What are the special values of WPF's Binding e

问题:

I already know about the Binding.DoNothing that one can return from an IValueConverter implementation to signify that no other operation should take place.

However, I cannot find a reference, or documentation nicely summing out, what the other special values are - such as returning the fallback value. What are they?

回答1:

Binding.DoNothing is an object instance that you actively return from a value converter; it instructs the binding engine to not update the value of the target property at all. Here's a nice example by Josh Smith of what you might use this for.

FallbackValue is a property that you set on bindings; it allows you to specify the value to be applied to the target property if:

• the binding source cannot be resolved (e.g. wrong binding path), or
• the binding property value is equal to DependencyProperty.UnsetValue, or
• a value converter used for the binding throws an exception, or
• a value converter used for the binding returns DependencyProperty.UnsetValue, or
• the value produced by the binding pipeline is not valid for the target property (e.g. wrong type)

TargetNullValue is also a property you set on bindings; it allows you to specify the value to be applied to the target property if the value of the source property is null. For example, if you bind a text box to a string property TargetNullValue lets you pick what appears in the text box if the source string is null.