Question

I have most of the Label objects in my app bound such that they can be replaced from a webservice.

I store my replacements in a Dictionary. The replacements vary based on the Client that loaded for the app.

The problem I am having is that most of the time I don’t replace the labels, and the binding failure is throwing an exception that is very slow.

Here is an example: (Note this code functions correctly, it is just slow if it can’t find a match.)

<Label Content="_Gender"
       Helpers:CaptionHelper.PropertyName="Content"
       Helpers:CaptionHelper.PropertyValue="{Binding Labels[Gender], 
                                                     FallbackValue=_Gender}"
       Target="{Binding ElementName=cbGender}" />

When the binding tries to lookup Lables[Gender], I get this in my output window:

System.Windows.Data Warning: 17 : Cannot get 'Item[]' value (type 'String') from 'Labels' (type 'Dictionary`2'). BindingExpression:Path=Labels[Gender]; DataItem='MyViewMV'
      (HashCode=63776763); target element is 'Label' (Name=''); target property is 'PropertyValue'(type 'String') TargetInvocationException:'System.Reflection.TargetInvocationException: 
           Exception has been thrown by the target of an invocation. ---> System.Collections.Generic.KeyNotFoundException: 
           The given key was not present in the dictionary.
   at System.Collections.Generic.Dictionary`2.get_Item(TKey key)
   --- End of inner exception stack trace ---
   at System.RuntimeMethodHandle._InvokeMethodFast(Params Truncated)
   at System.Reflection.RuntimeMethodInfo.Invoke(Params Truncated)
   at System.Reflection.RuntimeMethodInfo.Invoke(Params Truncated)
   at MS.Internal.Data.PropertyPathWorker.GetValue(Params Truncated)
   at MS.Internal.Data.PropertyPathWorker.RawValue(Params Truncated)'

Since I have a lot of these, it is taking about a full second to run through them all.

Is there a way to make the binding not throw an exception when the dictionary look up fails? Or somehow fail quicker?

Answer 1

If you have the option to, I would change your IDictionary implementation to one that returns null and instead use TargetNullValue (or even be IDictionary<TKey, object> and return DependencyProperty.UnsetValue if you still use FallbackValue):

public class PassthruDictionary<TKey, TValue> : IDictionary<TKey, TValue>
{
    private Dictionary<TKey, TValue> instance;

    // ... other stuff

    public TValue this[TKey key]
    {
        get
        {
            TValue value;
            if (instance.TryGetValue(key, out value))
            {
                return value;
            }
            else
            {
                return default(TValue);
            }
        }
        // ... more
    }
}