem to set the visibility correctly.

回答1:

You cannot use logical or code-expressions in bindings (it expects a PropertyPath). Either use a converter or triggers, which is what i would do:

<YourControl.Style>                     
    <Style TargetType="YourControl">
        <Setter Property="Visibility" Value="Collapsed" />
        <Style.Triggers>
            <DataTrigger Binding="{Binding MyList.Count}" Value="0">
                <Setter Property="Visibility" Value="Visible" />
            </DataTrigger>
        </Style.Triggers>
    </Style>
</YourControl.Style>

(You can of course refactor the style into a resource if you wish.)

回答2:

There is three ways:

1. to use Triggers mentioned by H.B.
2. to use convertors by implementing IValueConverter in a class and setting the Converter property of Binding to an instance of IValueConverter in that class
3. to define a property in your ViewModel to directly return the Visibility state.

You could always use Triggers method and it always is a good approach. The third method is useful(and is best) when you are using MVVM pattern (and you are not restricting yourself from referencing UI related assemblies in your ViewModel) I suggest using Triggers, but if you dont want to make your xaml dirty by that much markup codes use converters.

回答3:

You should use a converter, which converts the Count property to a Visibility value, or perhaps a new "HasItems" boolean property to a Visibility value. We use something, for example, called boolToVisibilityConvert, to handle jobs like this.

I can give you more precise details, if you need them.