在XAML中创建集合并绑定单个集合项

Question

I've created a custom control which, when bound to a custom collection of objects, displays the content of those objects.

Usually, I can use this control by simply going:

<local:CustomCollectionDisplayer DataContext="{Binding Source={x:Static Application.Current}, Path=SomeObject.InstanceOfCustomeCollectionOfCustomItems}" />

My problem now comes where I want to recycle this control to show only a single object. In the xaml, I want to make a custom collection where the only item in the collection is bound to that single object.

The code looks like this:

<local:CustomCollectionDisplayer>
    <local:CustomCollectionDisplayer.DataContext>
        <local:CustomCollection>
            <local:CustomItem Reference="{Binding Source={x:Static Application.Current}, Path=SomeObject.InstanceOfCustomItem}"/>-->
        </local:CustomCollection>
    </local:CustomCollectionDisplayer.DataContext>
</local:CustomCollectionDisplayer>

Obviously, there's no 'Reference' property which I can use to make the CustomItem in the collection point to the instance of CustomItem in 'SomeClass'. How can I achieve this without creating a dummy CustomCollection containing this CustomItem in my object viewmodel?

Answer 1

There already is a x:Reference markup extension , but it is very limited as it only gets objects by name. You could write your own markup-extension which can get properties. eg

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows.Markup;
using System.ComponentModel;

namespace Test.MarkupExtensions
{
    [ContentProperty("Object")]
    public class GetExtension : MarkupExtension
    {
        public object Object { get; set; }
        public string PropertyName { get; set; }

        public GetExtension() { }
        public GetExtension(string propertyName)
            : this()
        {
            if (propertyName == null)
                throw new ArgumentNullException("propertyName");
            PropertyName = propertyName;
        }

        public override object ProvideValue(IServiceProvider serviceProvider)
        {
            if (PropertyName == null)
                throw new InvalidOperationException("PropertyName cannot be null");
            if (Object == null)
            {
                var target = (IProvideValueTarget)serviceProvider.GetService(typeof(IProvideValueTarget));
                Object = target.TargetObject;
            }
            var prop = Object.GetType().GetProperty(PropertyName);
            if (prop == null)
                throw new Exception(String.Format("Property '{0}' not found on object of type {1}.", PropertyName, Object.GetType()));
            return prop.GetValue(Object, null);
        }
    }
}

Which could be used like this:

<local:CustomCollectionDisplayer>
    <local:CustomCollectionDisplayer.DataContext>
        <local:CustomCollection>
            <me:Get PropertyName="InstanceOfCustomItem">
                <me:Get PropertyName="SomeObject" Object="{x:Static Application.Current}"/>
            </me:Get>
        </local:CustomCollection>
    </local:CustomCollectionDisplayer.DataContext>
</local:CustomCollectionDisplayer>

You could also resolve a whole PropertyPath at once in the extension if you prefer that, this is just a sketchy example.

---

Another option is to bind the DataContext directly to the object and wrap it in the collection using a Converter .