在DataTemplateSelector上使用DataTemplates的WPF字典的键始终为空值

Question

I have a dictionary on my DataTemplateSelector that holds a list of datatemplates. The constructor creates the dictionary holding DataTemplate properties as the value but the dictionary is always reflecting null values, meanwhile I can look at the property when the SelectTemplate is called and the property has the correct DataTemplate in it. What is going on that the dictionary (which I thought was reference type) always show the initial 'null' value when the dictionary was created and not the object?

Is there anyway I can get this to work without scrapping the dictionary and using a switch? I was using it to avoid a giant switch statement, but I'm not sure why this isn't working.

View:

  <selector:NodePropertyGridTemplateSelector x:Key="sel">
     <selector:NodePropertyGridTemplateSelector.PageLoadedDataTemplate>
                <DataTemplate>
                   ...
                </DataTemplate>
            </selector:NodePropertyGridTemplateSelector.PageLoadedDataTemplate>
    </selector:NodePropertyGridTemplateSelector>

DataTemplateSelector:

 public class NodePropertyGridTemplateSelector : DataTemplateSelector
{
    private Dictionary<string, DataTemplate> _dictionary;

    public NodePropertyGridTemplateSelector()
    {
        _dictionary = new Dictionary<string, DataTemplate>();
        _dictionary.Add("PageLoaded", PageLoadedDataTemplate);
    }

    public override DataTemplate SelectTemplate(object item, System.Windows.DependencyObject container)
    {
        if (item != null)
        {
            return (DataTemplate)_dictionary["PageLoaded"];

            //return PageLoadedDataTemplate;  this works fine. The above dictionary is null for the value at this key.
        }
        return base.SelectTemplate(item, container);
    }

    public DataTemplate PageLoadedDataTemplate { get; set; }

Answer 1

When the constructor of NodePropertyGridTemplateSelector executes, the content of the property PageLoadedDataTemplate is still null , and this null value is being added to the dictionary.

Later - after the constructor returned and the NodePropertyGridTemplateSelector instance has been created - another value (a reference to a DataTemplate object) is being assigned to PageLoadedDataTemplate . However, while the content of the PageLoadedDataTemplate property is now different, the content stored in the dictionary still remains null .

To have the dictionary always reflect the current content of the PageLoadedDataTemplate property, update the dictionary in the property setter. By doing so, the getter must also be implemented - which should simply fetch and return the respective value stored in the dictionary.

public class NodePropertyGridTemplateSelector : DataTemplateSelector
{
    private readonly Dictionary<string, DataTemplate> _dictionary = new Dictionary<string, DataTemplate>();

    public override DataTemplate SelectTemplate(object item, System.Windows.DependencyObject container)
    {
        if (item != null)
        {
            DataTemplate dt;
            if (_dictionary.TryGetValue("PageLoaded", out dt))
                return dt;
        }
        return base.SelectTemplate(item, container);
    }

    public DataTemplate PageLoadedDataTemplate
    {
        get
        {
            DataTemplate dt;
            return _dictionary.TryGetValue("PageLoaded", out dt) ? dt : null;
        }

        set
        {
            if (value == null) _dictionary.Remove("PageLoaded");
            else _dictionary["PageLoaded"] = value;
        }
    }
}

The example code given here avoids null values being stored in the dictionary. If it the dictionary (or the property) should/can be accessed from different threads in a concurrent manner, it might be safer to use ConcurrentDictionary .